akbasic/TODO.md

Record

Outstanding work is in the issue tracker, not in this file: https://source.starfort.tech/andrew/akbasic/issues

This file was the implementation plan for the Go → C port of deps/basicinterpret, written to be executed by AI agents rather than read for inspiration. The port is done, so what it is now is the record: the design decisions that are settled, the deviations from the reference and why each was taken, the defects that were found and fixed, and the reasoning behind the measurements.

Each open item that moved leaves a line saying what it was and which issue carries it, because an entry explaining why a defect matters is worth keeping beside the work it constrains — but the tracking happens there, not here.

Issues are labelled by kind and blast radius, and milestoned by what they can land in: 0.1.x for anything that changes no public contract, 0.2.0 for new verbs and observable behaviour changes, 1.0.0 for the design decisions. Everything filed carries status::grooming.

Two gaps this file recorded and deliberately did not file — the missing HUD anchors and a dismissable dialog — are now libakgl #79 and #80. §7's rule is right that changing a dependency is that repository's decision; it does not follow that reporting the gap is.

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0. Agent protocol

0.1 The Go reference is deprecated. Stop matching it.

deps/basicinterpret is a dead project. It will not be updated, and this interpreter is no longer required to reproduce its behaviour. Recorded here first because it silently reverses the premise several sections of this file were written on, and because an agent that reads them without this will park work that is no longer blocked.

What it changes:

• §6 is now an ordinary defect list. "Port the behaviour first so the port is provably faithful" is retired. Fix them because they are wrong, not when fidelity permits.
• §1.8's message-text contract is now a convention. Improving a message is allowed; it costs a golden file, which is a cost rather than a veto.
• §5's bar drops from "defensible against the golden suite" to defensible on its own merits.
• tests/reference/ becomes a regression suite rather than a specification. Diverging from it is allowed and must be deliberate and recorded — see its README.

What it does not change:

• The corpus stays and stays green. Forty-one real BASIC programs with known-good output are worth having whatever their provenance, and an unexplained change there is still a red flag.
• The Go source stays readable as documentation. It remains the best answer to "what did the original actually do here", which is a question worth being able to answer even once the answer stops being binding.
• Nothing about the ak* house rules, which never came from the reference.

Read these before touching anything, in this order:

1. MAINTENANCE.md in this repository — project goals, the libakerror convention, the error-code range map, and the dependency versions.
2. deps/libakerror/AGENTS.md — the ATTEMPT/CLEANUP/PROCESS/HANDLE/FINISH protocol.
3. deps/libakerror/UPGRADING.md — 1.0.0's status registry. Required before writing an error code; the mechanism it replaced is gone.
4. deps/libakstdlib's issue tracker — the defects and gaps in the library this port calls into. §1.9 below says which calls are cleared for use; that section is not optional reading, it bans a family of functions the port would otherwise reach for by reflex.
5. deps/libakgl/AGENTS.md — the no-malloc rule and the commit co-author requirement.
6. deps/basicinterpret/README.md — the language reference and the unimplemented list. Still worth reading for the verb set and the semantics; no longer binding, per §0.1.

Rules for working this file:

• Do not mark an item done until its acceptance command passes on a clean out-of-tree build. "It compiles" is not acceptance.
• Do update the tracker in the same commit as the work: close the issue, or replace it with the defect it uncovered. Outstanding items live there; this file holds the record.
• Do add the agent program name, model name and version as a commit co-author. That rule comes from libakgl and applies here.
• Never hand-edit generated output. build/ trees and the generated akerror.h are off-limits; change the generator.
• Never reformat a file you are not otherwise changing.
• When a step is blocked because libakgl cannot supply a capability, do not work around it here. File it against libakgl in its issue tracker -- what the BASIC verb requires, what the akgl_* entry point should look like, and what tests would cover it -- and note the block in §7 below.

Style, restated so nobody has to go look: C99, 4-space indent, tabs at width 8 (stroustrup), function-body braces in column 0 on their own line, control braces on the same line, always brace, spaces inside control-flow parens — if ( x == y ) {. Pointer star binds to the identifier: char *name. Prefix is akbasic_ for functions, akbasic_TypeName for types, AKBASIC_UPPER_SNAKE for macros. static helpers drop the prefix. Parameter names must match between header and source.

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1. Design decisions already made

These are settled. Do not relitigate them mid-port; if evidence says one is wrong, say so in a commit that changes it deliberately, and update this section.

1.1 Reflection becomes one aligned dispatch table

The Go runtime resolves verbs with reflect.MethodByName("Command" + NAME) (basicruntime.go:401), functions with "Function" + NAME, and special parse paths with "ParseCommand" + NAME (basicparser.go:103). C has no reflection and we are not adding any. Replace all three with one static table in src/verbs.c, sorted by name, searched with bsearch(3):

/* name        token type              parse handler          exec handler        */
{ "AUTO",      AKBASIC_TOK_CMDIMM,     NULL,                  cmd_auto      },
{ "DATA",      AKBASIC_TOK_COMMAND,    parse_data,            cmd_data      },
{ "DEF",       AKBASIC_TOK_COMMAND,    parse_def,             cmd_def       },

A NULL parse handler means "parse the rval as a plain expression", which is exactly what commandByReflection returning (nil, nil) means today. Adding a verb is adding one row plus two functions. Keep the table column-aligned and one row per verb — it is a table, so it gets laid out as one.

This also kills the Go scanner's three separate maps (reservedwords, commands, functions, basicscanner.go:64-67): the token type lives in the same row.

1.2 Strings are fixed-size and live inline

libakstdlib has no string type. libakgl has akgl_String but it is PATH_MAX bytes, refcounted, and pool-allocated — wrong shape for a value that gets copied on every assignment, and it would drag a libakgl dependency into the core interpreter.

Define in include/akbasic/types.h:

#define AKBASIC_MAX_STRING_LENGTH    256    /* matches AKBASIC_MAX_LINE_LENGTH */

and give akbasic_Value a char stringval[AKBASIC_MAX_STRING_LENGTH] inline. clone() becomes a struct assignment. No allocator, no refcount, no lifetime question.

Tradeoff, stated: every akbasic_Value is ~300 bytes, so one environment's values[AKBASIC_MAX_VALUES] pool is ~19KB, and 32 environments is ~610KB of BSS. That is fine on a PC and is the price of never calling malloc. If it ever isn't fine, the knob is AKBASIC_MAX_STRING_LENGTH, not the allocator.

Truncation is an error, not a silent clamp: FAIL_RETURN(e, AKBASIC_ERR_VALUE, ...).

1.3 Maps become fixed-capacity open-addressed tables

Five Go maps need replacing:

Go site	Purpose	C replacement
BasicScanner.reservedwords/commands/functions	keyword → token type	the §1.1 static table + bsearch
BasicEnvironment.variables	name → *BasicVariable	akbasic_SymbolTable, capacity AKBASIC_MAX_VARIABLES
BasicEnvironment.functions	name → *BasicFunctionDef	akbasic_SymbolTable, capacity AKBASIC_MAX_FUNCTIONS
BasicEnvironment.labels	name → line number	akbasic_SymbolTable, capacity AKBASIC_MAX_LABELS

One implementation, src/symtab.c, keyed by aksl_strhash_djb2() (already in libakstdlib) with linear probing and a fixed slot array. Use the existing hash; do not write another one. Table full is an error, not a resize.

Caveat, recorded upstream: the wrapper sign-extends char, so a high-bit byte hashes wrong — "\xff\xfe" returns 5859874 where the unsigned char answer is 5868578. BASIC identifiers are 7-bit ASCII (the scanner only accepts IsLetter/IsDigit plus a type suffix), so this cannot bite the symbol tables. It would bite if anyone later keys a table on a string literal or a filename. Do not work around it here; it is already filed upstream.

1.4 Environments come from a pool and are released

Go calls new(BasicEnvironment) at basicruntime.go:121 and basicparser_commands.go:124 and never frees one. A long-running GOSUB or FOR in Go leaks; the GC eventually catches some of it, and nothing in the tests notices.

C gets HEAP_ENVIRONMENT[AKBASIC_MAX_ENVIRONMENTS] with akbasic_env_acquire() / akbasic_env_release(), in the shape of akgl_heap_next_*. akbasic_runtime_prev_environment() must release the environment it pops. Pool exhaustion is AKBASIC_ERR_ENVIRONMENT, reported with the current line number.

Watch the one place this is not a clean stack: userFunction (basicruntime.go:348) stores a BasicEnvironment by value inside BasicFunctionDef and re-init()s it on every call. In C the funcdef holds an akbasic_Environment * acquired at DEF time and reset per call — it is owned by the funcdef, not the pool's free list, until the funcdef dies.

1.5 Output goes through a text sink backend

Write() and Println() (basicruntime_graphics.go:140,148) mirror every line to stdout and to an SDL surface. That mirror is the only reason the golden-file suite works. Do not reproduce it as a hardcoded pair of calls.

Define a record of function pointers, populated by an initializer — the house pattern:

typedef struct akbasic_TextSink
{
    void *self;
    akerr_ErrorContext AKERR_NOIGNORE *(*write)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*writeln)(struct akbasic_TextSink *self, char *text);
    akerr_ErrorContext AKERR_NOIGNORE *(*readline)(struct akbasic_TextSink *self, char *dest, size_t len);
    akerr_ErrorContext AKERR_NOIGNORE *(*clear)(struct akbasic_TextSink *self);
} akbasic_TextSink;

akbasic_sink_init_stdio() is in the library and akbasic_sink_init_akgl() is in the akgl-backed module. The driver is supposed to pick: a default build selects stdio, while an AKBASIC_WITH_AKGL build selects the SDL text path and mirrors its output to stdout. It does not do that yet; §3 records the missing standalone frontend. Cursor arithmetic, wrapping and scrolling belong to the akgl sink, not to the interpreter.

1.6 The interpreter steps; it does not run

Go's run() (basicruntime.go:682) is a for {} that owns the process until MODE_QUIT. Goal 3 forbids that: a host game must be able to bound execution.

