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Document structures: a chapter, the architecture, and the differences

docs/16-structures.md is the feature: records, nesting, copy-on-assign, strict
pointers, lists, what is checked and what is not, and how a host shares its own
C structs. Every example in it is executed by docs_examples and byte-compared,
including the refusals -- so a message that changes fails the suite rather than
quietly making the chapter wrong.

The chapter makes one contrast explicitly, because it is the question a reader
will actually have: a misspelled *field* is refused and a misspelled *variable*
still prints zero. The rule underneath is that what the program declared gets
checked and what it did not gets shrugged at -- a variable's name is never
declared, a TYPE's field list is. Structures end up the strictest thing in the
language, not from a higher standard but because they are the only named thing
whose valid spellings are written down.

Chapter 14 gains the layout: an instance is a contiguous run of value slots with
a diagram of where the fields sit, the three-pass prescan and why each pass
exists, why the copy cannot live in akbasic_value_clone(), and why the render
depth bound is four rather than eight. Chapter 3 gains the @ suffix, chapter 13
records that all of this is an addition BASIC 7.0 has nothing like, and the verb
reference gains TYPE, POINT and DIM ... AS.

MAINTENANCE.md gains the two rules that are on a maintainer rather than on a
test: a structure copy must not go through clone, and a field chain gets its own
leaf field. TODO.md section 5 records what was invented and the three limits
that are ours, and section 8 records the two defects the work exposed.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2026-08-01 12:03:02 -04:00

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13. Differences from BASIC 7.0

If you already write Commodore BASIC, this is the chapter to read first. Everything here is deliberate, and everything is recorded in the repository's TODO.md with the reasoning — this is the short version.

The language

Variables carry a type suffix, and the suffixes differ

	Integer	Float	String
C128	`A%`	`A`	`A$`
akbasic	`A#`	`A%`	`A$`

There is no such thing as an unsuffixed variable. A bare name is a label.

`=` works in a condition, and `==` works everywhere

IF A = 5 THEN does what you expect. == also means equality and is what the older programs in this repository use. Outside a condition = is assignment, as always.

`AND`, `OR` and `NOT` in conditions

These work, and a condition is a whole expression rather than a single comparison, so IF A = 1 AND B = 2 THEN parses. Truth is nonzero, so IF A THEN works too.

`MID` and `INSTR` count from zero

A C128 counts from one. A failed INSTR gives -1 rather than 0.

`THEN` needs a verb

IF X THEN 100 is not a jump. Write IF X THEN GOTO 100.

Strings are 255 characters and cannot contain a quote

There is no escape character.

Numbers

Integers are 64-bit and floats are IEEE doubles, so PRINT 1.5 gives 1.500000. A leading zero is not octal; 0x is hexadecimal.

Structures are an addition

BASIC 7.0 has no records at all. TYPE/END TYPE, DIM X@ AS T, the @ suffix, . and ->, PTR TO and POINT ... AT are all new here, and Chapter 16 is the whole of it. Nothing about it changes an existing program.

Two consequences a C128 programmer should know. A structure assignment copies, like every other assignment; sharing is spelled POINT. And a type name is a bare word, so it shares a namespace with verbs and labels — TYPE POINT is refused because POINT is now a verb.

Block structure

A whole loop on one line does not loop.

10 FOR I# = 1 TO 3 : PRINT I# : NEXT I#
20 PRINT "DONE"

DONE

The loop prints nothing. Block skipping walks source lines, so a NEXT on the same line as its FOR is never reached. The same applies to DO/LOOP. Write loops across lines.

Two known defects in `FOR`

Both are recorded, both have tests asserting the correct behaviour, and both are waiting on a decision rather than on work:

A step that overshoots runs the body one extra time. FOR I = 1 TO 9 STEP 3 runs with 1, 4, 7 and 10.
FOR I = 1 TO 1 does not run its body at all, where every other BASIC runs it once.

The two errors cancel out for a step of 1, which is why they went unnoticed. Fixing them would change the output of a checked-in acceptance file, which is not something this project does silently.

A loop counter does not survive its loop. It lives in the loop's own scope, so reading it afterwards gives zero.

Direct mode

A statement typed with no line number runs immediately, as it should. This was not true until recently — the interpreter used to file everything but a handful of verbs as program text.

Errors

ER and EL are ER# and EL#, ordinary global variables. ER# holds this interpreter's error code, which bears no relation to a Commodore error number. Print ERR(ER#) for the text, and see Chapter 15 for the whole list.

Graphics

A coordinate is a window pixel, not one of 320 by 200. A C128 listing therefore draws in the top-left corner of a larger window; SCALE 1, 319, 199 gives it the whole window back. RGR(1) and RGR(2) report the size, and are ours rather than 7.0's — where 7.0's RGR takes only field 0, the GRAPHIC mode.
SSHAPE puts a handle in the string, not the pixels. You can pass it to GSHAPE and SPRSAV; you cannot save it or take its LEN.
WIDTH is emulated by drawing parallel passes.
Drawing does not persist across frames. The verbs draw straight to the renderer, so anything drawn is overwritten on the next frame unless the program redraws it. This is a defect, not a design.
CHAR ignores its colour argument and needs a text device with a cursor.

Sound

PLAY and SOUND do not block. The statement after them runs immediately.
FILTER is refused. There is no filter stage to configure.
PLAY's M is accepted and does nothing.
TEMPO's calibration is a choice, not a transcription.

Sprites

Coordinates are window pixels, not the VIC-II's raster space, and SCALE does not apply to them.
SPRSAV takes an integer array, not a string, for the data form — a string here cannot hold a zero byte. It also takes an image file path, which a C128 cannot.
A sprite loaded from a file keeps the image's own size, not 24 by 21.
MOVSPR's speed unit is a choice. The manual does not say what a unit is worth.
Collision is by bounding box, not by pixel, and only type 1 exists.
Priority and multicolour are recorded but not drawn.
SPRDEF is out of scope.

Files

PRINT # and INPUT # need a space before the #. PRINT#1 scans as a variable name.
RECORD counts lines, not fixed-length records.
BLOAD requires a length.
HEADER, COLLECT, BACKUP and BOOT are refused. They operate on a physical disk.
DIRECTORY is refused pending a wrapper in the standard library.

Machine

SYS is refused. There is no 6502 and no ROM.
FETCH and STASH are the same byte copy. There is no expansion RAM to tell them apart.
POKE, PEEK and POINTER use real process addresses. A wrong one is a segmentation fault, not an error message.
BANK, FAST and MONITOR do not exist.

Formatting

PRINT USING renders one field per statement. PRINT USING "### ###"; A, B is not supported.
Exponential fields (^^^^) are not implemented.

Console

SLEEP and WAIT hold the program without blocking the host. SLEEP with no host clock does nothing at all rather than waiting forever.
TI and TI$ are TI# and TI$, refreshed once per step.
WAIT polls ordinary process memory. Nothing changes it but the host.
KEY stores macros and nothing expands them.

Limits


Source lines	9999
Line length	255
String length	255
Variables	128
Array elements	1024 per array, 4096 in total
Scopes	32
Labels	64
`DATA` items	512
File channels	10
Operations per line	roughly 16

Every one is a fixed pool. Nothing in the interpreter calls malloc, which is what makes it safe to embed in a game that cannot afford a surprise allocation.

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