An enemy can walk around a wall without pretending that moving towards the player is already pathfinding.
Use it in a project
A 4×4 row-major grid uses zero for traversable and nonzero for blocked cells. FROM and TO must be 0–15. Return FOUND=1, shortest four-connected DISTANCE and the first MOVE (1 left, 2 right, 3 up, 4 down). Already at a clear destination returns distance zero and move zero. Blocked or unreachable endpoints return FOUND=0, DISTANCE=255 and MOVE=0 with VALID=1. Bad indices return VALID=0.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: MOVE, DISTANCE, FOUND, VALID, FROM, TO, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15. They occupy 22 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5, T6, T7, EXPR_SAVE, V0, V1, V2, V3, V4, V5, V6, V7, V8, V9, V10, V11, V12, V13, V14, V15 (25 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call bounded_grid_path_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 bounded_grid_path_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 | FROM, TO and C0–C15 |
| Read after service | MOVE, DISTANCE, FOUND and VALID |
Build distance layers outwards from the destination. A cell can join only the next layer, so scanning order cannot turn a long route into a falsely short one. A neighbour one layer closer supplies a valid first move.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Find the First Step Along a Shortest Grid Path. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
bounded_grid_path_init:
move.b #0,d0
move.b d0,MOVE
move.b d0,DISTANCE
move.b d0,FOUND
move.b d0,VALID
move.b d0,FROM
move.b d0,TO
move.b d0,C0
move.b d0,C1
move.b d0,C2
move.b d0,C3
move.b d0,C4
move.b d0,C5
move.b d0,C6
move.b d0,C7
move.b d0,C8
move.b d0,C9
move.b d0,C10
move.b d0,C11
move.b d0,C12
move.b d0,C13
move.b d0,C14
move.b d0,C15
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
move.b d0,T4
move.b d0,T5
move.b d0,T6
move.b d0,T7
move.b d0,EXPR_SAVE
move.b d0,V0
move.b d0,V1
move.b d0,V2
move.b d0,V3
move.b d0,V4
move.b d0,V5
move.b d0,V6
move.b d0,V7
move.b d0,V8
move.b d0,V9
move.b d0,V10
move.b d0,V11
move.b d0,V12
move.b d0,V13
move.b d0,V14
move.b d0,V15
rts
bounded_grid_path_step:
; MOVE = 0
move.b #0,d0
move.b d0,MOVE
; DISTANCE = 255
move.b #255,d0
move.b d0,DISTANCE
; FOUND = 0
move.b #0,d0
move.b d0,FOUND
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if FROM >= 16:
move.b FROM,d0
cmp.b #16,d0
bcc bounded_grid_path_L1
jmp bounded_grid_path_L2
bounded_grid_path_L1:
; return
rts
jmp bounded_grid_path_L3
bounded_grid_path_L2:
bounded_grid_path_L3:
; if TO >= 16:
move.b TO,d0
cmp.b #16,d0
bcc bounded_grid_path_L4
jmp bounded_grid_path_L5
bounded_grid_path_L4:
; return
rts
jmp bounded_grid_path_L6
bounded_grid_path_L5:
bounded_grid_path_L6:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = C[FROM]
moveq #0,d1
move.b FROM,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T0
; if T0 != 0:
move.b T0,d0
cmp.b #0,d0
bne bounded_grid_path_L7
jmp bounded_grid_path_L8
bounded_grid_path_L7:
; return
rts
jmp bounded_grid_path_L9
bounded_grid_path_L8:
bounded_grid_path_L9:
; T0 = C[TO]
moveq #0,d1
move.b TO,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T0
; if T0 != 0:
move.b T0,d0
cmp.b #0,d0
bne bounded_grid_path_L10
jmp bounded_grid_path_L11
bounded_grid_path_L10:
; return
rts
jmp bounded_grid_path_L12
bounded_grid_path_L11:
bounded_grid_path_L12:
; T0 = 0