The library exposes:

akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_step(akbasic_Runtime *obj);
akerr_ErrorContext AKERR_NOIGNORE *akbasic_runtime_run(akbasic_Runtime *obj, int maxsteps);

_step() does exactly what one iteration of Go's for {} body does and returns. _run(obj, maxsteps) loops _step() until the mode is AKBASIC_MODE_QUIT or maxsteps steps have elapsed; maxsteps <= 0 means unbounded, which is what the standalone driver passes. This is a deliberate restructure, and it must not change a single byte of golden output.

QUIT sets AKBASIC_MODE_QUIT and returns. Nothing in the library calls exit(), abort(), or FINISH_NORETURN. FINISH_NORETURN appears exactly once in the tree, in src/main.c.

1.7 Error codes are absolute, reserved from the 1.0.0 registry

deps/libakerror is at 1.0.0. Read deps/libakerror/UPGRADING.md before writing an error code; MAINTENANCE.md's error-code section is the condensed version. Three things this changes from what you may have seen in an older draft or in libakgl:

• AKERR_MAX_ERR_VALUE no longer exists. The name registry is sparse and takes any int. There is nothing for a consumer to size, and no compile definition to set. Anywhere you find one, delete it.
• __AKERR_ERROR_NAMES no longer exists. The table is private.
• Codes are absolute integer constants at 256 or above, never AKERR_LAST_ERRNO_VALUE + N. The offset scheme is what produced the live libakgl collision documented in MAINTENANCE.md.

akbasic owns 512–767 per the range map in MAINTENANCE.md. Declare it as an enum, so the values stay compile-time integer constants (HANDLE expands to case labels, which require that) and adding a code does not mean renumbering an offset:

#define AKBASIC_OWNER "akbasic"

enum {
    AKBASIC_ERR_BASE        = 512,   /** Start of akbasic's reserved status range */
    AKBASIC_ERR_SYNTAX      = AKBASIC_ERR_BASE,
                                     /** Parse-time grammar violation */
    AKBASIC_ERR_TYPE,                /** Incompatible types in an operation */
    AKBASIC_ERR_UNDEFINED,           /** Reference to an undefined verb, function or label */
    AKBASIC_ERR_BOUNDS,              /** Array subscript or pool index out of range */
    AKBASIC_ERR_ENVIRONMENT,         /** Environment pool exhausted or orphaned environment */
    AKBASIC_ERR_VALUE,               /** A value was malformed, truncated or unconvertible */
    AKBASIC_ERR_STATE,               /** A verb ran outside the block structure it requires */
    AKBASIC_ERR_LIMIT       = AKBASIC_ERR_BASE + 256
};

akbasic_init() reserves the whole 256 in one call and then names each code. Both registry calls return akerr_ErrorContext * and are AKERR_NOIGNORE, so a collision is an ordinary error — PASS it and let it propagate out of init:

akerr_ErrorContext AKERR_NOIGNORE *akbasic_init(void)
{
    PREPARE_ERROR(errctx);

    PASS(errctx, akerr_reserve_status_range(AKBASIC_ERR_BASE,
					    AKBASIC_ERR_LIMIT - AKBASIC_ERR_BASE,
					    AKBASIC_OWNER));
    PASS(errctx, akerr_register_status_name(AKBASIC_OWNER, AKBASIC_ERR_SYNTAX,
					    "Syntax Error"));
    /* ... one per code ... */
    SUCCEED_RETURN(errctx);
}

Reserve the whole range in one call — a subset or superset of your own range raises AKERR_STATUS_RANGE_OVERLAP, not a no-op. Use akerr_register_status_name(), never the two-argument akerr_name_for_status(status, name) set path: the owned form is the one that catches a component writing into a range that is not its own, and the two-argument form is what let libakgl silently clobber libakerror's names.

There is no startup ceiling to assert any more — the BSS-overflow hazard the old draft defended against was deleted along with AKERR_MAX_ERR_VALUE. What replaces it is the reservation itself: if akbasic_init() returns an error, something else owns part of 512–767 and the process must not continue as though it does not.

Do not call akerr_init() first. Every registry entry point calls it, and since 1.0.0 it no longer clears reservations made before it ran.

1.8 Error message text is a convention, no longer a contract

tests/language/array_outofbounds.txt is, verbatim:

? 20 : RUNTIME ERROR Variable index access out of bounds at dimension 0: 4 (max 2)\n\n

The trailing double newline is real: basicError builds a string ending in \n and hands it to Println, which adds another.

This used to be a hard contract and is now a default. The Go implementation is deprecated and will not be updated, so the two projects are no longer required to match — see §0.1. What survives is the practical half: these strings and this newline behaviour are what every expectation in tests/reference/ was written against, so changing one means changing golden files, and that is worth doing on purpose rather than by accident. A message that reads awkwardly may now be improved; do it deliberately, move the expectations in the same commit, and add a line to §5.

Numeric formatting still matches the reference: integers via %" PRId64 ", floats via %f (Go's %f and C's %f both give six decimals — tests/reference/language/arithmetic/float.txt confirms). No reason to change it, which is different from not being allowed to.

1.9 Which libakstdlib calls are cleared for use — the bans are lifted

deps/libakstdlib is at 0.2.0, and that release fixed all six of the confirmed defects this section was built around, plus seventeen contract gaps. The table of bans that used to live here is gone because it is no longer true, and leaving it would send an agent around a workaround for a function that now works.

What changed, in the four that mattered to this port:

Was banned	Now
aksl_atoi / _atol / _atoll / _atof	Every one takes a dest pointer and raises AKERR_VALUE on no digits or trailing junk, ERANGE on overflow — exactly the contract this section demanded
aksl_list_append / _iterate	Append no longer truncates to two nodes; iterate no longer skips the first half
aksl_tree_iterate	AKERR_ITERATOR_BREAK stops the traversal
aksl_realpath	Rewritten; no longer reads uninitialised memory on its error path

Two signature changes reach this repository. aksl_fread and aksl_fwrite now take a required size_t *nmemb_out and report a short transfer as AKERR_IO instead of a silent success — so a DSAVE onto a full disk is now an error a program sees, where before it reported nothing. src/runtime_commands.c passes the count and discards it, which is correct: the library does the noticing now.

src/convert.c is gone. It existed only because aksl_ato* could not report a conversion failure, and its own note said what to do when that changed: "When it grows it, delete src/convert.c and switch the call sites over." That is done. Six call sites now go straight to the library — newLiteralInt and newLiteralFloat in src/grammar.c, the line number in src/scanner.c, FunctionVAL in src/runtime_functions.c, INPUT in src/runtime_commands.c, and GSHAPE's handle in src/runtime_graphics.c.

Three things that fell out of it, recorded because each was checked rather than assumed:

• aksl_strtoll(str, NULL, base, &dest) is the exact replacement, not aksl_atoll, wherever a base is involved. The ato* forms are base 10; src/grammar.c picks base 8 or 16 from the lexeme's prefix. A NULL endptr is what makes trailing junk an error rather than a stopping point, and that is the whole contract this port needs.
• The raised status changed from AKBASIC_ERR_VALUE to AKERR_VALUE, and the message text with it — VAL("garbage") now says no digits in "garbage". §1.8 makes message text part of the acceptance contract, so this was checked against the corpus first: no golden file contained a conversion message, which is also why §1.9 used to ask for one. Two now exist, tests/language/numeric/val_refuses_garbage and octal_literal, so the next change to that text has to move a golden file.
• tests/convert.c became tests/numeric_contract.c. The assertions did not become worthless when the wrapper went away — they became assertions about a contract this port depends on and no longer owns, which is worth pinning exactly because a regression in it would make VAL("garbage") return 0 silently. Same reasoning libakstdlib's own tests/test_status_registry.c gives for pinning that it reserves no status range.

One caveat survives the upgrade unchanged: aksl_strhash_djb2 still sign-extends char, so a high-bit byte hashes differently from the unsigned char answer. BASIC identifiers are 7-bit ASCII so the symbol tables cannot reach it — see §1.3, which is still accurate.

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2. What exists — the core port is complete and green

Phases 0 through 6 of the original plan are done. The interpreter builds clean under -Wall -Wextra, passes the reference's entire corpus, and passes under ASan and UBSan.

It did reproduce the reference byte for byte, and that claim is retired rather than broken: §0.1 released it, and one case has since diverged deliberately (§6 item 16, listed in tests/reference/README.md). Everything else still matches, which is worth knowing but is no longer a gate.

cmake -S . -B build && cmake --build build --parallel
ctest --test-dir build --output-on-failure          # 59/59
cmake -S . -B build-asan -DAKBASIC_SANITIZE=ON      # 59/59
cmake -S . -B build-cov  -DAKBASIC_COVERAGE=ON      # 92.3% line, 96.9% function

Module	Source	Reference
Symbol table	src/symtab.c	the five Go maps
Value	src/value.c	basicvalue.go
Variable	src/variable.c	basicvariable.go
Tokens and AST leaves	src/grammar.c	basicgrammar.go + basicparser.go
Dispatch table	src/verbs.c	the three reflection lookups
Scanner	src/scanner.c	basicscanner.go
Parser	src/parser.c, src/parser_commands.c	basicparser*.go
Environment	src/environment.c	basicenvironment.go
Runtime	src/runtime.c	basicruntime.go minus SDL
Verbs and functions	src/runtime_commands.c, src/runtime_functions.c	basicruntime_{commands,functions}.go
Text sink	src/sink_stdio.c	basicruntime_graphics.go's stdout mirror
Stdio driver	src/main.c	main.go minus its SDL frontend
Embedding examples	examples/embed.c, examples/hostvars.c	(new)

akbasic_runtime_load(rt, source) was added while writing the README's embedding section: a host usually holds its script as a string and wants the sink reserved for output, and the only path that existed — AKBASIC_MODE_RUNSTREAM reading through the sink's readline — forces a game to point its output device at its source text. examples/embed.c is the code the README quotes, built by every build and registered as a CTest case so a signature change breaks the build rather than rotting the document.

The acceptance suite is the reference's own corpus, checked in at tests/reference/. All 41 .bas files are registered as individual CTest cases and byte-compared against their .txt — including the trailing double newline on an error line (§1.8).

It was driven in place out of deps/basicinterpret until 2026-07-31, on the reasoning that copying a submodule's corpus guarantees drift. That reasoning was sound and was overruled deliberately: the Go dependency is being deprecated, and a build that cannot run its own acceptance suite without cloning the implementation it replaced is not finished. The copy is byte-identical to basicinterpreter@d76162c, and tests/reference/README.md records the provenance, the cost of the drift nobody is watching for now, and the rule that those expectations are never edited to suit this interpreter.

