Andrew Kesterson
5c5bf63356
The graphics verbs documented a coordinate transform that did not exist. With SCALE off a coordinate went straight to akgl_draw_* as a pixel address, so an 800x600 window drew a C128 listing into its corner and left the rest unused -- while the chapter said coordinates were 320x200 and stretching to fit was the host's business. akbasic_GraphicsBackend gains a size entry point, require_graphics() asks it before every verb that draws so a resized window is honoured between two statements, and 320x200 becomes the fallback for a backend that leaves it NULL. It is the record's one optional member, so a host written against the old header keeps the behaviour it had. SCALE now maps onto the device, and RGR(1)/RGR(2) report the drawing surface so a program can use a window whose size it did not choose. RGR(0) is BASIC 7.0's own field, the GRAPHIC mode. SCALE also mapped xmax onto the width rather than onto the last pixel, so SCALE 1, 319, 199 followed by DRAW 1, 319, 199 drew nothing at all -- one pixel past the surface. Fixed in the same line, because it is what makes "SCALE gives a C128 listing the whole window" true rather than nearly true. The akgl test renders against a 128x128 target, deliberately smaller than the old constants: a SCALE still dividing by them misses it entirely rather than landing somewhere plausible. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
173 lines
6.0 KiB
Markdown
173 lines
6.0 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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```basic
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10 FOR I# = 1 TO 3 : PRINT I# : NEXT I#
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20 PRINT "DONE"
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```
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```output
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DONE
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```
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The loop 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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- **A coordinate is a window pixel, not one of 320 by 200.** A C128 listing therefore
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draws in the top-left corner of a larger window; `SCALE 1, 319, 199` gives it the
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whole window back. `RGR(1)` and `RGR(2)` report the size, and are ours rather than
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7.0's — where 7.0's `RGR` takes only field 0, the `GRAPHIC` mode.
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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 window pixels**, not the VIC-II's raster space, and `SCALE` does
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not apply to them.
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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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