Andrew Kesterson
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Organised the way the C128 Programmer's Reference Guide is: the language first, then each hardware area, then the reference sections. One markdown file per chapter. The verb and function references are generated from the interpreter's own dispatch table, with an assertion that every row is described, so they cannot drift out of step with what the program accepts. 98 verbs and 30 functions. Every example was run before it was written down, which caught three claims that were wrong: a whole FOR loop on one line prints nothing rather than looping once, MID and INSTR count from zero where a C128 counts from one, and a multi-line DEF returns a value the caller has to assign away. Chapter 13 is the list a BASIC 7.0 programmer needs -- roughly sixty documented differences, including the two known FOR defects and the fact that drawing does not survive a frame. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
164 lines
5.7 KiB
Markdown
164 lines
5.7 KiB
Markdown
# 13. Differences from BASIC 7.0
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If you already write Commodore BASIC, this is the chapter to read first. Everything
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here is deliberate, and everything is recorded in the repository's `TODO.md` with the
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reasoning — this is the short version.
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## The language
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### Variables carry a type suffix, and the suffixes differ
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| | Integer | Float | String |
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|---|---|---|---|
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| C128 | `A%` | `A` | `A$` |
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| akbasic | `A#` | `A%` | `A$` |
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There is **no such thing as an unsuffixed variable**. A bare name is a label.
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### `=` works in a condition, and `==` works everywhere
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`IF A = 5 THEN` does what you expect. `==` also means equality and is what the older
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programs in this repository use. Outside a condition `=` is assignment, as always.
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### `AND`, `OR` and `NOT` in conditions
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These work, and a condition is a whole expression rather than a single comparison, so
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`IF A = 1 AND B = 2 THEN` parses. Truth is nonzero, so `IF A THEN` works too.
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### `MID` and `INSTR` count from zero
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A C128 counts from one. A failed `INSTR` gives -1 rather than 0.
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### `THEN` needs a verb
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`IF X THEN 100` is not a jump. Write `IF X THEN GOTO 100`.
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### Strings are 255 characters and cannot contain a quote
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There is no escape character.
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### Numbers
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Integers are 64-bit and floats are IEEE doubles, so `PRINT 1.5` gives `1.500000`. A
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leading zero is not octal; `0x` is hexadecimal.
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## Block structure
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**A whole loop on one line does not loop.**
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```
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10 FOR I# = 1 TO 3 : PRINT I# : NEXT I#
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```
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prints nothing. Block skipping walks source *lines*, so a `NEXT` on the same line as
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its `FOR` is never reached. The same applies to `DO`/`LOOP`. Write loops across lines.
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## Two known defects in `FOR`
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Both are recorded, both have tests asserting the correct behaviour, and both are
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waiting on a decision rather than on work:
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- **A step that overshoots runs the body one extra time.** `FOR I = 1 TO 9 STEP 3` runs
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with 1, 4, 7 *and 10*.
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- **`FOR I = 1 TO 1` does not run its body at all**, where every other BASIC runs it
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once.
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The two errors cancel out for a step of 1, which is why they went unnoticed. Fixing
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them would change the output of a checked-in acceptance file, which is not something
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this project does silently.
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**A loop counter does not survive its loop.** It lives in the loop's own scope, so
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reading it afterwards gives zero.
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## Direct mode
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A statement typed with no line number runs immediately, as it should. This was not
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true until recently — the interpreter used to file everything but a handful of verbs as
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program text.
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## Errors
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`ER` and `EL` are **`ER#` and `EL#`**, ordinary global variables. `ER#` holds this
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interpreter's error code, which bears no relation to a Commodore error number. Print
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`ERR(ER#)` for the text.
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## Graphics
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- **Coordinates are always 320 by 200**, whatever the window size.
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- **`SSHAPE` puts a handle in the string, not the pixels.** You can pass it to `GSHAPE`
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and `SPRSAV`; you cannot save it or take its `LEN`.
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- **`WIDTH` is emulated** by drawing parallel passes.
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- **Drawing does not persist across frames.** The verbs draw straight to the renderer,
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so anything drawn is overwritten on the next frame unless the program redraws it.
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This is a defect, not a design.
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- **`CHAR` ignores its colour argument** and needs a text device with a cursor.
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## Sound
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- **`PLAY` and `SOUND` do not block.** The statement after them runs immediately.
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- **`FILTER` is refused.** There is no filter stage to configure.
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- **`PLAY`'s `M` is accepted and does nothing.**
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- **`TEMPO`'s calibration is a choice**, not a transcription.
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## Sprites
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- **Coordinates are the 320 by 200 drawing space**, not the VIC-II's raster space.
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- **`SPRSAV` takes an integer array**, not a string, for the data form — a string here
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cannot hold a zero byte. It also takes an **image file path**, which a C128 cannot.
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- **A sprite loaded from a file keeps the image's own size**, not 24 by 21.
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- **`MOVSPR`'s speed unit is a choice.** The manual does not say what a unit is worth.
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- **Collision is by bounding box**, not by pixel, and only type 1 exists.
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- **Priority and multicolour are recorded but not drawn.**
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- **`SPRDEF` is out of scope.**
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## Files
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- **`PRINT #` and `INPUT #` need a space before the `#`.** `PRINT#1` scans as a
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variable name.
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- **`RECORD` counts lines**, not fixed-length records.
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- **`BLOAD` requires a length.**
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- **`HEADER`, `COLLECT`, `BACKUP` and `BOOT` are refused.** They operate on a physical
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disk.
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- **`DIRECTORY` is refused** pending a wrapper in the standard library.
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## Machine
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- **`SYS` is refused.** There is no 6502 and no ROM.
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- **`FETCH` and `STASH` are the same byte copy.** There is no expansion RAM to tell
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them apart.
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- **`POKE`, `PEEK` and `POINTER` use real process addresses.** A wrong one is a
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segmentation fault, not an error message.
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- **`BANK`, `FAST` and `MONITOR` do not exist.**
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## Formatting
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- **`PRINT USING` renders one field per statement.** `PRINT USING "### ###"; A, B` is
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not supported.
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- **Exponential fields (`^^^^`) are not implemented.**
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## Console
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- **`SLEEP` and `WAIT` hold the program without blocking the host.** `SLEEP` with no
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host clock does nothing at all rather than waiting forever.
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- **`TI` and `TI$` are `TI#` and `TI$`**, refreshed once per step.
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- **`WAIT` polls ordinary process memory.** Nothing changes it but the host.
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- **`KEY` stores macros and nothing expands them.**
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## Limits
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| Source lines | 9999 |
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| Line length | 255 |
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| String length | 255 |
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| Variables | 128 |
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| Array elements | 1024 per array, 4096 in total |
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| Scopes | 32 |
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| Labels | 64 |
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| `DATA` items | 512 |
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| File channels | 10 |
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| Operations per line | roughly 16 |
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Every one is a fixed pool. Nothing in the interpreter calls `malloc`, which is what
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makes it safe to embed in a game that cannot afford a surprise allocation.
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