A fast object can cross a wall between frames even when neither endpoint overlaps it.
Use it in a project
Move an interval [OLD_POSITION,OLD_POSITION+SIZE) towards TARGET against [WALL_START,WALL_END). SIZE must be nonzero; both proposed endpoints plus SIZE must fit in a byte, the wall must be nonempty, and the old interval must not overlap it. Return POSITION clamped to first contact and HIT=1 when motion reaches or crosses that face. Invalid requests leave POSITION=OLD_POSITION and VALID=0.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: POSITION, HIT, VALID, OLD_POSITION, TARGET, SIZE, WALL_START, WALL_END. They occupy 8 contiguous bytes. Also reserve the private workspace: T0, T1 (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call swept_axis_collision_init once at the intended reset boundary. It clears inputs as well as outputs and workspace. Set your starting state and configuration afterwards, then call swept_axis_collision_step whenever the described event occurs. The helper reads no device, waits for no frame and installs no interrupt handler.
| Ownership | Fields |
|---|---|
| Retain between calls | none |
| Supply before service | OLD_POSITION, TARGET, SIZE and wall edges |
| Read after service | POSITION, HIT and VALID |
When moving right, test the path of the right edge against the wall’s left face. Moving left tests the object’s left edge against the wall’s right face. This avoids sampling intermediate pixels.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Sweep an Interval Along One Axis. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
swept_axis_collision_init:
move.b #0,d0
move.b d0,POSITION
move.b d0,HIT
move.b d0,VALID
move.b d0,OLD_POSITION
move.b d0,TARGET
move.b d0,SIZE
move.b d0,WALL_START
move.b d0,WALL_END
move.b d0,T0
move.b d0,T1
rts
swept_axis_collision_step:
; POSITION = OLD_POSITION
move.b OLD_POSITION,d0
move.b d0,POSITION
; HIT = 0
move.b #0,d0
move.b d0,HIT
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if SIZE == 0:
move.b SIZE,d0
cmp.b #0,d0
beq swept_axis_collision_L1
jmp swept_axis_collision_L2
swept_axis_collision_L1:
; return
rts
jmp swept_axis_collision_L3
swept_axis_collision_L2:
swept_axis_collision_L3:
; if WALL_START >= WALL_END:
move.b WALL_START,d0
move.b WALL_END,d1
cmp.b d1,d0
bcc swept_axis_collision_L4
jmp swept_axis_collision_L5
swept_axis_collision_L4:
; return
rts
jmp swept_axis_collision_L6
swept_axis_collision_L5:
swept_axis_collision_L6:
; T0 = OLD_POSITION + SIZE
move.b OLD_POSITION,d0
move.b SIZE,d1
add.b d1,d0
move.b d0,T0
; if T0 < OLD_POSITION:
move.b T0,d0
move.b OLD_POSITION,d1
cmp.b d1,d0
bcs swept_axis_collision_L7
jmp swept_axis_collision_L8
swept_axis_collision_L7:
; return
rts
jmp swept_axis_collision_L9
swept_axis_collision_L8:
swept_axis_collision_L9:
; T1 = TARGET + SIZE
move.b TARGET,d0
move.b SIZE,d1
add.b d1,d0
move.b d0,T1
; if T1 < TARGET:
move.b T1,d0
move.b TARGET,d1
cmp.b d1,d0
bcs swept_axis_collision_L10
jmp swept_axis_collision_L11
swept_axis_collision_L10:
; return
rts
jmp swept_axis_collision_L12
swept_axis_collision_L11:
swept_axis_collision_L12:
; if OLD_POSITION < WALL_END:
move.b OLD_POSITION,d0
move.b WALL_END,d1
cmp.b d1,d0
bcs swept_axis_collision_L13
jmp swept_axis_collision_L14
swept_axis_collision_L13:
; if T0 > WALL_START:
move.b T0,d0
move.b WALL_START,d1
cmp.b d1,d0
bhi swept_axis_collision_L16
jmp swept_axis_collision_L17
swept_axis_collision_L16:
; return
rts
jmp swept_axis_collision_L18
swept_axis_collision_L17:
swept_axis_collision_L18:
jmp swept_axis_collision_L15
swept_axis_collision_L14:
swept_axis_collision_L15:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; POSITION = TARGET
move.b TARGET,d0
move.b d0,POSITION
; if TARGET > OLD_POSITION:
move.b TARGET,d0
move.b OLD_POSITION,d1
cmp.b d1,d0
bhi swept_axis_collision_L19
jmp swept_axis_collision_L20
swept_axis_collision_L19:
; if T0 <= WALL_START:
move.b T0,d0
move.b WALL_START,d1
cmp.b d1,d0
bls swept_axis_collision_L22
jmp swept_axis_collision_L23
swept_axis_collision_L22:
; if T1 >= WALL_START:
move.b T1,d0
move.b WALL_START,d1
cmp.b d1,d0
bcc swept_axis_collision_L25
jmp swept_axis_collision_L26
swept_axis_collision_L25:
; POSITION = WALL_START - SIZE
move.b WALL_START,d0
move.b SIZE,d1
sub.b d1,d0
move.b d0,POSITION
; HIT = 1
move.b #1,d0
move.b d0,HIT
jmp swept_axis_collision_L27
swept_axis_collision_L26:
swept_axis_collision_L27:
jmp swept_axis_collision_L24
swept_axis_collision_L23:
swept_axis_collision_L24:
jmp swept_axis_collision_L21
swept_axis_collision_L20:
swept_axis_collision_L21:
; if TARGET < OLD_POSITION:
move.b TARGET,d0
move.b OLD_POSITION,d1
cmp.b d1,d0
bcs swept_axis_collision_L28
jmp swept_axis_collision_L29
swept_axis_collision_L28:
; if OLD_POSITION >= WALL_END:
move.b OLD_POSITION,d0
move.b WALL_END,d1
cmp.b d1,d0
bcc swept_axis_collision_L31
jmp swept_axis_collision_L32
swept_axis_collision_L31:
; if TARGET <= WALL_END:
move.b TARGET,d0
move.b WALL_END,d1
cmp.b d1,d0
bls swept_axis_collision_L34
jmp swept_axis_collision_L35
swept_axis_collision_L34:
; POSITION = WALL_END
move.b WALL_END,d0
move.b d0,POSITION
; HIT = 1
move.b #1,d0
move.b d0,HIT
jmp swept_axis_collision_L36
swept_axis_collision_L35:
swept_axis_collision_L36:
jmp swept_axis_collision_L33
swept_axis_collision_L32:
swept_axis_collision_L33:
jmp swept_axis_collision_L30
swept_axis_collision_L29:
swept_axis_collision_L30:
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four return-address stack bytes. The helper makes no nested calls and adds no other stack use. Its named scratch is private workspace, not a pointer, a ROM variable or shared zero-page space that the game may borrow concurrently. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. Give each block one mutation owner.
