A trailing companion needs where the leader was, not another guess at where it might be going.
Use it in a project
An eight-entry ring stores byte X/Y pairs. HEAD is the next write index 0–7; COUNT is 0–8. PUSH nonzero appends the supplied position, advances HEAD modulo eight and saturates COUNT at eight. Then query LAG entries back from newest: zero means newest. A lag outside COUNT returns VALID=0 and zero outputs but does not undo a valid append. Invalid HEAD/COUNT rejects before writing.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: OUT_X, OUT_Y, VALID, HEAD, COUNT, INPUT_X, INPUT_Y, LAG, PUSH, HX0, HX1, HX2, HX3, HX4, HX5, HX6, HX7, HY0, HY1, HY2, HY3, HY4, HY5, HY6, HY7. They occupy 25 contiguous bytes. Also reserve the private workspace: T0, EXPR_SAVE (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call motion_history_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 motion_history_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 | HEAD, COUNT and HX0–HX7/HY0–HY7 |
| Supply before service | INPUT_X, INPUT_Y, LAG and PUSH |
| Read after service | OUT_X, OUT_Y and VALID |
The write cursor wraps cheaply because capacity is a power of two. A lag query subtracts from the next-write cursor rather than treating physical array order as chronological order.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Keep a Small Position History. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
motion_history_init:
move.b #0,d0
move.b d0,OUT_X
move.b d0,OUT_Y
move.b d0,VALID
move.b d0,HEAD
move.b d0,COUNT
move.b d0,INPUT_X
move.b d0,INPUT_Y
move.b d0,LAG
move.b d0,PUSH
move.b d0,HX0
move.b d0,HX1
move.b d0,HX2
move.b d0,HX3
move.b d0,HX4
move.b d0,HX5
move.b d0,HX6
move.b d0,HX7
move.b d0,HY0
move.b d0,HY1
move.b d0,HY2
move.b d0,HY3
move.b d0,HY4
move.b d0,HY5
move.b d0,HY6
move.b d0,HY7
move.b d0,T0
move.b d0,EXPR_SAVE
rts
motion_history_step:
; OUT_X = 0
move.b #0,d0
move.b d0,OUT_X
; OUT_Y = 0
move.b #0,d0
move.b d0,OUT_Y
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if HEAD >= 8:
move.b HEAD,d0
cmp.b #8,d0
bcc motion_history_L1
jmp motion_history_L2
motion_history_L1:
; return
rts
jmp motion_history_L3
motion_history_L2:
motion_history_L3:
; if COUNT > 8:
move.b COUNT,d0
cmp.b #8,d0
bhi motion_history_L4
jmp motion_history_L5
motion_history_L4:
; return
rts
jmp motion_history_L6
motion_history_L5:
motion_history_L6:
; if PUSH != 0:
move.b PUSH,d0
cmp.b #0,d0
bne motion_history_L7
jmp motion_history_L8
motion_history_L7:
; HX[HEAD] = INPUT_X
move.b INPUT_X,d0
move.b d0,d2
moveq #0,d1
move.b HEAD,d1
lea HX0,a0
move.b d2,0(a0,d1.w)
; HY[HEAD] = INPUT_Y
move.b INPUT_Y,d0
move.b d0,d2
moveq #0,d1
move.b HEAD,d1
lea HY0,a0
move.b d2,0(a0,d1.w)
; HEAD = HEAD + 1
move.b HEAD,d0
add.b #1,d0
move.b d0,HEAD
; HEAD = HEAD & 7
move.b HEAD,d0
and.b #7,d0
move.b d0,HEAD
; if COUNT < 8:
move.b COUNT,d0
cmp.b #8,d0
bcs motion_history_L10
jmp motion_history_L11
motion_history_L10:
; COUNT = COUNT + 1
move.b COUNT,d0
add.b #1,d0
move.b d0,COUNT
jmp motion_history_L12
motion_history_L11:
motion_history_L12:
jmp motion_history_L9
motion_history_L8:
motion_history_L9:
; if LAG >= COUNT:
move.b LAG,d0
move.b COUNT,d1
cmp.b d1,d0
bcc motion_history_L13
jmp motion_history_L14
motion_history_L13:
; return
rts
jmp motion_history_L15
motion_history_L14:
motion_history_L15:
; T0 = HEAD - 1
move.b HEAD,d0
sub.b #1,d0
move.b d0,T0
; T0 = T0 - LAG
move.b T0,d0
move.b LAG,d1
sub.b d1,d0
move.b d0,T0
; T0 = T0 & 7
move.b T0,d0
and.b #7,d0
move.b d0,T0
; OUT_X = HX[T0]
moveq #0,d1
move.b T0,d1
lea HX0,a0
move.b 0(a0,d1.w),d0
move.b d0,OUT_X
; OUT_Y = HY[T0]
moveq #0,d1
move.b T0,d1
lea HY0,a0
move.b 0(a0,d1.w),d0
move.b d0,OUT_Y
; VALID = 1
move.b #1,d0
move.b d0,VALID
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:
OUT_X: dc.b 0
OUT_Y: dc.b 0
VALID: dc.b 0
HEAD: dc.b 0
COUNT: dc.b 0
INPUT_X: dc.b 0
INPUT_Y: dc.b 0
LAG: dc.b 0
PUSH: dc.b 0
HX0: dc.b 0
HX1: dc.b 0
HX2: dc.b 0
HX3: dc.b 0
HX4: dc.b 0
HX5: dc.b 0
HX6: dc.b 0
HX7: dc.b 0