Nothing in the build or the suite needs deps/basicinterpret any more, and that is checked rather than assumed — both configurations were configured, built and run from scratch with the submodule moved out of the tree. The Commodore font moved too, to assets/fonts/, which was the other thing tying the build to it; assets/fonts/PROVENANCE.md carries the licence question that came with it.

Eighteen unit tests cover the modules the corpus cannot reach on its own. tests/known_reference_defects.c is registered in AKBASIC_KNOWN_FAILING_TESTS and asserts the correct contract for six of the defects in §6; when one is fixed CTest reports "unexpectedly passed", which is the cue to split that assertion out and strike the item.

Two budget numbers moved during implementation and are worth knowing: AKBASIC_MAX_ARRAY_ELEMENTS (1024) caps one array and AKBASIC_MAX_ARRAY_VALUES (4096) caps all of them together, drawn from an akbasic_ValuePool the runtime owns. The reference had no such ceiling because it called make().

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3. The akgl-backed sink, devices and standalone frontend — done

-DAKBASIC_WITH_AKGL=ON builds and its suite passes, and the build option now changes what the executable does: an AKGL build of basic opens a window, draws BASIC output into it, pumps SDL events, lets you type at it, and still puts every byte on stdout. Four adaptors in the akbasic_akgl target, which is the only thing here that links SDL, are complete and tested:

File	Backs
src/sink_akgl.c	the §1.5 text sink, over akgl_text_measure and akgl_text_rendertextat
src/graphics_akgl.c	akbasic_GraphicsBackend, over akgl_draw_*, and the SSHAPE surface pool
src/audio_akgl.c	akbasic_AudioBackend, over akgl_audio_*
src/input_akgl.c	akbasic_InputBackend, over akgl_controller_poll_key

The character grid comes from akgl_text_measure(font, "A", &w, &h) — the direct equivalent of the font.SizeUTF8("A") at basicruntime.go:96, and the call this was blocked on until 42b60f7. Wrapping is done on the character grid rather than by handing SDL_ttf a wraplength, because the cursor has to land somewhere definite: a program that PRINTs a long string and then PRINTs again expects the second to start on the row after the first ended, and only the code that placed the characters knows which row that is.

The interpreter owns no window, renderer or event loop. Every initializer takes something the host already made. Each calls akgl_error_init() first: akgl_game_init() would have, but we drive subsystems directly and never call it, and a code raised before that registration carries no name into its stack trace. It is idempotent.

Acceptance: tests/akgl_backends.c, a 128x128 software renderer under the dummy video driver read back with SDL_RenderReadPixels — the pattern deps/libakgl/tests/draw.c established, which needs no display and no offscreen harness. Registered as the akgl_backends CTest case, and only when AKBASIC_WITH_AKGL is on.

The standalone frontend

The standalone executable is itself a host, and that half of the Go frontend is now ported — src/frontend_akgl.c, in its own akbasic_frontend target. The separation is in the build graph and not only in a comment: akbasic is the interpreter, akbasic_akgl is the four adaptors that draw through somebody else's renderer, and akbasic_frontend is the one thing in the repository that creates a window. A game embedding the interpreter links the first two and not the third.

Piece	Where
Window, renderer, font, the four adaptors	akbasic_frontend_akgl_init()
Sink and devices onto a runtime	akbasic_frontend_akgl_attach()
One frame: pump events, draw the text layer, present	akbasic_frontend_akgl_pump()
The bounded frame loop	akbasic_frontend_akgl_drive()
stdout mirror	src/sink_tee.c, in the core library

Four things are worth knowing about how it came out.

1. The stdout mirror is a sink, not a second write. §1.5 made the sink boundary precisely so the interpreter would never carry the reference's hardcoded pair of calls, and §3 item 5 said it again. akbasic_sink_init_tee() composes two sinks into one and takes readline from whichever of the two was named as the reader — which is the whole difference between the two modes: a program read from a file comes in through the stdio half, and typed lines come in through the akgl half's line editor. It lives in the core library and is tested there (tests/sink_tee.c), because composing two function-pointer records needs no SDL.

2. The line editor borrows the host's loop a frame at a time. readline has to wait for a typed line, and §1.6 forbids the library blocking — and a sink that sat on the keyboard with no way to pump events would deadlock the process on its first INPUT. So the host installs an akbasic_AkglPump with akbasic_sink_akgl_set_pump(), and the editor calls it between keystrokes. The loop is still the host's. akbasic_frontend_akgl_pump() has that exact signature and is installed as-is. Without a pump the sink reports EOF, which is what it did before and what a host that wants no editor still gets.

3. The frame loop is bounded, and that is what makes the close button work. 256 steps per frame (AKBASIC_FRONTEND_STEPS_PER_FRAME), then pump and present. 10 GOTO 10 under an unbounded run() would own the process forever; tests/akgl_frontend.c runs exactly that and asserts the window close ends it.

4. The frame is not cleared. The graphics verbs draw straight to the renderer rather than into a display list, so clearing would wipe every DRAW at the end of the frame it was issued in. The text layer is drawn over whatever is there, which is also how a C128 stacks its text plane on its bitmap plane.

Acceptance: two things, and the second one is the stronger.

• tests/akgl_frontend.c, registered as the akgl_frontend CTest case: a program run through the frontend under the dummy driver, asserting the mirrored stdout byte for byte and reading the renderer back to prove the same characters were drawn in the Commodore font; synthetic key events pushed into SDL's own queue and read back through the frontend's input backend, so every link the host owns is in the path; the line editor's fold to upper case, its backspace and its escape; a whole REPL session typed at the window and ended with a typed QUIT; and the window-close path.
• The entire golden corpus is driven through the AKGL binary. A -DAKBASIC_WITH_AKGL=ON build registers the same 41 upstream cases against the same executable, which now opens a window to run them, and all 41 still match byte for byte. That is a much broader statement than any hand-written frontend test: the SDL path changes no observable output anywhere in the corpus.

Three no_device.bas local cases are deliberately not registered in an AKGL build. Their premise is a driver that was given no devices, which is true of the stdio driver and deliberately false of this one. The refusal path they cover is asserted directly against the backend records in tests/devices.c, which both configurations build.

Manual check against a real display, 2026-07-31. The dummy driver cannot prove presentation, so build-akgl/basic was run against X display :0 on a program that prints a line, draws a BOX and then loops. xwininfo found a real 800x600 window titled BASIC; the window was captured with import and measured: the first text row carries glyph pixels (mean 65/255), the box's left edge at x=100 is a solid white line (mean 255), the box interior is black — it outlines rather than fills — and an empty corner is black. stdout carried HELLO WORLD at the same time. QUIT exits cleanly with status 0.

The typing half is no longer manual, and it is no longer a gap. It used to read "somebody should type at it once", because no key-synthesis tool was installed. xdotool now is, and tests/akgl_typing.sh is registered as the akgl_typing CTest case: it starts the driver under a pty, waits for the window with xdotool search --sync, gives it keyboard focus, types a program containing a string literal and a lower-case one, and polls the mirrored stdout for what they print.

That case earns its keep twice over. It is the only test that covers the path upstream of SDL — X11 → SDL composition → libakgl's ring → the editor — and everything else synthesises SDL events, which is precisely how the missing SDL_StartTextInput() shipped with a green suite. Confirmed by reverting both halves of that fix and watching it fail with the same symptom that was reported: READY, and nothing after it.

It needs a real X server, a window manager and xdotool, and it steals keyboard focus for about fifteen seconds. Absent any of those it reports CTest Skipped rather than failing, which is what it does in CI. AKBASIC_SKIP_INTERACTIVE=1 skips it deliberately.

The five things that had to be worked around — all gone

Every one was filed against libakgl and every one is resolved in its 0.3.0. What is left here is the note that they existed, because the pattern is the point: file it upstream, comment the workaround at its site with the words "filed upstream", delete it when it lands.

Was	Resolved by
An embedded libakgl required its vendored SDL, SDL_image, SDL_mixer, SDL_ttf and jansson to be installed, so our CMakeLists.txt declared all five by hand first	It adds them whether or not it is top-level
akgl/controller.h did not compile on its own — it used akgl_Actor * and included nothing that declared it	It includes <akgl/actor.h>
There was no way to bind a 2D backend to a renderer you already had, so the six vtable pointers were assigned by hand in two files	akgl_render_bind2d()
akgl_text_rendertextat() dereferenced an unset draw_texture, so the first PRINT through an unbound backend segfaulted	It checks, and reports AKERR_NULLPOINTER like the draw entry points
SOUND's frequency sweep had no equivalent and was refused	akgl_audio_sweep()

0.3.0 also arrived with a regression that this repository is the one that found, because it is the only consumer that embeds libakgl alongside other projects: the commit that made the vendored dependencies unconditional also made its add_test() shadow unconditional, and CMake chains command overrides exactly one level deep. Two projects in one tree cannot both shadow add_test — the second rebinds _add_test to the first and the builtin becomes unreachable to everybody, so our own registrations recursed to CMake's depth limit. Fixed upstream by putting the top-level guard back; filed as libakgl defect 27, with the two-line CMake program that demonstrates the one-level limit.

Still missing from the sink

Nothing that blocks a verb, and as of libakgl 0.3.0 nothing that blocks a character either. The ring now carries the composed UTF-8 text SDL worked out from each keystroke, so the editor takes that in preference to the keycode and shifted characters, keyboard layouts, compose keys and dead keys all work. A double quote can be typed, so a BASIC string literal can be typed, which was the sharp end of the old limitation — it used to mean a program with a string in it had to be passed on the command line. Letters are no longer folded to upper case, because there is no longer any reason to.

The host has to turn text input on, and forgetting to is how this broke once. SDL3 has it off by default and per-window, so it is akbasic_frontend_akgl_init() that calls SDL_StartTextInput() — akgl/controller.h says as much. Without it SDL emits no SDL_EVENT_TEXT_INPUT at all, every keystroke reaches the ring with an empty text, and an editor that reads that as "not a character" is silently dead: nothing echoes in the window, and nothing reaches stdout either, because RUN can never be typed.

