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:
lda #0
sta POSITION
sta MINIMUM
sta MAXIMUM
sta DISTANCE
sta DIRECTION
sta BOUNCES
sta VALID
sta T0
rts
reflected_axis_step:
; BOUNCES = 0
lda #0
sta BOUNCES
; VALID = 0
lda #0
sta VALID
; if MINIMUM >= MAXIMUM:
lda MINIMUM
cmp MAXIMUM
bcs reflected_axis_L4
reflected_axis_L5:
jmp reflected_axis_L2
reflected_axis_L4:
jmp reflected_axis_L1
reflected_axis_L1:
; return
rts
jmp reflected_axis_L3
reflected_axis_L2:
reflected_axis_L3:
; if POSITION < MINIMUM:
lda POSITION
cmp MINIMUM
bcc reflected_axis_L9
reflected_axis_L10:
jmp reflected_axis_L7
reflected_axis_L9:
jmp reflected_axis_L6
reflected_axis_L6:
; return
rts
jmp reflected_axis_L8
reflected_axis_L7:
reflected_axis_L8:
; if POSITION > MAXIMUM:
lda POSITION
cmp MAXIMUM
beq reflected_axis_L15
bcs reflected_axis_L14
reflected_axis_L15:
jmp reflected_axis_L12
reflected_axis_L14:
jmp reflected_axis_L11
reflected_axis_L11:
; return
rts
jmp reflected_axis_L13
reflected_axis_L12:
reflected_axis_L13:
; if DIRECTION >= 2:
lda DIRECTION
cmp #2
bcs reflected_axis_L19
reflected_axis_L20:
jmp reflected_axis_L17
reflected_axis_L19:
jmp reflected_axis_L16
reflected_axis_L16:
; return
rts
jmp reflected_axis_L18
reflected_axis_L17:
reflected_axis_L18:
; VALID = 1
lda #1
sta VALID
; T0 = DISTANCE
lda DISTANCE
sta T0
; while T0 != 0:
reflected_axis_L21:
lda T0
cmp #0
bne reflected_axis_L24
reflected_axis_L25:
jmp reflected_axis_L23
reflected_axis_L24:
jmp reflected_axis_L22
reflected_axis_L22:
; if DIRECTION == 0:
lda DIRECTION
cmp #0
beq reflected_axis_L29
reflected_axis_L30:
jmp reflected_axis_L27
reflected_axis_L29:
jmp reflected_axis_L26
reflected_axis_L26:
; if POSITION == MINIMUM:
lda POSITION
cmp MINIMUM
beq reflected_axis_L34
reflected_axis_L35:
jmp reflected_axis_L32
reflected_axis_L34:
jmp reflected_axis_L31
reflected_axis_L31:
; DIRECTION = 1
lda #1
sta DIRECTION
; BOUNCES = BOUNCES + 1
lda BOUNCES
clc
adc #1
sta BOUNCES
jmp reflected_axis_L33
reflected_axis_L32:
reflected_axis_L33:
jmp reflected_axis_L28
reflected_axis_L27:
; if POSITION == MAXIMUM:
lda POSITION
cmp MAXIMUM
beq reflected_axis_L39
reflected_axis_L40:
jmp reflected_axis_L37
reflected_axis_L39:
jmp reflected_axis_L36
reflected_axis_L36:
; DIRECTION = 0
lda #0
sta DIRECTION
; BOUNCES = BOUNCES + 1
lda BOUNCES
clc
adc #1
sta BOUNCES
jmp reflected_axis_L38
reflected_axis_L37:
reflected_axis_L38:
reflected_axis_L28:
; if DIRECTION == 0:
lda DIRECTION
cmp #0
beq reflected_axis_L44
reflected_axis_L45:
jmp reflected_axis_L42
reflected_axis_L44:
jmp reflected_axis_L41
reflected_axis_L41:
; POSITION = POSITION - 1
lda POSITION
sec
sbc #1
sta POSITION
jmp reflected_axis_L43
reflected_axis_L42:
; POSITION = POSITION + 1
lda POSITION
clc
adc #1
sta POSITION
reflected_axis_L43:
; T0 = T0 - 1
lda T0
sec
sbc #1
sta T0
jmp reflected_axis_L21
reflected_axis_L23:
rts
component_end:Machine contract
A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. A direct JSR needs two 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.
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. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
MINIMUM=$81
MAXIMUM=$82
DISTANCE=$83
DIRECTION=$84
BOUNCES=$85
VALID=$86
T0=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0A client retaining state between calls
preview_rules:
.if CUSTOM
; POSITION = 2
lda #2
sta POSITION
; MINIMUM = 0
lda #0
sta MINIMUM
; MAXIMUM = 3
lda #3
sta MAXIMUM
; DISTANCE = 7
lda #7
sta DISTANCE
; DIRECTION = 1
lda #1
sta DIRECTION
.if BROKEN
; DISTANCE = 1
lda #1
sta DISTANCE
.endif
jsr checked_step
jsr emit_state
; DISTANCE = 14
lda #14
sta DISTANCE
.if BROKEN
; DISTANCE = 1
lda #1
sta DISTANCE
.endif
jsr checked_step
jsr emit_state
rts
.else
; POSITION = 8
lda #8
sta POSITION
; MINIMUM = 0
lda #0
sta MINIMUM
; MAXIMUM = 10
lda #10
sta MAXIMUM
; DISTANCE = 7
lda #7
sta DISTANCE
; DIRECTION = 1
lda #1
sta DIRECTION
.if BROKEN
; DISTANCE = 1
lda #1
sta DISTANCE
.endif
jsr checked_step
jsr emit_state
; DISTANCE = 14
lda #14
sta DISTANCE
.if BROKEN
; DISTANCE = 1
lda #1
sta DISTANCE
.endif
jsr checked_step
jsr 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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
MINIMUM=$81
MAXIMUM=$82
DISTANCE=$83
DIRECTION=$84
BOUNCES=$85
VALID=$86
T0=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0The 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
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
MINIMUM=$81
MAXIMUM=$82
DISTANCE=$83
DIRECTION=$84
BOUNCES=$85
VALID=$86
T0=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
preview=OUTPUT+1400
ready=OUTPUT+1408
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"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0
Cost and alternatives
The measured helper extent is 220 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 NTSC NROM, background only, DMC/NMI disabled, using Asm198x 0.0.72, Emu198x 0.44.2. 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.