A fast ball should use the distance left after a bounce rather than sticking to the wall.
Use it in a project
MINIMUM must be below MAXIMUM, POSITION inside them and DIRECTION either 0 towards the minimum or 1 towards the maximum. Consume DISTANCE byte-sized unit steps, reflecting before a step that would leave the interval. BOUNCES counts reflections; landing exactly on an endpoint does not reflect until movement next needs to leave it. Invalid input leaves position/direction unchanged.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: POSITION, MINIMUM, MAXIMUM, DISTANCE, DIRECTION, BOUNCES, VALID. They occupy 7 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call reflected_axis_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 reflected_axis_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 | POSITION and DIRECTION |
| Supply before service | MINIMUM, MAXIMUM and DISTANCE |
| Read after service | BOUNCES and VALID |
Each unit of travel checks whether the current direction would leave the interval. Reflect first if it would, then consume that unit. Remaining travel keeps moving, so a large step can bounce more than once.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Reflect Movement at Axis Bounds. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
reflected_axis_init:
move.b #0,d0
move.b d0,POSITION
move.b d0,MINIMUM
move.b d0,MAXIMUM
move.b d0,DISTANCE
move.b d0,DIRECTION
move.b d0,BOUNCES
move.b d0,VALID
move.b d0,T0
rts
reflected_axis_step:
; BOUNCES = 0
move.b #0,d0
move.b d0,BOUNCES
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if MINIMUM >= MAXIMUM:
move.b MINIMUM,d0
move.b MAXIMUM,d1
cmp.b d1,d0
bcc reflected_axis_L1
jmp reflected_axis_L2
reflected_axis_L1:
; return
rts
jmp reflected_axis_L3
reflected_axis_L2:
reflected_axis_L3:
; if POSITION < MINIMUM:
move.b POSITION,d0
move.b MINIMUM,d1
cmp.b d1,d0
bcs reflected_axis_L4
jmp reflected_axis_L5
reflected_axis_L4:
; return
rts
jmp reflected_axis_L6
reflected_axis_L5:
reflected_axis_L6:
; if POSITION > MAXIMUM:
move.b POSITION,d0
move.b MAXIMUM,d1
cmp.b d1,d0
bhi reflected_axis_L7
jmp reflected_axis_L8
reflected_axis_L7:
; return
rts
jmp reflected_axis_L9
reflected_axis_L8:
reflected_axis_L9:
; if DIRECTION >= 2:
move.b DIRECTION,d0
cmp.b #2,d0
bcc reflected_axis_L10
jmp reflected_axis_L11
reflected_axis_L10:
; return
rts
jmp reflected_axis_L12
reflected_axis_L11:
reflected_axis_L12:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = DISTANCE
move.b DISTANCE,d0
move.b d0,T0
; while T0 != 0:
reflected_axis_L13:
move.b T0,d0
cmp.b #0,d0
bne reflected_axis_L14
jmp reflected_axis_L15
reflected_axis_L14:
; if DIRECTION == 0:
move.b DIRECTION,d0
cmp.b #0,d0
beq reflected_axis_L16
jmp reflected_axis_L17
reflected_axis_L16:
; if POSITION == MINIMUM:
move.b POSITION,d0
move.b MINIMUM,d1
cmp.b d1,d0
beq reflected_axis_L19
jmp reflected_axis_L20
reflected_axis_L19:
; DIRECTION = 1
move.b #1,d0
move.b d0,DIRECTION
; BOUNCES = BOUNCES + 1
move.b BOUNCES,d0
add.b #1,d0
move.b d0,BOUNCES
jmp reflected_axis_L21
reflected_axis_L20:
reflected_axis_L21:
jmp reflected_axis_L18
reflected_axis_L17:
; if POSITION == MAXIMUM:
move.b POSITION,d0
move.b MAXIMUM,d1
cmp.b d1,d0
beq reflected_axis_L22
jmp reflected_axis_L23
reflected_axis_L22:
; DIRECTION = 0
move.b #0,d0
move.b d0,DIRECTION
; BOUNCES = BOUNCES + 1
move.b BOUNCES,d0
add.b #1,d0
move.b d0,BOUNCES
jmp reflected_axis_L24
reflected_axis_L23:
reflected_axis_L24:
reflected_axis_L18:
; if DIRECTION == 0:
move.b DIRECTION,d0
cmp.b #0,d0
beq reflected_axis_L25
jmp reflected_axis_L26
reflected_axis_L25:
; POSITION = POSITION - 1
move.b POSITION,d0
sub.b #1,d0
move.b d0,POSITION
jmp reflected_axis_L27
reflected_axis_L26:
; POSITION = POSITION + 1
move.b POSITION,d0
add.b #1,d0
move.b d0,POSITION
reflected_axis_L27:
; T0 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T0
jmp reflected_axis_L13
reflected_axis_L15:
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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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
MINIMUM: dc.b 0
MAXIMUM: dc.b 0
DISTANCE: dc.b 0
DIRECTION: dc.b 0
BOUNCES: dc.b 0
VALID: dc.b 0
T0: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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
; POSITION = 2
move.b #2,d0
move.b d0,POSITION
; MINIMUM = 0
move.b #0,d0
move.b d0,MINIMUM
; MAXIMUM = 3
move.b #3,d0
move.b d0,MAXIMUM
; DISTANCE = 7
move.b #7,d0
move.b d0,DISTANCE
; DIRECTION = 1
move.b #1,d0
move.b d0,DIRECTION
ifne BROKEN
; DISTANCE = 1
move.b #1,d0
move.b d0,DISTANCE
endif
bsr checked_step
bsr emit_state
; DISTANCE = 14
move.b #14,d0
move.b d0,DISTANCE
ifne BROKEN
; DISTANCE = 1
move.b #1,d0
move.b d0,DISTANCE
endif
bsr checked_step
bsr emit_state
rts
else
; POSITION = 8
move.b #8,d0
move.b d0,POSITION
; MINIMUM = 0
move.b #0,d0
move.b d0,MINIMUM
; MAXIMUM = 10
move.b #10,d0
move.b d0,MAXIMUM
; DISTANCE = 7
move.b #7,d0
move.b d0,DISTANCE
; DIRECTION = 1
move.b #1,d0
move.b d0,DIRECTION
ifne BROKEN
; DISTANCE = 1
move.b #1,d0
move.b d0,DISTANCE
endif
bsr checked_step
bsr emit_state
; DISTANCE = 14
move.b #14,d0
move.b d0,DISTANCE
ifne BROKEN
; DISTANCE = 1
move.b #1,d0
move.b d0,DISTANCE
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Use a narrower interval, exercising repeated reflection. 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
MINIMUM: dc.b 0
MAXIMUM: dc.b 0
DISTANCE: dc.b 0
DIRECTION: dc.b 0
BOUNCES: dc.b 0
VALID: dc.b 0
T0: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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 01 00 03 0E 00 05 01 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller reduces DISTANCE to one. It loses travel remaining after a bounce and changes the motion speed. Pass the full intended travel; use a faster algorithm if the bounded unit-step loop is too expensive.
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
MINIMUM: dc.b 0
MAXIMUM: dc.b 0
DISTANCE: dc.b 0
DIRECTION: dc.b 0
BOUNCES: dc.b 0
VALID: dc.b 0
T0: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",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 350 code bytes; public state is 7 bytes and private scratch is 1 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.
At most 255 unit iterations. This is readable and preserves overshoot, but a large-distance folded-coordinate version is the next optimisation to investigate.
This readable version costs up to 255 loop iterations. A folded-coordinate or quotient/remainder version can handle large travel more cheaply, but needs wider arithmetic and careful endpoint direction rules. It models a point and fixed walls, not swept shape collision or damping.
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
Fractional positions: Sub-unit movement and boundary response solve different jobs. This byte-distance routine needs an explicit unit conversion before being combined with an 8.8 position.