The editor therefore falls back to the keycode when a keystroke carries no composed text, folded to upper case. A worse keyboard is a great deal better than no keyboard, and it means a host that embeds these adaptors and forgets SDL_StartTextInput() gets a usable editor rather than a dead one. Both halves are asserted in tests/akgl_frontend.c, and both were checked by reverting each in turn and watching the test fail.

Worth knowing about the test suite that missed this: every other keyboard test pushes SDL_EVENT_TEXT_INPUT into SDL's queue by hand, which is what a real keyboard produces — once text input has been started. Synthesising the end of a chain cannot test the beginning of it. The new test asserts SDL_TextInputActive() directly for that reason, and the whole frontend suite has been run against a real X11 window as well as the dummy driver.

One limit remains, and it is a choice rather than a gap:

• No cursor movement within a line. Backspace and escape only. The arrow keys stay in the ring for a script's own GET loop, which is the more valuable use of them.

A non-ASCII character is accepted by the keyboard and then dropped rather than stored, which is §1.2 rather than the editor: the grid is a byte per cell and a value's string is a fixed 256 bytes, so a multi-byte character has nowhere to go. Dropping it is honest where storing half of it is not.

WINDOW and KEY in group E want a text-window rectangle and a function-key table on top of this, which is ordinary work here rather than anything blocked.

---

4. Remaining work: language completion (goal 2)

The work queue is the "What Isn't Implemented" list in deps/basicinterpret/README.md. Each verb is: table row (§1.1) → parse handler if it needs one → exec handler → unit test → a new .bas/.txt pair. Both kinds of test; they answer different questions.

Order by what unblocks the most, and by what does not need libakgl to grow first:

Group	Verbs	Blocked on
A. Structure	DO, LOOP, WHILE, UNTIL, ON, BEGIN, BEND, END	done — src/runtime_structure.c, tests/structure_verbs.c
B. Housekeeping	NEW, CLR, CONT, SWAP, TRON, TROFF, HELP	done — src/runtime_housekeeping.c, tests/housekeeping_verbs.c
B′. Housekeeping, deferred	RESTORE, RENUMBER	done — src/data.c and src/renumber.c, tests/read_data.c and tests/renumber.c
C. Errors	TRAP, RESUME, ER, ERR	done — src/runtime_trap.c, tests/trap_verbs.c. ER/EL are ER#/EL#; see §5
D. Strings/format	USING, PUDEF, WIDTH, CHAR	done — src/format.c, src/runtime_format.c, tests/format_verbs.c
E. Console	WINDOW, KEY, SLEEP, WAIT, TI	done — src/runtime_console.c, tests/console_verbs.c
F. Disk	all 21	done — src/runtime_disk.c, tests/disk_verbs.c. Five refuse for want of a drive and DIRECTORY for want of an upstream wrapper; see §5
G. Graphics	GRAPHIC, DRAW, BOX, CIRCLE, PAINT, COLOR, SCALE, SSHAPE, GSHAPE, LOCATE	done — src/runtime_graphics.c, tests/graphics_verbs.c
H. Sprites	SPRITE, MOVSPR, SPRCOLOR, SPRSAV, COLLISION	done — src/runtime_sprite.c, src/sprite_akgl.c, tests/sprite_verbs.c. SPRDEF is out of scope; see below
I. Audio	PLAY, SOUND, ENVELOPE, VOL, TEMPO	done — src/runtime_audio.c, src/play.c, tests/audio_verbs.c
I′. Audio, still gapped	FILTER	libakgl has no filter stage and SDL3 supplies no primitive; refused with AKBASIC_ERR_DEVICE. SOUND's sweep arguments used to be here and landed with akgl_audio_sweep in libakgl 0.3.0
E′. Console input	GET, GETKEY, SCNCLR	done — src/runtime_input.c, tests/input_verbs.c
J. Machine	SYS, FETCH, STASH	done — src/runtime_machine.c, tests/machine_verbs.c. SYS is refused by name; see §5

RESTORE and RENUMBER are deferred, and neither is a small job.

• RESTORE needs a DATA pointer, and there isn't one. Done. Every DATA statement is pre-scanned into one flat list before the program runs -- src/data.c, called from akbasic_runtime_set_mode() beside the label prescan -- and READ walks a cursor along it. RESTORE resets the cursor; RESTORE (line) moves it to the first item on or after that line.

  It fixed two defects on the way, both of which were consequences of READ not reading. A DATA line above its READ is now found, where skipping forward from the READ used to run off the end of the program in silence. And the lines between a READ and its DATA now execute, where the skip used to swallow them -- a C128 runs them, because READ takes the next item and execution carries on. The one corpus case has its DATA immediately after its READ, so neither was observable there. tests/read_data.c.

  DATA at run time is now a no-op: it is a declaration, and reaching the statement means walking past it.

• RENUMBER has to rewrite references or it is worse than useless. Done. src/renumber.c builds the whole old-to-new map first, then rewrites every line -- including lines before the renumbered region, which can branch into it -- and only then moves them.

  The rewrite is textual and understands exactly one thing about the rest of the language: where a string literal starts and stops, so PRINT "GOTO 10" survives. GOTO, GOSUB, RUN, RESTORE and TRAP targets are rewritten, and so is the comma-separated list after ON X GOTO -- for free, since the targets follow the GOTO. COLLISION is special-cased because its handler is its second argument.

  Two deliberate non-rewrites. A target naming a line that does not exist is left alone: GOTO 9999 in a program with no line 9999 is already broken, and inventing a destination would hide that. And THEN/ELSE do not take bare line numbers in this dialect -- the corpus writes THEN GOTO 100 -- so a number after THEN is an expression, not a target.

  A renumbering that would land a moved line on a kept one is refused before anything is written, because losing a line to a renumbering is not recoverable.

  A program written with labels needs none of this. GOTO DONE is unaffected by any renumbering, which is the strongest argument for LABEL there is.

LOCATE used to appear in both E and G. It belongs to G alone: in BASIC 7.0 LOCATE moves the graphics pixel cursor that DRAW starts from, and the text cursor verb is CHAR, which is already in group D.

Out of scope, and staying that way — the reference marks these as incompatible with a modern PC and that reasoning stands: BANK (no bank switching), FAST (irrelevant CPU speed control), MONITOR (no machine-language monitor). SPRDEF joins them on the same reasoning: it is not a programmable verb but an interactive full-screen sprite editor, driven by single keystrokes and its own cursor. A program cannot call it usefully and this interpreter does not own the screen it would take over. The three SPRSAV source forms are what replace it.

The libakgl JSON layer turned out not to be in the way at all. Group H was parked on "check the sprite/actor API before filing", and the concern was that libakgl's sprites are described by JSON documents that would be cumbersome to reach through BASIC. akgl_sprite_load_json() is a thin wrapper over akgl_spritesheet_initialize + akgl_sprite_initialize + writes to public struct fields, and every field it fills from a document — frame list, animation speed, loop flags, state-to-sprite map — is something a Commodore sprite does not have. So src/sprite_akgl.c builds the same four objects directly. The one field that did survive into the verb syntax is the spritesheet's filename: SPRSAV "ship.png", 1 goes through akgl_path_relative() and akgl_spritesheet_initialize(), which is the same pair of calls the JSON loader makes.

Also on the queue, from the reference's own defect list:

• Multiple statements per line Done. akbasic_parser_parse() consumes a run of COLON tokens before each statement and hands the caller a NULL leaf when nothing followed them, which is how an empty statement — a trailing separator, or :: — stays legal rather than becoming an error. tests/parser_commands.c counts statements per line; tests/language/statements/multiple_per_line.bas asserts what actually runs.

  One thing about it needed a decision rather than a transcription, and it is recorded as deviation 31 in §5: BASIC 7.0 scopes everything after THEN to the condition, so IF C THEN A : B must run neither A nor B when C is false. The parser takes exactly one statement per arm, so the rest of the line reaches the statement loop as ordinary statements and the branch has to tell the loop whether to run them.

  A known limitation, not a defect: a whole FOR/NEXT on one line (FOR I# = 1 TO 3 : PRINT I# : NEXT I#) does not loop. The block structure is the reference's waitingForCommand model (§1.6), which skips forward by source line to the verb it is waiting for, so a NEXT on the same line as its FOR is never reached. Fixing it means restructuring control flow to work on statements rather than lines, which is a deliberate piece of work and wants its own commit. Group A's DO/LOOP will meet the same wall.

• Array references in parameter lists Done, and it turned out to be §6 item 13 a third time rather than a separate defect. An identifier's subscript list moved to .expr, and akbasic_ASTLeaf grew a next field so an argument list chains through a link of its own. That last part is the real fix: the reference chains arguments through each argument's own .right, and every leaf type that can be an argument already uses .right for something, so moving one operand out of the way only shifted the collision along. Covered by tests/language/arrays_in_parameter_lists.bas and tests/runtime_evaluate.c.

What a demoscene program wanted and could not have

examples/megademo/megademo.bas was written to push the interpreter to its edges, and these are the edges it hit. None of them blocked the demo — every one has a workaround and the program carries all six — but each workaround is a routine every future game will carry too, and that is the argument for the verb.

Ordered by how much BASIC each one would delete: RND and ASC (#16), the unwritable palette (#17), GSHAPE's missing blend modes (#18), no way to ask whether PLAY has drained (#19), PLAY being monophonic (#20), DATA being too small a pipe for assets (#21), and the TI# spin that starves the audio queue (#22).

The last of those is the one worth reading before writing a game loop. Polling TI# does not merely waste CPU: the host calls settime() once per runtime_run() batch, so a batch of spin statements drops the note release rate below what a sixteenth-note tune needs, the queue never reclaims slots until it drains completely, and the backlog compounds until PLAY queue is full. Reproduced both ways in isolation. Sleep a frame instead.

---

5. Deliberate deviations from the reference

Keep this list current. Most of these change structure without changing observable output; the ones that do change output say so and carry the golden file they moved.

The bar has dropped since this list was started. It used to be "defensible against the golden suite", because matching the Go implementation byte for byte was goal 1's headline claim. That implementation is now deprecated (§0.1), so the bar is the ordinary one: defensible on its own merits, recorded here, and tested.