move.b #0,d0
move.b d0,T0
; while T0 < 16:
bounded_grid_path_L13:
move.b T0,d0
cmp.b #16,d0
bcs bounded_grid_path_L14
jmp bounded_grid_path_L15
bounded_grid_path_L14:
; V[T0] = 255
move.b #255,d0
move.b d0,d2
moveq #0,d1
move.b T0,d1
lea V0,a0
move.b d2,0(a0,d1.w)
; T0 = T0 + 1
move.b T0,d0
add.b #1,d0
move.b d0,T0
jmp bounded_grid_path_L13
bounded_grid_path_L15:
; V[TO] = 0
move.b #0,d0
move.b d0,d2
moveq #0,d1
move.b TO,d1
lea V0,a0
move.b d2,0(a0,d1.w)
; T0 = 1
move.b #1,d0
move.b d0,T0
; while T0 < 16:
bounded_grid_path_L16:
move.b T0,d0
cmp.b #16,d0
bcs bounded_grid_path_L17
jmp bounded_grid_path_L18
bounded_grid_path_L17:
; T1 = 0
move.b #0,d0
move.b d0,T1
; while T1 < 16:
bounded_grid_path_L19:
move.b T1,d0
cmp.b #16,d0
bcs bounded_grid_path_L20
jmp bounded_grid_path_L21
bounded_grid_path_L20:
; T2 = C[T1]
moveq #0,d1
move.b T1,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T2
; T3 = V[T1]
moveq #0,d1
move.b T1,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T2 == 0:
move.b T2,d0
cmp.b #0,d0
beq bounded_grid_path_L22
jmp bounded_grid_path_L23
bounded_grid_path_L22:
; if T3 == 255:
move.b T3,d0
cmp.b #255,d0
beq bounded_grid_path_L25
jmp bounded_grid_path_L26
bounded_grid_path_L25:
; T4 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T4
; T5 = T1 & 3
move.b T1,d0
and.b #3,d0
move.b d0,T5
; if T5 != 0:
move.b T5,d0
cmp.b #0,d0
bne bounded_grid_path_L28
jmp bounded_grid_path_L29
bounded_grid_path_L28:
; T6 = T1 - 1
move.b T1,d0
sub.b #1,d0
move.b d0,T6
; T7 = V[T6]
moveq #0,d1
move.b T6,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T7
; if T7 == T4:
move.b T7,d0
move.b T4,d1
cmp.b d1,d0
beq bounded_grid_path_L31
jmp bounded_grid_path_L32
bounded_grid_path_L31:
; V[T1] = T0
move.b T0,d0
move.b d0,d2
moveq #0,d1
move.b T1,d1
lea V0,a0
move.b d2,0(a0,d1.w)
jmp bounded_grid_path_L33
bounded_grid_path_L32:
bounded_grid_path_L33:
jmp bounded_grid_path_L30
bounded_grid_path_L29:
bounded_grid_path_L30:
; if T5 != 3:
move.b T5,d0
cmp.b #3,d0
bne bounded_grid_path_L34
jmp bounded_grid_path_L35
bounded_grid_path_L34:
; T6 = T1 + 1
move.b T1,d0
add.b #1,d0
move.b d0,T6
; T7 = V[T6]
moveq #0,d1
move.b T6,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T7
; if T7 == T4:
move.b T7,d0
move.b T4,d1
cmp.b d1,d0
beq bounded_grid_path_L37
jmp bounded_grid_path_L38
bounded_grid_path_L37:
; V[T1] = T0
move.b T0,d0
move.b d0,d2
moveq #0,d1
move.b T1,d1
lea V0,a0
move.b d2,0(a0,d1.w)
jmp bounded_grid_path_L39
bounded_grid_path_L38:
bounded_grid_path_L39:
jmp bounded_grid_path_L36
bounded_grid_path_L35:
bounded_grid_path_L36:
; if T1 >= 4:
move.b T1,d0
cmp.b #4,d0
bcc bounded_grid_path_L40
jmp bounded_grid_path_L41
bounded_grid_path_L40:
; T6 = T1 - 4
move.b T1,d0
sub.b #4,d0
move.b d0,T6
; T7 = V[T6]
moveq #0,d1
move.b T6,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T7
; if T7 == T4:
move.b T7,d0
move.b T4,d1
cmp.b d1,d0
beq bounded_grid_path_L43
jmp bounded_grid_path_L44
bounded_grid_path_L43:
; V[T1] = T0
move.b T0,d0
move.b d0,d2
moveq #0,d1
move.b T1,d1
lea V0,a0
move.b d2,0(a0,d1.w)
jmp bounded_grid_path_L45
bounded_grid_path_L44:
bounded_grid_path_L45:
jmp bounded_grid_path_L42
bounded_grid_path_L41:
bounded_grid_path_L42:
; if T1 < 12:
move.b T1,d0
cmp.b #12,d0
bcs bounded_grid_path_L46
jmp bounded_grid_path_L47
bounded_grid_path_L46:
; T6 = T1 + 4
move.b T1,d0
add.b #4,d0
move.b d0,T6
; T7 = V[T6]
moveq #0,d1