The A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
Adapt the caller
demo.asm supplies the bindings and include order. exercise.inc first runs table cases, then a separate client retaining state between calls. A game can call the public step entry directly and remove the diagnostic packet and display. Do not copy the diagnostic’s hardware takeover into an existing game loop.
Build with make ASM198X=/path/to/asm198x and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
POSITION: dc.b 0
HIT: dc.b 0
VALID: dc.b 0
OLD_POSITION: dc.b 0
TARGET: dc.b 0
SIZE: dc.b 0
WALL_START: dc.b 0
WALL_END: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; OLD_POSITION = 20
move.b #20,d0
move.b d0,OLD_POSITION
; TARGET = 2
move.b #2,d0
move.b d0,TARGET
; SIZE = 2
move.b #2,d0
move.b d0,SIZE
; WALL_START = 10
move.b #10,d0
move.b d0,WALL_START
; WALL_END = 12
move.b #12,d0
move.b d0,WALL_END
ifne BROKEN
; TARGET = OLD_POSITION
move.b OLD_POSITION,d0
move.b d0,TARGET
endif
bsr checked_step
bsr emit_state
rts
else
; OLD_POSITION = 2
move.b #2,d0
move.b d0,OLD_POSITION
; TARGET = 20
move.b #20,d0
move.b d0,TARGET
; SIZE = 3
move.b #3,d0
move.b d0,SIZE
; WALL_START = 10
move.b #10,d0
move.b d0,WALL_START
; WALL_END = 12
move.b #12,d0
move.b d0,WALL_END
ifne BROKEN
; TARGET = OLD_POSITION
move.b OLD_POSITION,d0
move.b d0,TARGET
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Sweep leftwards towards the opposite wall face. The maintained custom caller changes its inputs or client sequence rather than hiding a different implementation behind the same label. Compare every state of the two clients, not only their final display.
Custom client selection
section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
POSITION: dc.b 0
HIT: dc.b 0
VALID: dc.b 0
OLD_POSITION: dc.b 0
TARGET: dc.b 0
SIZE: dc.b 0
WALL_START: dc.b 0
WALL_END: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The custom diagnostic row is 0C 01 01 14 02 02 0A 0C. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller passes the old position as the target. Nothing moves, so no sweep can detect contact. Keep the requested endpoint distinct from the current position.
The BROKEN branch lives in the sequential client, not the reusable helper. wrong.asm selects it; repaired.asm restores the normal call sequence. The repaired executable matches the normal program (for Spectrum, compare loaded CODE rather than the tape filename).
Select the deliberately wrong client
section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
POSITION: dc.b 0
HIT: dc.b 0
VALID: dc.b 0
OLD_POSITION: dc.b 0
TARGET: dc.b 0
SIZE: dc.b 0
WALL_START: dc.b 0
WALL_END: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 442 code bytes; public state is 8 bytes and private scratch is 2 bytes. There are no immutable helper tables. Caller data, the test cases, return-address stack, alignment and display buffers cost additional space. These are assembled sizes, not cycle measurements.
Constant work for one wall. More obstacles require a broad phase and nearest-contact selection; subdividing movement is simpler but costs more as speed increases.
This is one axis and one static obstacle, with no initial penetration. A full 2D swept solver needs the earliest contact across candidates and a policy for the remaining movement.
Execution evidence
Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. Each executes 64 boundary/varied/error table cases, dirty initialisation and the separate retained-state client. An independent mathematical/container model checks every public byte after every call. This is bounded example coverage, not exhaustive correctness for every possible state.
The records retain loaded immutable code, completion, register sentinels, full packets, source/tool/firmware hashes and captures. The public gate validates these retained observations and rejects altered evidence; it does not run restricted firmware. No firmware is distributed. The eight displayed bytes are the first eight public fields, zero-padded when shorter; the packet includes every field.
Where it fits
Rectangle overlap: Endpoint overlap is useful for slow movement but does not detect a crossed obstacle.