HY0: dc.b 0
HY1: dc.b 0
HY2: dc.b 0
HY3: dc.b 0
HY4: dc.b 0
HY5: dc.b 0
HY6: dc.b 0
HY7: dc.b 0
T0: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",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
; INPUT_X = 0
move.b #0,d0
move.b d0,INPUT_X
; INPUT_Y = 1
move.b #1,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 1
move.b #1,d0
move.b d0,INPUT_X
; INPUT_Y = 2
move.b #2,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 2
move.b #2,d0
move.b d0,INPUT_X
; INPUT_Y = 3
move.b #3,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 3
move.b #3,d0
move.b d0,INPUT_X
; INPUT_Y = 4
move.b #4,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 4
move.b #4,d0
move.b d0,INPUT_X
; INPUT_Y = 5
move.b #5,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 5
move.b #5,d0
move.b d0,INPUT_X
; INPUT_Y = 6
move.b #6,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 6
move.b #6,d0
move.b d0,INPUT_X
; INPUT_Y = 7
move.b #7,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 7
move.b #7,d0
move.b d0,INPUT_X
; INPUT_Y = 8
move.b #8,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 8
move.b #8,d0
move.b d0,INPUT_X
; INPUT_Y = 9
move.b #9,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; PUSH = 0
move.b #0,d0
move.b d0,PUSH
; LAG = 7
move.b #7,d0
move.b d0,LAG
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
rts
else
; INPUT_X = 10
move.b #10,d0
move.b d0,INPUT_X
; INPUT_Y = 20
move.b #20,d0
move.b d0,INPUT_Y
; PUSH = 1
move.b #1,d0
move.b d0,PUSH
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 30
move.b #30,d0
move.b d0,INPUT_X
; INPUT_Y = 40
move.b #40,d0
move.b d0,INPUT_Y
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; INPUT_X = 50
move.b #50,d0
move.b d0,INPUT_X
; INPUT_Y = 60
move.b #60,d0
move.b d0,INPUT_Y
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
; PUSH = 0
move.b #0,d0
move.b d0,PUSH
; LAG = 2
move.b #2,d0
move.b d0,LAG
ifne BROKEN
; HEAD = 0
move.b #0,d0
move.b d0,HEAD
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Append nine positions, then query the oldest retained entry after wrap. 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:
OUT_X: dc.b 0
OUT_Y: dc.b 0
VALID: dc.b 0
HEAD: dc.b 0
COUNT: dc.b 0
INPUT_X: dc.b 0
INPUT_Y: dc.b 0
LAG: dc.b 0
PUSH: dc.b 0
HX0: dc.b 0
HX1: dc.b 0
HX2: dc.b 0
HX3: dc.b 0
HX4: dc.b 0
HX5: dc.b 0
HX6: dc.b 0
HX7: dc.b 0
HY0: dc.b 0
HY1: dc.b 0
HY2: dc.b 0
HY3: dc.b 0
HY4: dc.b 0
HY5: dc.b 0
HY6: dc.b 0
HY7: dc.b 0
T0: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",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 02 01 01 08 08 09 07. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller resets HEAD on every service. New positions overwrite one slot and older queries read unrelated entries. Keep the ring cursor alongside the stored history.
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:
OUT_X: dc.b 0
OUT_Y: dc.b 0
VALID: dc.b 0
HEAD: dc.b 0
COUNT: dc.b 0
INPUT_X: dc.b 0
INPUT_Y: dc.b 0
LAG: dc.b 0
PUSH: dc.b 0
HX0: dc.b 0
HX1: dc.b 0
HX2: dc.b 0
HX3: dc.b 0
HX4: dc.b 0
HX5: dc.b 0
HX6: dc.b 0
HX7: dc.b 0
HY0: dc.b 0
HY1: dc.b 0
HY2: dc.b 0
HY3: dc.b 0
HY4: dc.b 0
HY5: dc.b 0
HY6: dc.b 0
HY7: dc.b 0
T0: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XWORLDKIT000",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 466 code bytes; public state is 25 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 without shifting old positions. Eight pairs cost sixteen data bytes. A timestamped history costs more storage but can answer elapsed-time rather than sample-age queries.
These are serviced samples, not time-stamped seconds. Variable update spacing changes the time represented by a lag. Interpolation, full-state rewind and collision-safe followers require additional state and rules.
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 40 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
Event queues: Both use bounded storage, but a history retains recent samples for age queries rather than consuming queued events.