move.b T6,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T7
; if T7 == T4:
move.b T7,d0
move.b T4,d1
cmp.b d1,d0
beq bounded_grid_path_L49
jmp bounded_grid_path_L50
bounded_grid_path_L49:
; V[T1] = T0
move.b T0,d0
move.b d0,d2
moveq #0,d1
move.b T1,d1
lea V0,a0
move.b d2,0(a0,d1.w)
jmp bounded_grid_path_L51
bounded_grid_path_L50:
bounded_grid_path_L51:
jmp bounded_grid_path_L48
bounded_grid_path_L47:
bounded_grid_path_L48:
jmp bounded_grid_path_L27
bounded_grid_path_L26:
bounded_grid_path_L27:
jmp bounded_grid_path_L24
bounded_grid_path_L23:
bounded_grid_path_L24:
; T1 = T1 + 1
move.b T1,d0
add.b #1,d0
move.b d0,T1
jmp bounded_grid_path_L19
bounded_grid_path_L21:
; T0 = T0 + 1
move.b T0,d0
add.b #1,d0
move.b d0,T0
jmp bounded_grid_path_L16
bounded_grid_path_L18:
; DISTANCE = V[FROM]
moveq #0,d1
move.b FROM,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,DISTANCE
; if DISTANCE == 255:
move.b DISTANCE,d0
cmp.b #255,d0
beq bounded_grid_path_L52
jmp bounded_grid_path_L53
bounded_grid_path_L52:
; return
rts
jmp bounded_grid_path_L54
bounded_grid_path_L53:
bounded_grid_path_L54:
; FOUND = 1
move.b #1,d0
move.b d0,FOUND
; if DISTANCE == 0:
move.b DISTANCE,d0
cmp.b #0,d0
beq bounded_grid_path_L55
jmp bounded_grid_path_L56
bounded_grid_path_L55:
; return
rts
jmp bounded_grid_path_L57
bounded_grid_path_L56:
bounded_grid_path_L57:
; T0 = DISTANCE - 1
move.b DISTANCE,d0
sub.b #1,d0
move.b d0,T0
; T1 = FROM & 3
move.b FROM,d0
and.b #3,d0
move.b d0,T1
; if T1 != 0:
move.b T1,d0
cmp.b #0,d0
bne bounded_grid_path_L58
jmp bounded_grid_path_L59
bounded_grid_path_L58:
; T2 = FROM - 1
move.b FROM,d0
sub.b #1,d0
move.b d0,T2
; T3 = V[T2]
moveq #0,d1
move.b T2,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 == T0:
move.b T3,d0
move.b T0,d1
cmp.b d1,d0
beq bounded_grid_path_L61
jmp bounded_grid_path_L62
bounded_grid_path_L61:
; MOVE = 1
move.b #1,d0
move.b d0,MOVE
; return
rts
jmp bounded_grid_path_L63
bounded_grid_path_L62:
bounded_grid_path_L63:
jmp bounded_grid_path_L60
bounded_grid_path_L59:
bounded_grid_path_L60:
; if T1 != 3:
move.b T1,d0
cmp.b #3,d0
bne bounded_grid_path_L64
jmp bounded_grid_path_L65
bounded_grid_path_L64:
; T2 = FROM + 1
move.b FROM,d0
add.b #1,d0
move.b d0,T2
; T3 = V[T2]
moveq #0,d1
move.b T2,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 == T0:
move.b T3,d0
move.b T0,d1
cmp.b d1,d0
beq bounded_grid_path_L67
jmp bounded_grid_path_L68
bounded_grid_path_L67:
; MOVE = 2
move.b #2,d0
move.b d0,MOVE
; return
rts
jmp bounded_grid_path_L69
bounded_grid_path_L68:
bounded_grid_path_L69:
jmp bounded_grid_path_L66
bounded_grid_path_L65:
bounded_grid_path_L66:
; if FROM >= 4:
move.b FROM,d0
cmp.b #4,d0
bcc bounded_grid_path_L70
jmp bounded_grid_path_L71
bounded_grid_path_L70:
; T2 = FROM - 4
move.b FROM,d0
sub.b #4,d0
move.b d0,T2
; T3 = V[T2]
moveq #0,d1
move.b T2,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 == T0:
move.b T3,d0
move.b T0,d1
cmp.b d1,d0
beq bounded_grid_path_L73
jmp bounded_grid_path_L74
bounded_grid_path_L73:
; MOVE = 3
move.b #3,d0
move.b d0,MOVE
; return
rts
jmp bounded_grid_path_L75
bounded_grid_path_L74:
bounded_grid_path_L75:
jmp bounded_grid_path_L72
bounded_grid_path_L71:
bounded_grid_path_L72:
; if FROM < 12:
move.b FROM,d0
cmp.b #12,d0
bcs bounded_grid_path_L76
jmp bounded_grid_path_L77
bounded_grid_path_L76:
; T2 = FROM + 4
move.b FROM,d0
add.b #4,d0
move.b d0,T2
; T3 = V[T2]
moveq #0,d1
move.b T2,d1
lea V0,a0