1. Reflection → static dispatch table (§1.1).
2. Go maps → fixed open-addressed tables (§1.3).
3. Unbounded new(BasicEnvironment) → pool with release (§1.4).
4. Hardcoded stdout+SDL mirror → text sink backend (§1.5).
5. run() owns the process → step() / bounded run() (§1.6).
6. panic() → FAIL_RETURN; no library call terminates the process (§1.6).
7. debug.PrintStack() on parse error → the akerr stack trace only (§3.1 of the original plan).
8. BasicEnvironment.eval_clone_identifiers deleted as dead state (§4.2 of the original plan).
9. BasicValue.name and BasicEnvironment.update() deleted as dead: nothing ever writes the field a non-empty string, and update() — its only reader — has no callers.
10. The DEF-statement bootstrap is gone. The reference declares every builtin by running a BASIC program of DEF lines through the interpreter at startup and then nulling out the expressions (basicruntime_functions.go:14); here a builtin's name, arity and handler are a row in the dispatch table, and MOD, SPC and STR are ordinary native handlers. This removes the need to run the interpreter before the interpreter is ready.
11. Undefined behaviour the reference reaches by a defined route is refused rather than inherited: a shift count outside 0..63, a negative string multiplier, and integer division by zero all raise. Go defines all three (or panics); C does not. No golden case exercises any of them, so observable behaviour is unchanged.
12. CommandEXIT clears the pending NEXT wait before popping. The reference does not, which leaves the parent waiting for a NEXT that never arrives (§6 item 8) — in C that is a hang rather than a misbehaviour, and no golden case depends on it.

Deviations in the verbs the reference never implemented

Items 1–12 above are deviations from ported code. These are deviations from Commodore BASIC 7.0, in verbs the reference lists as unimplemented and which therefore had no Go behaviour to port. They matter to somebody typing in a listing out of a C128 manual.

13. Hardware verbs reach a device through a backend record, and refuse when there is none. akbasic_GraphicsBackend, _AudioBackend and _InputBackend are records of function pointers on the runtime; all three may be NULL, which is what the current stdio-only standalone driver gives them. That is correct for a default build and unfinished for an AKBASIC_WITH_AKGL build; see §3. A verb that needs a device it was not given raises AKBASIC_ERR_DEVICE naming itself. COLOR, LOCATE, SCALE, ENVELOPE, TEMPO and VOL deliberately do not require one — they change interpreter state, so a program can configure itself before the host lends it a renderer.

14. CIRCLE is a polygon, always. BASIC 7.0's CIRCLE takes two radii, a start and end angle, a rotation and a degree increment, which makes it an inc-degree polygon by definition. akgl_draw_circle exists and is deliberately not used: it draws one radius, full sweep, unrotated, so it could serve only the case where every optional argument is defaulted — and a shape that changed character depending on whether the two radii happened to be equal is worse than one that is uniformly a polygon.

15. SSHAPE stores a handle in the string, not the pixels. A real C128 packs the region's bitmap into the string variable. Here a value's string is a fixed 256 bytes (§1.2) and a saved region is a device surface, so the variable gets SHAPE:<n> and the surface stays in a fixed pool on the backend. GSHAPE accepts only a string carrying that prefix, so a hand-written one is refused rather than pasting an unrelated slot. What this costs: a shape cannot be written to disk, cannot be concatenated, and LEN of it is not its size. Nothing in BASIC does any of those to a shape string except a program deliberately poking at it.

16. BOX cannot fill at all, and the plan for it is to spell fill as a negative angle. 7.0's last argument selects outline or fill and sits after the rotation, which would make a filled box a seven-argument call — one more than akbasic_cmd_box() collects. Spelling it as a negative rotation instead is the intended answer and is not implemented: today a rotation of zero outlines through the renderer's rectangle and any other rotation, negative included, draws four lines.

    This entry used to claim in bold that it fills, with the body then saying it was filed rather than implemented — a headline that contradicted its own paragraph, which is exactly how a reader ends up believing a feature exists. Found by writing a documentation figure for BOX and getting an outline back. PAINT is the fill a program has today.

    akbasic_GraphicsBackend::filled_rect is dead from the language's side as a direct consequence: src/graphics_akgl.c implements it and tests/mockdevice.h records it, but no BASIC verb reaches it. It is the entry point this fix would call, so it is waiting rather than unused — worth knowing before somebody tidies it away.

17. GRAPHIC records its mode but honours only one consequence of it. 7.0's five modes differ in bitmap resolution and in whether the bottom of the screen stays text. Neither means anything against a host's renderer, whose size the interpreter does not own. The mode is stored, an out-of-range one is refused because that is a typo worth catching, and the one observable rule is that mode 0 is text. The mode is readable as RGR(0), which is 7.0's whole RGR and the one field of ours that is.

18. A coordinate reaching a backend is a device pixel, and the whole of the host's window is reachable from BASIC. This used to read "coordinates are BASIC's 320x200 space and scaling to the window is the host's job", on the reasoning that the interpreter cannot ask the renderer how big it is without owning it. That reasoning was wrong twice over, which is what §8 item 9 came to say.

    It was wrong about the mechanism: not owning a thing is no bar to asking it, and the backend record is how everything else here asks. akbasic_GraphicsBackend grew a size entry point, require_graphics() calls it before every verb that draws -- so a resized window is honoured between two statements rather than at attach -- and 320x200 became the fallback for a backend that leaves size NULL. It is the record's one optional entry point, so a host written against the old header keeps working and gets exactly the behaviour it used to get.

    And it was wrong about the description: nothing ever stretched anything. With SCALE off a coordinate was passed straight to akgl_draw_* as a pixel address, so an 800x600 window drew a C128 listing into its top-left 320x200 corner and left the rest unused. The documentation described a coordinate transform that did not exist.

    What changed observably is SCALE, which now maps user coordinates onto the device rather than onto the constants, and RGR(1) / RGR(2), which report the drawing surface's width and height so a program can use a window whose size it did not choose. A C128 listing draws in the corner as it always did; SCALE 1, 319, 199 gives it the whole window.

    One fix rode along, in the same line of code and too small to defer. SCALE mapped xmax onto the width, which put the user space's far corner one pixel past the surface: SCALE 1, 319, 199 then DRAW 1, 319, 199 drew nothing at all. It maps onto the last pixel now, which is what makes that sentence above true rather than nearly true. tests/graphics_verbs.c and tests/akgl_backends.c, the latter against a 128x128 target chosen because it is smaller than the old constants -- a SCALE still dividing by them misses it entirely rather than landing somewhere plausible.

19. PLAY does not block. On a C128 PLAY holds the program until the last note ends. §1.6 forbids that outright, so PLAY parses the string into a fixed queue and returns; akbasic_runtime_step() releases one note at a time against whatever time the host last passed to akbasic_runtime_settime(). What this changes for a program: the statement after a PLAY runs immediately rather than a bar later, so a listing that relied on PLAY for timing will race ahead. A program that wants to wait should test the queue rather than assume the verb waits for it.

20. The host owns the clock, and an unset clock rushes the music. The library reads no clock — it owns no loop and must not block. A host calls akbasic_runtime_settime() once a frame; the standalone driver calls it every step off CLOCK_MONOTONIC. Left unset it reads zero forever, so every duration has already expired and the queue drains as fast as step() is called. Audible, deterministic, and never a hang, which is the right way for that to fail.

21. PLAY's M (measure) is a no-op, and SOUND's sweep runs once rather than oscillating. M synchronises the three voices on a C128; with one sequential queue there is nothing to synchronise, and a listing full of them should still play, so it is accepted and ignored.

    The sweep used to be refused outright — there was no akgl_audio_* equivalent and faking one would have tied audible pitch to how often the host calls us. libakgl 0.3.0 added akgl_audio_sweep() and SOUND voice, freq, dur, dir, min, step now reaches it. What does not survive is dir 3, "oscillate": akgl_audio_sweep runs one pass between two endpoints, and a real oscillation needs the mixer to turn around at them. What this changes for a program: a dir 3 siren rises or falls once instead of warbling. dir 1 and 2 are exact, and so is the direction — both the SID and akgl_audio_sweep take it from which endpoint is higher, so a min above the starting frequency rises whatever dir says.

    step is converted as a delta rather than a position: it is a register increment, and the register-to-hertz table maps positions, so it goes across as the distance between register 0 and register step. A step that converts to zero would never arrive and is raised to one hertz, which is the smallest move that still gets there.

    A backend whose record has no sweep — a host written against libakgl 0.2.0 — still refuses a swept note with AKBASIC_ERR_DEVICE and plays a held one normally. tests/audio_verbs.c asserts both halves.

22. TEMPO's constant is a calibration choice, not a transcription. BASIC 7.0 documents TEMPO as a relative duration from 1 to 255 defaulting to 8, and does not publish what a whole note actually lasts. src/audio_tables.c uses 16000 ms at TEMPO 1, which puts a default-tempo quarter note at 500 ms — 120 beats per minute. If the real constant turns up, that is the one line to change.

23. GETKEY holds the step loop rather than blocking. A C128 sits on the keyboard inside GETKEY until somebody types. §1.6 forbids the library blocking, so the verb sets a flag and akbasic_runtime_step() declines to advance the program until a key arrives. Every step still returns and a bounded akbasic_runtime_run() still comes back, so a host keeps its frame rate; the program simply does not move past the GETKEY, which is the part a program author cares about. The PLAY queue is serviced before this check on purpose — music should keep playing while a program waits for a keypress. Withdrawing the input device while a GETKEY is holding releases it rather than wedging the script forever.

24. GET and GETKEY into a numeric variable yield the key code. BASIC 7.0's GET takes a string variable. Accepting an integer one as well, and giving it the raw code, is what a program testing for cursor or function keys needs and costs nothing. No key is code zero, matching what a C128 reports. A float variable is refused: it is neither a character nor a code.

Deviations in the standalone frontend

Items 1–12 are deviations from ported interpreter code and 13–24 from BASIC 7.0. These four are deviations from the reference's program: main.go and the SDL half of basicruntime_graphics.go.

25. The frame loop is bounded and the reference's is not. Go's run() is a for {} that owns the process until MODE_QUIT (basicruntime.go:682), which is why the reference's window never answers its close button. Here the driver runs AKBASIC_FRONTEND_STEPS_PER_FRAME steps, pumps, presents, and goes round again. What this changes for a program: nothing observable — a bounded run reproduces an unbounded one exactly, and the whole golden corpus is driven through this loop to prove it. What it changes for a user is that the window closes when asked.

