A sentry looking down a corridor must stop at the first wall.
Use it in a project
On a 4×4 row-major grid, FROM and TO must be 0–15 and share a row or column. Zero grid bytes are transparent; nonzero bytes block. Exclude the observer cell and include the destination. A cell sees itself. Non-cardinal or out-of-range requests return VALID=0 and VISIBLE=0.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: FROM, TO, VISIBLE, VALID, C0, C1, C2, C3, C4, C5, C6, C7, C8, C9, C10, C11, C12, C13, C14, C15. They occupy 20 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call cardinal_sight_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 cardinal_sight_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 inside the helper |
| Supply before service | FROM, TO and C0–C15 |
| Read after service | VISIBLE and VALID |
A shared row has stride one; a shared column has stride four. Walk from the cell after the observer to the destination, stopping at a blocker. Validating alignment prevents a row walk from wrapping into the next row.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Test Sight Along a Grid Row or Column. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
cardinal_sight_init:
move.b #0,d0
move.b d0,FROM
move.b d0,TO
move.b d0,VISIBLE
move.b d0,VALID
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
rts
cardinal_sight_step:
; VISIBLE = 0
move.b #0,d0
move.b d0,VISIBLE
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if FROM >= 16:
move.b FROM,d0
cmp.b #16,d0
bcc cardinal_sight_L1
jmp cardinal_sight_L2
cardinal_sight_L1:
; return
rts
jmp cardinal_sight_L3
cardinal_sight_L2:
cardinal_sight_L3:
; if TO >= 16:
move.b TO,d0
cmp.b #16,d0
bcc cardinal_sight_L4
jmp cardinal_sight_L5
cardinal_sight_L4:
; return
rts
jmp cardinal_sight_L6
cardinal_sight_L5:
cardinal_sight_L6:
; if FROM == TO:
move.b FROM,d0
move.b TO,d1
cmp.b d1,d0
beq cardinal_sight_L7
jmp cardinal_sight_L8
cardinal_sight_L7:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; VISIBLE = 1
move.b #1,d0
move.b d0,VISIBLE
; return
rts
jmp cardinal_sight_L9
cardinal_sight_L8:
cardinal_sight_L9:
; T0 = FROM >> 2
move.b FROM,d0
lsr.b #2,d0
move.b d0,T0
; T1 = TO >> 2
move.b TO,d0
lsr.b #2,d0
move.b d0,T1
; T2 = 1
move.b #1,d0
move.b d0,T2
; if T0 != T1:
move.b T0,d0
move.b T1,d1
cmp.b d1,d0
bne cardinal_sight_L10
jmp cardinal_sight_L11
cardinal_sight_L10:
; T0 = FROM & 3
move.b FROM,d0
and.b #3,d0
move.b d0,T0
; T1 = TO & 3
move.b TO,d0
and.b #3,d0
move.b d0,T1
; if T0 != T1:
move.b T0,d0
move.b T1,d1
cmp.b d1,d0
bne cardinal_sight_L13
jmp cardinal_sight_L14
cardinal_sight_L13:
; return
rts
jmp cardinal_sight_L15
cardinal_sight_L14:
cardinal_sight_L15:
; T2 = 4
move.b #4,d0
move.b d0,T2
jmp cardinal_sight_L12
cardinal_sight_L11:
cardinal_sight_L12:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = FROM
move.b FROM,d0
move.b d0,T0
; while T0 != TO:
cardinal_sight_L16:
move.b T0,d0
move.b TO,d1
cmp.b d1,d0
bne cardinal_sight_L17
jmp cardinal_sight_L18
cardinal_sight_L17:
; if T0 < TO:
move.b T0,d0
move.b TO,d1
cmp.b d1,d0
bcs cardinal_sight_L19
jmp cardinal_sight_L20
cardinal_sight_L19:
; T0 = T0 + T2
move.b T0,d0
move.b T2,d1
add.b d1,d0
move.b d0,T0
jmp cardinal_sight_L21
cardinal_sight_L20:
; T0 = T0 - T2
move.b T0,d0
move.b T2,d1
sub.b d1,d0
move.b d0,T0
cardinal_sight_L21:
; T1 = C[T0]
moveq #0,d1
move.b T0,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T1
; if T1 != 0:
move.b T1,d0
cmp.b #0,d0
bne cardinal_sight_L22
jmp cardinal_sight_L23
cardinal_sight_L22:
; return
rts
jmp cardinal_sight_L24
cardinal_sight_L23:
cardinal_sight_L24:
jmp cardinal_sight_L16
cardinal_sight_L18:
; VISIBLE = 1
move.b #1,d0
move.b d0,VISIBLE
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:
FROM: dc.b 0
TO: dc.b 0
VISIBLE: dc.b 0
VALID: 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
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
; FROM = 0
move.b #0,d0
move.b d0,FROM
; TO = 12
move.b #12,d0
move.b d0,TO
; C2 = 1
move.b #1,d0
move.b d0,C2
; C8 = 1
move.b #1,d0
move.b d0,C8
ifne BROKEN
; C2 = 0
move.b #0,d0
move.b d0,C2
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
; C2 = 1
move.b #1,d0
move.b d0,C2
ifne BROKEN
; C2 = 0
move.b #0,d0
move.b d0,C2
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Look down the first column, with a blocker at cell eight. 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:
FROM: dc.b 0
TO: dc.b 0
VISIBLE: dc.b 0
VALID: 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
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 00 0C 00 01 00 00 01 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears C2 before querying sight. A real blocker disappears and the observer sees through it. Keep the query read-only with respect to the collision grid.
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:
FROM: dc.b 0
TO: dc.b 0
VISIBLE: dc.b 0
VALID: 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
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 502 code bytes; public state is 20 bytes and private scratch is 3 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 three destination-inclusive probes on a valid 4×4 cardinal query. General line traversal needs more arithmetic and an explicit corner policy.
This is explicitly cardinal sight, not a raycaster or arbitrary-angle field of view. Use a grid line traversal for diagonal rays and choose corner visibility deliberately. A renderer does not establish whether game logic should see through a tile.
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 50 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
Tile properties: Use a visibility-blocking property when visual tile IDs alone do not tell you whether a corridor is transparent.