move.b 0(a0,d1.w),d0
move.b d0,T3
; if T3 == T0:
move.b T3,d0
move.b T0,d1
cmp.b d1,d0
beq bounded_grid_path_L79
jmp bounded_grid_path_L80
bounded_grid_path_L79:
; MOVE = 4
move.b #4,d0
move.b d0,MOVE
jmp bounded_grid_path_L81
bounded_grid_path_L80:
bounded_grid_path_L81:
jmp bounded_grid_path_L78
bounded_grid_path_L77:
bounded_grid_path_L78:
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:
MOVE: dc.b 0
DISTANCE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
FROM: dc.b 0
TO: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
EXPR_SAVE: dc.b 0
V0: dc.b 0
V1: dc.b 0
V2: dc.b 0
V3: dc.b 0
V4: dc.b 0
V5: dc.b 0
V6: dc.b 0
V7: dc.b 0
V8: dc.b 0
V9: dc.b 0
V10: dc.b 0
V11: dc.b 0
V12: dc.b 0
V13: dc.b 0
V14: dc.b 0
V15: 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
; FROM = 0
move.b #0,d0
move.b d0,FROM
; TO = 15
move.b #15,d0
move.b d0,TO
ifne BROKEN
; C1 = 0
move.b #0,d0
move.b d0,C1
endif
bsr checked_step
bsr emit_state
rts
else
; FROM = 0
move.b #0,d0
move.b d0,FROM
; TO = 3
move.b #3,d0
move.b d0,TO
; C1 = 1
move.b #1,d0
move.b d0,C1
ifne BROKEN
; C1 = 0
move.b #0,d0
move.b d0,C1
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Search across a clear board to the opposite corner. 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:
MOVE: dc.b 0
DISTANCE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
FROM: dc.b 0
TO: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
EXPR_SAVE: dc.b 0
V0: dc.b 0
V1: dc.b 0
V2: dc.b 0
V3: dc.b 0
V4: dc.b 0
V5: dc.b 0
V6: dc.b 0
V7: dc.b 0
V8: dc.b 0
V9: dc.b 0
V10: dc.b 0
V11: dc.b 0
V12: dc.b 0
V13: dc.b 0
V14: dc.b 0
V15: 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 02 06 01 01 00 0F 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller removes the obstacle before searching. The returned first step goes through the cell that should be blocked. A path query should read the collision map, not clear cells to make a route possible.
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:
MOVE: dc.b 0
DISTANCE: dc.b 0
FOUND: dc.b 0
VALID: dc.b 0
FROM: dc.b 0
TO: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
C4: dc.b 0
C5: dc.b 0
C6: dc.b 0
C7: dc.b 0
C8: dc.b 0
C9: dc.b 0
C10: dc.b 0
C11: dc.b 0
C12: dc.b 0
C13: dc.b 0
C14: dc.b 0
C15: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
EXPR_SAVE: dc.b 0
V0: dc.b 0
V1: dc.b 0
V2: dc.b 0
V3: dc.b 0
V4: dc.b 0
V5: dc.b 0
V6: dc.b 0
V7: dc.b 0
V8: dc.b 0
V9: dc.b 0
V10: dc.b 0
V11: dc.b 0
V12: dc.b 0
V13: dc.b 0
V14: dc.b 0
V15: 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 1492 code bytes; public state is 22 bytes and private scratch is 25 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.
Fifteen layers scan sixteen cells each, using sixteen private distance bytes. A queue-based breadth-first search avoids many repeated probes on larger boards but needs a bounded queue. Incremental search can spread work across frames.
This unit-cost four-neighbour search chooses the first available downhill neighbour in left/right/up/down order. It returns a first step, not a full persistent path. Diagonals, terrain costs and moving obstacles require different rules or a new query.
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 45 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
Grid sight: Seeing the target and finding a walkable route are different questions; a clear ray does not replace a path search.