26. The text layer repaints every row it owns, every frame. Not just the rows that changed.

    That was tried — a drawn[] array marking which rows had carried glyphs, so an untouched row could be skipped — and it is wrong on real hardware. SDL_RenderPresent swaps buffers, so a frame does not inherit the frame before it; it inherits the one two back, and on some backends something undefined. A row erased once is clean in one buffer and still dirty in the other, and presenting alternates between them.

    That is what a backspace across a line wrap looked like: the first character of the wrapped tail flickering in and out forever after the rest had gone. It reproduces on X11 and never under the dummy driver or a software renderer, which is exactly why the suite was green through two rounds of fixing it. tests/akgl_frontend.c now paints stale pixels by hand — which is what a swapped-in buffer hands back — so the case is covered without needing real hardware.

    There is no cheaper correct answer available: a frame either owns every pixel it presents or it inherits pixels it cannot reason about.

    What this costs, and it is more than it used to. The host still does not call SDL_RenderClear, but the text area is now repainted in full every frame, so anything a graphics verb drew underneath it is erased every frame rather than only where text lands. In the standalone driver the text area is the whole window, so DRAW and PRINT no longer coexist there.

    And the same buffer swap means the graphics verbs were never reliable across frames anyway, which is worth stating plainly rather than leaving as a surprise: a one-shot DRAW lands in one buffer and the next present shows the other. It looked fine in a single screenshot and would have flickered exactly as the text did. Making graphics survive needs a persistent surface the frame is composited from, which is real work and its own commit — filed here rather than half-done.

27. The cursor is a blinking block. The reference draws an underscore glyph (drawCursor, basicruntime_graphics.go:33), and so did this until the underscore turned out to sit under the text being typed and make it hard to read. A block occupies the cell after the text instead of the space beneath it.

    Half a second per blink (AKBASIC_SINK_CURSOR_BLINK_MS), which is about a Commodore's and slow enough to read under. akbasic_AkglSink.cursorperiodms set to zero holds it solid, which is what makes a frame deterministic for a test.

    Two details that are decisions rather than accidents. The clock is SDL's, not the host's: everywhere else the host owns the clock because the library owns no loop and must not block, but this is an adaptor that already links SDL and threading a timestamp through the sink interface to animate a cursor would be a poor trade. And a line that exactly fills a row leaves the cursor one column past the end, because putchar_at wraps only when the next character arrives — the cursor is drawn at the start of the next row instead, which is where the next character will actually land and where a C128 puts it.

28. The window is closed by the host, and that is not QUIT. Closing the window stops the frontend and leaves obj->mode alone; it does not set AKBASIC_MODE_QUIT. The distinction is invisible to the standalone program, which exits either way, and load-bearing for an embedding host: a game that closes its own window has not decided that the script is finished, and a script that ran QUIT has. tests/akgl_frontend.c asserts both halves.

29. Piped input still works in an AKGL build. With a terminal on stdin the window is the console and lines come from its editor; piped or redirected, they come from the pipe. This is not in the reference, which always reads os.Stdin in REPL mode. It exists so basic < program.bas behaves the same in both builds — and so the golden corpus can be driven through the SDL binary, which is where most of the frontend's real coverage comes from.

Deviations that change what a program is allowed to do

30. A verb name with a type suffix is not a variable name. PRINT$ = 1 is refused where the reference accepts it, because its reserved-word check searched the keyword tables with the suffix still attached and therefore never matched (§6 item 16). A real C128 refuses it too, so this is the reference being wrong rather than this interpreter being strict.

    What it costs a program: a listing that used INPUT$, LEN# or GOTO% as a variable stops parsing, and the fix is to rename the variable. One case in the reference's own corpus did exactly that; see tests/reference/README.md.

Deviations in statement separation

31. A branch decides who owns the rest of its line. BASIC 7.0 scopes every statement after THEN to the condition, and the reference has no opinion on the matter because it never consumed the COLON token at all. The parser here takes exactly one statement for each arm, so the rest of the line arrives at the statement loop as ordinary top-level statements and something has to say whether to run them. The BRANCH case in akbasic_runtime_evaluate() sets runtime->skiprestofline, and the two statement loops stop on it.

    The rule is not "skip when false", which is the reason this is written down:

    Line	Condition	What runs
    IF C THEN A : B	true	A, B
    IF C THEN A : B	false	nothing
    IF C THEN A ELSE B : D	true	A
    IF C THEN A ELSE B : D	false	B, D

    The remainder always belongs to whichever arm was written last, so it is skipped exactly when that arm is the one not taken — with an ELSE the last arm is ELSE, without one it is THEN. That is one line of code and it reads as an oddity without the table above it.

Deviations in the structure and error verbs (groups A and C)

61. Structures are an addition, not a port. BASIC 7.0 has no records at all, so TYPE/END TYPE, DIM X@ AS T, . and ->, PTR TO and POINT ... AT are invented here. The @ suffix was not: the Go reference reserved IDENTIFIER_STRUCT and never used it, and src/grammar.c rendered such a leaf as "NOT IMPLEMENTED" until this landed. docs/16-structures.md is the whole feature.

    Assignment copies and POINT shares, which is the decision everything else rests on. A structure is a value like every other value here, so a program that never writes POINT can never be surprised by aliasing — and the two field operators are kept apart so a reader always knows from the spelling which they are looking at. . on a pointer and -> on a value are both errors, each naming the other.

    A declared type is what buys the storage model. An instance has a known slot count and is laid out exactly as an array is, out of the same value pool, so nothing new holds data — only a table of descriptors. It also bounds copy depth statically, since a TYPE cannot contain itself by value.

    Three limits are ours and are stated rather than derived: 16 types, 16 fields per type, and four levels of nesting in PRINT. The last is not a safety bound on copying — copy stops at pointers by construction — but on rendering, which must follow a pointer and would not come back on a cycle. Four rather than eight because the bound has to bite before the 256-byte render buffer does, or a cycle stops because it ran out of room rather than because it was told to.

    A type name shares a namespace with verbs and labels, since all three are bare words. Refused at declaration with a message that says so, because the parser's own answer was "Expected expression or literal" pointing at the line rather than the problem. Field names follow the same reserved-word rule variable names already do, enforced by the loader's scan — TO@ is a bad field name for exactly the reason TO# is a bad variable name.

62. A host's C struct is the same thing with its bytes somewhere else. akbasic_host_register_type() puts it in the same table a TYPE fills, so copy, PTR TO, . and -> all work across the boundary with no second set of rules — and the language's own copy-versus-POINT distinction turns out to be exactly the snapshot-versus-share distinction a host needs, so there is one API rather than two.

    A binding takes a shadow run of slots; a read refreshes from host memory and a write converts back. Conversion refuses rather than truncates — 70000 into an int16_t names the field — and a field name's suffix must agree with the C type it describes, refused at registration.

    It is the only pointer this interpreter holds that it did not allocate, and that is the one thing a host has to think about. akbasic_host_unbind() exists for it. examples/hoststruct.c is a working host, built and run by every build.

63. A whole loop on one line does not loop. DO : PRINT 1 : LOOP runs once, exactly as FOR I=1 TO 3 : PRINT I : NEXT I does. Block skipping walks source lines, which is the reference's waitingForCommand model (§1.6), so a LOOP on the same line as its DO is never reached as a separate step. Fixing it means making the skip operate on statements rather than lines — a deliberate piece of work, already noted in §4 against FOR.

64. ER and EL are ER# and EL#. A C128 exposes them as bare reserved names. This dialect cannot: an identifier carries its type in a suffix, and a name without one is a label. They are written into the global scope when a trap fires, so PRINT ER# reads the way the rest of the language does and needs no new syntax.

    ER# carries the libakerror status code, not a Commodore error number — 515 for an out-of-bounds subscript rather than C128 error 9. There is no correspondence to reproduce: the errors this interpreter raises are not the errors a 1985 ROM raised. ERR(ER#) gives the name, and that is the part a program should be printing anyway.

65. END does not arm CONT. A C128 lets CONT resume after END as well as after STOP. Here END means the program finished, and continuing from a finished program resumes at whatever line happens to follow — so CONT refuses instead. STOP still arms it.

Deviations in the format verbs (group D)

47. PRINT USING renders one field per statement. BASIC 7.0 lets a format string hold several fields and PRINT USING take a list of values against them. Here the first field is used and any literal text around it is copied through, so PRINT USING "A: ###"; X works and PRINT USING "### ###"; X, Y does not. Multiple fields need PRINT to take a value list, which it does not yet — a separate piece of work from the field rendering itself, which src/format.c does properly.

    Exponential fields (^^^^) are not implemented either. Nothing in the language produces a value that needs one, since PRINT renders a float with %f.

48. WIDTH is emulated with parallel passes. It sets the thickness of drawn lines, and neither the graphics backend record nor akgl_draw_line takes a thickness — so a WIDTH 2 line is drawn twice, offset one pixel along whichever axis the line does not mostly run along. Visually right at the only other value the verb accepts; a general thickness would want a real perpendicular offset and an API that takes one.

49. CHAR ignores its colour argument, and needs a sink that has a cursor. The text sink draws in one colour, chosen by the host when it built the sink, so a per-call colour would have to become part of the sink interface. The argument is accepted and ignored rather than refused, because refusing it would make every published CHAR line a syntax error.

    Positioning is a new optional moveto on the sink record. A stdio sink leaves it NULL and CHAR refuses by name — a terminal's cursor is not this library's to move and a pipe has no cursor at all — so CHAR works in an AKGL build and says why it cannot in a stdio one.

Deviations in the console verbs (group E)

50. SLEEP and WAIT hold the step loop; they do not block. §1.6 forbids blocking, so a waiting program is one that does not advance — akbasic_runtime_step() still returns, a bounded akbasic_runtime_run() still comes back on time, and an embedded game keeps its frame rate while a script sleeps.

    SLEEP with no host clock does nothing at all rather than waiting forever. A deadline computed from a clock that never advances is never reached, and the first version of this hung the test suite proving it.

