Andrew Kesterson
0679c3042b
You were right that this was still everywhere. The two `.bas` files carried their workarounds as present-tense statements about the interpreter -- "a name first created inside a GOSUB costs a value slot that is never handed back", "writing PAST the terminator of a short row draws nothing at all", "SSHAPE and GSHAPE ignore the subscript on a string array", "a FOR inside a BEGIN block does not survive the RETURN" -- and every one of those is now false. A comment that lies is worse than no comment, and these were the first thing anybody opening the files would read. **The characters game takes the geometry fix.** `CW# = 16` becomes `CW# = RGR(3)`, and the grid comes from `RWINDOW`, so the game fits whatever window and font the host gives it. That was the one thing in the listing that would break on a different font, and `RGR(3)` and `RWINDOW` were added to the interpreter because of it -- leaving the constant in place while Chapter 17 teaches the function was the inconsistency worth closing. `RGR(1)` still comes first so a build with no graphics device refuses by naming the device that is missing. Everything else in both listings keeps its shape and says why. Declaring every name up front, guarding loops with `GOTO`, six scalars for six brick stamps: none is forced any more and none costs anything, so they stay, with the comments marking which rules stand and which are history. The sprites header's five traps are marked FIXED where they are fixed. The chapters follow: Chapter 17 Step 2 no longer says "the listing still has `CW# = 16`", and Chapter 18's trap section is "five things that did not do what they looked like" with the two that still stand named up front rather than left to be counted. Verified: Chapter 17 Step 2's block is once again verbatim from the listing, so every quoted fragment in both chapters matches its source with no exceptions; both games run 40 seconds on the SDL frontend with no error line; both suites green in both configurations; `docs_examples` and `docs_screenshots --check` pass. 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.