Andrew Kesterson
91fadf032b
Eleven of the thirteen defects these two games found are fixed, and the chapters that taught around them said things that are no longer true. Chapter 17: the drawing verbs are no longer invisible, they are covered by a text layer that `WINDOW` can now shrink; the cell size is `RGR(3)` and the grid is `RWINDOW`, not a hand-measured constant; a character written past a short row's end lands. Chapter 18: traps 3 and 4 -- the skipped block that broke its caller's `RETURN`, and `SSHAPE` ignoring a subscript -- become history rather than warnings, and Step 3 no longer claims a sprite is the *only* way to put a picture up, only the one that costs nothing. **Neither `.bas` listing changes, and both READMEs now say why.** The character game still writes `CW# = 16` and the artwork game still keeps six brick stamps in six scalars. What those listings are worth is being what a program written against those constraints looks like, with comments explaining what each one was avoiding -- rewriting them to pretend the problems never existed would throw that away. So Chapter 17 Step 2 shows the `RGR(3)`/`RWINDOW` form and says plainly that the listing beside it does not use it. That is the one place in either chapter where a quoted fragment is not verbatim from the game. Also: the closing pointers now say which entries are struck and which stand, and the budgets table in Chapter 18 notes that several of those budgets were tighter when the game was written. TODO.md section 9 item 3 is updated rather than struck: the `WINDOW` half is fixed, the default text area owning the whole window is not, and whether that default is right is a question rather than a defect. Verified against the fixed interpreter: both games run 45 seconds on the SDL frontend with no error line and the score climbing; both suites are green in both configurations; `docs_examples` passes and `docs_screenshots --check` re-renders all thirteen figures byte-identically; coverage is 95.0% of lines against the 90 gate, with src/variable.c at 100%; and every other quoted fragment still appears verbatim in the listing it came from. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
The akbasic guide
akbasic is a BASIC interpreter in the style of Commodore BASIC 7.0 — the dialect the C128 shipped with — and Dartmouth BASIC. It runs programs from a file or from an interactive prompt, and it is also a C library you can link into a game so that players can script it.
These chapters follow the shape of the C128 Programmer's Reference Guide: the language first, then each hardware area, then the reference sections. If you know BASIC 7.0 you can skip to Chapter 13, which is the list of everything that behaves differently here and why. Chapter 14 is the odd one out: it is about the interpreter rather than the language, for anyone embedding it, debugging it or changing it.
Chapters 17 and 18 are tutorials rather than reference: they build one complete game twice, two different ways, and are where the rules that a real program runs into are collected.
Chapters
| 1. Introduction | What akbasic is, what it is not, and how to build it |
| 2. Getting started | The prompt, your first program, saving and loading |
| 3. The language | Variables, types, arrays, operators, expressions |
| 4. Control flow | IF, FOR, DO, GOSUB, labels, ON, error trapping |
| 5. Strings and formatting | String functions, PRINT USING, PUDEF |
| 6. Graphics | GRAPHIC, DRAW, BOX, CIRCLE, PAINT, shapes |
| 7. Sound | SOUND, PLAY, ENVELOPE, VOL, TEMPO |
| 8. Sprites | SPRITE, SPRSAV, MOVSPR, collision |
| 9. Files and disk | Channels, DOPEN, program storage |
| 10. Embedding | Driving the interpreter from C |
| 11. Verb reference | Every statement, alphabetically |
| 12. Function reference | Every function, alphabetically |
| 13. Differences from BASIC 7.0 | What a C128 programmer needs to know |
| 14. Architecture | How the interpreter is put together, how to debug it, how to change it |
| 15. Error codes | Appendix: every value ER# can hold and every error line the interpreter prints |
| 16. Structures | TYPE, records, strict pointers, and sharing a C struct with an embedding host |
| 17. Tutorial: Breakout | Build a whole game out of the text grid and two DATA sprites, a step at a time |
| 18. Tutorial: Breakout with artwork | Build it again out of loaded artwork, powerups and a drawn HUD |
The shortest possible start
$ cmake -S . -B build && cmake --build build
$ ./build/basic
10 FOR I# = 1 TO 5
20 PRINT "HELLO " + I#
30 NEXT I#
RUN
HELLO 1
HELLO 2
HELLO 3
HELLO 4
HELLO 5
READY
Two things in that program are not Commodore BASIC and will catch you out
immediately: variables carry a type suffix (I# is an integer) and +
concatenates a string with a number. Chapter 3 explains both.
Every example in these chapters is executed by the test suite and its output
compared byte for byte — see MAINTENANCE.md if you are editing them.