51. TI and TI$ are TI# and TI$. Same reason as ER#/EL#: no bare variable names. They are ordinary globals refreshed once per step from the host's clock, rather than pseudo-variables, because this dialect has no mechanism for a name that computes itself. A host that never sets a clock gets a stopped clock rather than a wrong one.

52. WAIT polls ordinary process memory. On a C128 the byte is a hardware register an interrupt is changing. Here nothing changes it but the host, another thread, or a POKE from a TRAP handler — so a program that waits on a byte nobody writes waits forever, which is exactly what the same program does on a C128 with the wrong address. Read through a volatile pointer, because the whole point is that something outside this program writes it.

53. KEY stores its macros and nothing expands them. The definitions are kept and bare KEY lists them, which is the half that is this library's business. Expanding a macro when a function key is pressed belongs to whatever owns the keyboard — the frontend's line editor — and the input backend delivers keycodes rather than editor commands.

Deviations in the machine verbs (group J)

54. FETCH and STASH are the same byte copy. On a C128 they differ by which side is the RAM Expansion Unit — STASH writes out to it, FETCH reads back. There is no expansion unit and there are no banks, so both are memmove between two addresses and saying so is better than inventing a distinction. memmove rather than memcpy because a program shifting a buffer along by a few bytes overlaps its own source, which is a normal ask.

    Addresses are real process addresses, which is the decision POKE, PEEK and POINTER already made. There is no bounds check and there cannot be one: a wrong address is a segmentation fault, not an error message.

55. SYS is refused by name. It calls machine code at an address. There is no 6502 and no ROM to call, so there is nothing to jump to and nothing sensible to emulate — and jumping to a real address in this process would be a way to crash it on purpose. It parses first and refuses second, so a listing containing SYS still loads and still lists; only running it fails, and it says why.

    Same reasoning as BANK, FAST and MONITOR, with one difference: those have no table row at all, and SYS has a handler because it is common enough in published listings to deserve a specific message rather than "Unknown command".

Deviations in the disk verbs (group F)

56. There is no 1541, so the verbs split three ways. The ones that mean something on a filesystem are implemented against aksl_f* — DOPEN, DCLOSE, APPEND, RECORD, SCRATCH, RENAME, COPY, CONCAT, BSAVE, BLOAD, VERIFY. The ones that are spellings of verbs already here are aliases: SAVE is DSAVE, LOAD is DLOAD, CATALOG is DIRECTORY, DVERIFY is VERIFY.

    HEADER, COLLECT, BACKUP and BOOT are refused by name, with the reason. Each operates on a physical disk — formatting one, validating its block allocation map, duplicating it, booting from it — and a filesystem has no equivalent that is not a lie. HEADER would have to mean "delete everything in this directory", which is a spectacularly bad thing to do to somebody who typed a C128 verb. DCLEAR is the exception: resetting a drive also closes its channels, and closing the channels is real, so that is what it does.

57. DIRECTORY is refused for want of an upstream wrapper. libakstdlib has no aksl_opendir, and this project's rule is that a missing capability is filed upstream rather than worked around — so it is filed against libakstdlib and the verb says so. Calling opendir(3) here would mean reporting through errno in a file where everything else reports through an akerr_ErrorContext *.

58. PRINT # and INPUT # need the space before the #. A C128 writes PRINT#1,A$. The scanner reads # as a type suffix, so PRINT# comes out as an identifier named PRINT with an integer suffix — and a verb name carrying a suffix is refused (deviation 30). With a space the # is its own token and the parse handler reads it.

59. RECORD counts lines, not fixed-length records. A C128's relative file has a record length fixed when the file was created; a filesystem file has none. So a record is a line and RECORD rewinds and reads forward to it, which is slower than a seek and is the only definition that does not invent a record length the file does not have.

60. BLOAD requires a length. A C128 reads until the file ends. Here the address is a real process address, so a file longer than the caller expected would write past whatever it was pointed at with no way to notice. Refusing to guess is the only safe answer.

Deviations in conditions

41. A lone = is equality inside a condition. IF A# = 2 THEN works, which it did not before: the scanner reads == as EQUAL and a single = as ASSIGNMENT, because at that point it cannot know whether it is looking at a statement or a condition, and the reference never resolved the ambiguity anywhere. A condition is the one context where it has an answer — BASIC has no assignment expression — so akbasic_Parser::comparing is set around the condition and the relation parser offers ASSIGNMENT as a comparison operator only while it is. == keeps working, and the whole checked-in corpus is written in it.

    Outside a condition = has to stay an assignment, which is what the flag is for: the first attempt put ASSIGNMENT in the operator list unconditionally, and FOR I# = 1 TO 5 promptly stopped initializing its counter.

42. A condition is a whole expression, so AND and OR work in one. IF A# = 5 AND B# = 3 THEN used to report "Incomplete IF statement". The reference parses a single relation after IF, and a relation sits below AND and OR in the grammar chain, so the AND was never consumed and the THEN check found the wrong token. akbasic_parse_if() calls akbasic_parser_expression() now.

43. Truth is nonzero, not "a boolean holding true". akbasic_value_is_truthy() is what a branch tests. Commodore BASIC has no boolean type: a comparison yields -1 or 0, AND and OR are the bitwise operators, and IF A THEN is legal for any numeric A. The reference tests boolvalue == -1 regardless of the value's type, so IF A# = 1 OR B# = 2 THEN — whose OR yields an integer — was silently always false, and so was IF A# THEN.

    The bitwise operators accept a truth value as an operand for the same reason: -1 is every bit set precisely so that AND and OR double as the logical pair. A string is still never true; a C128 raises a type mismatch there, which is a stricter answer this could adopt later.

Deviations in the sprite verbs (group H)

32. Labels are filed before the program runs, not as each LABEL executes. akbasic_runtime_scan_labels() walks the stored source on every entry into AKBASIC_MODE_RUN and files every LABEL <name> it finds, textually, without parsing the lines around it. The reference resolves a label only once its LABEL statement has run, so GOTO and GOSUB reached backwards and never forwards.

    This was not a nice-to-have. An interrupt handler by definition sits on a line normal flow does not fall into, so COLLISION 1, BUMPED could not be made to work at all under the old rule — neither resolving at arm time nor at fire time helps when the LABEL line is never executed. Forward GOTO is the improvement that came with it, covered by tests/language/statements/label_forward.bas.

    LABEL still executes and still files itself, so a name that appears twice resolves to the last one in the source until one of them runs. The scan is textual on purpose: parsing every line up front would raise on lines the program would never have reached.

33. Sprite coordinates are device pixels, not the VIC-II's 0–511 by 0–255. MOVSPR takes the same coordinate space DRAW does. A C128's sprite coordinates are the raster's, offset so that (24, 50) is the top-left of the visible screen; reproducing that would give the language two coordinate systems and make MOVSPR 1, 0, 0 put a sprite off-screen. Consistent with deviation 18, which is where "the same space DRAW uses" is defined — and it moved, so this one moved with it.

    SCALE deliberately does not apply to sprites. It is a graphics-verb transform and the sprite verbs do not call it, which was true before deviation 18 was rewritten and is worth writing down now that the two spaces can differ: with SCALE 1, 319, 199 on, a DRAW at 160,100 and a MOVSPR to 160,100 land in different places. Arguably they should agree; not changed here because it is a decision about what SCALE means rather than a slip, and it wants its own commit.

34. MOVSPR's speed unit is ours. BASIC 7.0 documents speed 0–15 with 15 fastest and says nothing about what a unit is worth. Here one unit is AKBASIC_SPRITE_SPEED_PIXELS_PER_SECOND (5) pixels per second, so speed 15 crosses the 320-pixel screen in about four seconds. Paced off akbasic_runtime_settime() from akbasic_runtime_step(), exactly as the PLAY queue is, so a host that never sets a clock gets a still picture rather than a hang — same trade as deviation 20.

35. SPRSAV takes an integer array where a C128 takes a string, and takes a file path as well. Three source forms, one of which is the C128's:

    Form	Source
    SPRSAV A$, n	a region SSHAPE saved. The C128 documents SPRSAV's string as the SSHAPE data format at a fixed 24×21, so sharing the shape pool is faithful rather than a shortcut
    SPRSAV A#, n	63 elements of a DIMmed integer array — the DATA-driven path a type-in listing uses
    SPRSAV "ship.png", n	an image file

    The array form exists because a string here cannot hold a sprite. A value carries a NUL-terminated char[256] (§1.2) and a pattern is 63 raw bytes including zeros, so the C128's own spelling is unrepresentable. DIM P#(63) is exactly the right size.

    The file form is a deliberate addition, not a fallback. A string source is a path unless it starts with SHAPE:, which is the prefix SSHAPE mints, so the two never collide. It resolves through akgl_path_relative() — the working directory first, then the directory the running program was loaded from — and loads through akgl_spritesheet_initialize(), which are the same two calls akgl_sprite_load_json() makes for a spritesheet. A sprite loaded this way takes the image's own size rather than being forced to 24×21; a modern PC has no reason to throw away art that is not sprite-shaped.

    akbasic_runtime_set_source_path() is what tells the interpreter where the program came from. Group F's disk verbs will want the same thing.

    The reverse direction is refused. SPRSAV n, A$ on a C128 copies a sprite out; here that would mean writing an image file, which is a disk operation, and group F is unimplemented as a whole. Refused by name rather than half-built.

36. COLLISION implements type 1 only, and types 2 and 3 are refused by name. Sprite-to-background needs the whole render target read back and compared against each sprite every frame; a light pen has no meaning on a machine with no light pen. Both raise AKBASIC_ERR_DEVICE with the reason rather than being accepted and never firing.

    Collision is bounding-box, not pixel. A C128's VIC-II collides on set pixels, so two sprites whose boxes overlap but whose art does not are reported as colliding here and would not be there. Pixel-exact collision would mean keeping every sprite's unpacked bitmap and testing the overlap rectangle a pixel at a time; the box test is four comparisons. akgl_collide_rectangles() is deliberately not used — it has a documented corner-containment defect that misses a plus-shaped overlap, and nothing in libakgl calls it.

37. A collision handler is entered between source lines, and must end in RETURN. akbasic_runtime_service_interrupts() runs at the top of akbasic_runtime_step() and injects what amounts to a GOSUB the program did not write. Between lines is the only safe place: a handler entered mid-statement would have to return into the middle of a line and the parser keeps no state that could resume there. That is the same granularity block skipping already works at (§1.6).

