Andrew Kesterson
cdaefcc941
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>
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8. Sprites
Eight sprites, numbered 1 to 8, as on a C128. They need the SDL build.
A sprite here is a real object in the host's graphics library, registered alongside whatever the host's own game is drawing — so a game that embeds this interpreter can see and manipulate a script's sprites.
Giving a sprite a picture
SPRSAV does it, and it takes three kinds of source.
From an image file
10 SPRSAV "ship.png", 1
The most useful form, and not one a C128 has. The path is tried against the working
directory first and then against the directory the running program was loaded from — so
a .bas stored beside its artwork works wherever you launch it from. Anything the
image library can decode will do.
A sprite loaded this way takes the image's own size. It is not forced to 24 by 21.
From a saved region
10 BOX 1, 0, 0, 24, 21
20 SSHAPE A$, 0, 0, 24, 21
30 SPRSAV A$, 1
Draw it with the graphics verbs, capture it, install it. The C128 documents SPRSAV's
string as the SSHAPE data format at a fixed 24 by 21, so sharing the mechanism is
faithful rather than a shortcut.
From data
10 DIM P#(63)
20 FOR I# = 0 TO 62
30 READ P#(I#)
40 NEXT I#
50 SPRSAV P#, 1
60 DATA 255, 129, 129, ...
63 bytes: three per row, twenty-one rows, most significant bit leftmost. This is the form a type-in listing uses.
It takes an integer array, not a string. A C128 puts the raw bytes in a string; a
string here is NUL-terminated, so it cannot hold a zero byte and therefore cannot hold
a sprite. DIM P#(63) is exactly the right size.
Writing a sprite back out — SPRSAV 1, A$ — is refused. That would be a disk
operation.
Showing and moving
10 SPRITE 1, 1, 3
20 MOVSPR 1, 100, 50
SPRITE n [,on] [,colour] [,priority] [,xexpand] [,yexpand] [,multicolour]. Only the
number is required and an argument you leave out is left alone, so SPRITE 1, 1
turns sprite 1 on without disturbing its colour.
MOVSPR has four forms, and the punctuation is what tells them apart:
| Form | What it does |
|---|---|
MOVSPR 1, 100, 50 |
put it at (100, 50) |
MOVSPR 1, +10, -20 |
move it by that much |
MOVSPR 1, 10 ; 90 |
move it 10 pixels along bearing 90 |
MOVSPR 1, 45 # 8 |
set it moving along bearing 45 at speed 8 |
A bearing is degrees clockwise from straight up, so 0 is north and 90 is east. In the polar form the distance comes first and the angle second — the opposite order from the continuous form, which is a trap worth remembering.
The continuous form does not move anything on the statement itself. The sprite moves as the program runs, paced by the host's clock. Speed 0 stops it.
Coordinates are the same 320 by 200 space the drawing verbs use, not the VIC-II's raster coordinates.
Collision
10 COLLISION 1, BUMPED
20 REM ... main loop ...
90 GOTO 20
100 LABEL BUMPED
110 PRINT "HIT: " + BUMP(1)
120 RETURN
COLLISION 1, target calls a subroutine when two sprites overlap. The handler is
entered between lines and must end in RETURN, exactly like a GOSUB body. Omitting
the target disarms it.
BUMP(1) returns a bitmask of which sprites have collided — bit 0 is sprite 1 — and
reading it clears it, which is what makes "has anything hit me since I last looked"
answerable.
Only type 1, sprite-to-sprite, is implemented. Types 2 and 3 are refused by name.
Collision is by bounding box, not by pixel: two sprites whose boxes overlap but whose artwork does not are reported as colliding.
Reading state back
| Function | Gives |
|---|---|
RSPPOS(n, 0) |
x |
RSPPOS(n, 1) |
y |
RSPPOS(n, 2) |
speed |
RSPRITE(n, f) |
one of SPRITE's settings, in SPRITE's own argument order |
RSPCOLOR(1) or RSPCOLOR(2) |
one of the shared multicolour registers |
These read the interpreter's own state rather than asking the device, so they work even with no device attached.
SPRDEF
Not implemented, and deliberately. It is an interactive full-screen sprite editor
driven by single keystrokes, not something a program can call — and this interpreter
does not own the screen it would take over. The three SPRSAV forms replace it.