    An interrupt does not interrupt an interrupt. A collision that is still true while its own handler runs would otherwise re-enter on the next line until the environment pool was gone. The event is not lost — it is taken as soon as the handler's RETURN lands.

38. Sprite priority is recorded and not honoured. SPRITE n,,,1 sets the bit, RSPRITE reads it back, and nothing draws differently. A C128 draws a low-priority sprite behind the bitmap plane; here the text layer, the drawing surface and the sprites share one render target and sprites are composited on top of it every frame, so there is nothing to go behind. Honouring it would mean a separate composited surface per plane — which is the same piece of work deviation 19 already needs.

39. Multicolour mode is recorded and not honoured. Same shape as priority: SPRITE's seventh argument and SPRCOLOR's two registers are kept, RSPRITE and RSPCOLOR read them back, and no sprite is drawn in multicolour. Multicolour packs two bitmap bits per pixel and selects between the sprite's own colour and the two shared registers; every SPRSAV source form here carries one bit per pixel or a full-colour image, so there is no second bit to select with. The argument positions are kept honest for the day a multicolour pattern format exists.

40. A sprite is a real libakgl actor, and it is drawn by a renderfunc of ours. Each of the eight becomes a SpriteSheet + Sprite + Character + Actor registered in AKGL_REGISTRY_ACTOR, so an embedding game sees BASIC's sprites alongside its own (goal 3). But akgl_actor_render() is not what draws them. There were two reasons and libakgl 0.5.0 fixed one: the default used to compute its destination height from the sprite's width, drawing a 24×21 Commodore sprite as a 24×24 square, and it now uses the height — libakgl defect 26, filed from here and closed there.

    The reason that remains is the one that keeps this file. An actor carries a single scalar scale applied to both axes, which cannot express SPRITE's separate x- and y-expand bits, so MOVSPR's twice-as-wide-same-height is unrepresentable. libakgl records it as open and names this interpreter as the caller that needs it; the fix is a design choice over there — scale_x/scale_y beside scale, or replacing scale and taking the ABI break while the major is still 0 — rather than a patch. Until then src/sprite_akgl.c installs its own renderfunc, which is libakgl's own extension point for exactly this.

    Worth keeping as a pattern: both defects were found by using the library for something it had not been used for, and both were filed rather than worked around silently. One came back fixed.

41. Line numbers are optional in a loaded program, and a blank line is not a program line. This one moved a golden file. A line with no number used to be filed under the loader's cursor unchanged — that is, on top of the line before it — so two unnumbered lines in a row silently lost the first, and a blank line erased whatever preceded it. The reference does the same, and tests/reference/language/arithmetic/integer.bas is the proof: four PRINT statements, an expectation with three values, and a trailing blank line that erased 40 PRINT 4 - 2 before the program ran. The expectation is now 4 4 2 2 and tests/reference/README.md records it.

    In its place: akbasic_runtime_file_line() (src/runtime.c) is the one implementation of the rule, shared by akbasic_runtime_load(), RUNSTREAM and DLOAD. A numbered line is filed under its number and moves the cursor; an unnumbered one takes the slot after it; blank lines are skipped by all three. A collision involving an assigned number is refused with AKBASIC_ERR_BOUNDS rather than overwriting. akbasic_SourceLine grew a numbered flag so deviation 64 can tell the two apart.

    The prompt is deliberately untouched. A line typed without a number is direct mode and runs now; that is the only thing separating program text from a statement at a REPL, and it is why this is a loading feature rather than a language-wide one. AUTO remains the way to enter program text without typing numbers.

    Consequence worth stating: an unnumbered program is capped at 9998 lines, the cap that already existed, and its assigned numbers are increment-1 — so LIST and DSAVE show a listing with no gaps to insert into. RENUMBER before DSAVE gives the gaps back, and marks every line numbered.

    Not changed: two lines that both carry the same written number still silently keep the last, as they always have. That is a separate decision with its own corpus risk and it is not this one.

42. Branching by number to a line the program did not number is refused before it runs. The other half of deviation 63, and the reason akbasic_SourceLine carries a flag rather than the loader just picking slots quietly. In a script written without line numbers GOTO 100 finds the hundredth line and branches there: plausible, silent and wrong, which is the worst thing to hand somebody at run time.

    akbasic_runtime_check_targets() is a fourth prescan, run beside the label, DATA and TYPE ones on every entry into AKBASIC_MODE_RUN — the earliest the check can be made and the only place all four ways a program arrives pass through. It refuses with AKBASIC_ERR_SYNTAX, naming the line and saying what to do instead:

    ? 1 : PARSE ERROR Line 1: branch to line 4, which the program did not number.
          Branch by LABEL, or RENUMBER first
    

    Two things are deliberately not refused. A target naming an empty line, for the same reason RENUMBER leaves one alone — GOTO 9999 in a program with no line 9999 is already broken and inventing a rule about it would hide that. And a target in a fully numbered program, which is every program that existed before this, so nothing checked in changed.

    It shares RENUMBER's walk rather than repeating it. src/renumber.c grew an akbasic_TargetWalk — a self pointer and a visit function — and rewrite_line() now takes one. RENUMBER's visitor substitutes the number a line moved to; the check's substitutes the number unchanged and raises if it names an assigned line. One walk, so the two cannot disagree about what a branch target is, which they would have within a release.

    Worth knowing: the check points environment->lineno at the line it is walking, so the ? N : prefix names the offending line. The other three prescans do not, so a malformed TYPE or a bad DATA item still reports whichever line the loader stopped on — usually the last line of the program. They work around it by putting Line %d: in the message text, which is why the message above says the number twice. Worth fixing in akbasic_runtime_set_mode()'s wrapper rather than in four places; not fixed here.

---

---

6. Reference defects — fix them; fidelity is no longer a reason not to

These are real bugs in deps/basicinterpret, reproduced faithfully during the port.

The rule that governed this section is gone. It used to read "port the behaviour first so the golden suite passes and the port is provably faithful, then fix each one deliberately" — and faithfulness was the reason several of these are still here. The Go implementation is deprecated and will not be updated (§0.1), so there is no longer anything to be faithful to. Each of these is now an ordinary defect, to be judged on whether the fix is right rather than on whether it matches.

What has not changed is the working method: one fix per commit, with a test that asserts the correct contract, and a golden file moved in the same commit if the fix changes output. AKBASIC_KNOWN_FAILING_TESTS is empty now and is kept declared for the next defect that has to be reproduced before it can be fixed.

1. basicvariable.go:176 — toString() tests len(self.values) == 0 and then indexes self.values[0]. Never ported. There is no akbasic_variable_to_string: rendering a value is akbasic_value_to_string(), which switches on the value type and has no count test to invert. The buggy function had no counterpart to reproduce.

2. basicvariable.go:108 — setBoolean builds a value with valuetype: TYPE_STRING. Fixed in the port. akbasic_value_set_bool() sets AKBASIC_TYPE_BOOLEAN, and tests/value_compare.c reads a boolean back through every comparison operator.

3. basicenvironment.go:157 — stopWaiting(command) ignores command. Fixed in the port. akbasic_environment_stop_waiting() walks the scope chain and clears the first environment actually waiting for that verb, so an inner block can no longer clear an outer block's wait. A miss is tolerated rather than raised, which is the one part of the suggested fix not taken: EXIT and the end of a DEF body both call it speculatively.

4. basicvalue.go:181 — mathPlus mutates self in place when self.mutable is true. Fixed. It clones like every other operator now, so A# + 1 cannot modify A#.

   The gate on this item was FOR/NEXT coverage, because NEXT's increment relied on the mutation to advance the counter. tests/for_next.c is that coverage -- STEP, a negative step, a float counter, a body that assigns to the counter, EXIT, and nesting, none of which the golden corpus reaches -- and akbasic_cmd_next() now writes the incremented value back with akbasic_variable_set_subscript(). Removing that write-back does not fail the suite, it hangs it: the counter stops advancing and the loop never ends.

5. basicvalue.go:191 and siblings — every binary operator adds both of the right-hand operand's numeric fields. Fixed. rval_as_int() and rval_as_float() select on the operand's type instead of summing intval + (int64_t)floatval.

   It was filed as a landmine with no consequence today, and that is why it needed a test written against the value API rather than against the language: no BASIC program can build a value carrying both fields, and the old code passes every other test in the tree. tests/value_arithmetic.c constructs one directly.

6. basicvalue.go, all comparisons — dest is cloned from self, so the resulting boolean inherits self.name. Moot. BasicValue.name was dropped as dead during the port along with BasicEnvironment.update(), its only reader — deviation 9 above. There is no name for a comparison result to inherit.

7. basicruntime_commands.go:484 — CommandIF dereferences expr.right after the loop guaranteed it is nil, so the "Malformed IF statement" check is unreachable. Fixed in the port. akbasic_parse_if() matches the THEN token and compares its lexeme directly, so the check runs on every IF.

8. basicruntime_commands.go:669 — CommandEXIT pops the environment without stopWaiting. Fixed, and then fixed again — see item 18, which is what clearing the wait left behind.

9. basicruntime.go:149 — newVariable() and BasicRuntime.variables[MAX_VARIABLES] are dead. Not ported as dead code. The pair is live here and is the storage model: akbasic_environment_create() takes a slot from akbasic_runtime_new_variable(), because a pool is what replaced Go's per-environment map (§1.3). The reference's versions were dead; these are not.

10. basicgrammar.go:224 — newLiteralInt selects base 8 whenever the lexeme starts with 0. Fixed. Base 10 unless prefixed 0x; a leading zero in a listing is padding, not a radix. PRINT 010 prints 10 and PRINT 08 parses. Nothing in the upstream corpus had a leading-zero literal, which is why nothing there noticed. Regression tests in tests/grammar_leaves.c and tests/language/numeric/octal_literal.bas, which was rewritten from pinning the defect to pinning the fix.

11. basicruntime.go:121 / basicparser_commands.go:124 — environments are never freed. Fixed in the port. They come from a fixed pool and akbasic_runtime_prev_environment() releases each one — deviation 3 above. An unreleased scope shows up as pool exhaustion rather than as unbounded growth, which is