Test Sight Along a Grid Row or Column on NES | Pattern Library

A sentry looking down a corridor must stop at the first wall.

NES shows the working diagnostic row 00 03 00 01 00 00 01 00. — A completed native caller displays 00 03 00 01 00 00 01 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

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

6502.inc

; 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:
    lda #0
    sta FROM
    sta TO
    sta VISIBLE
    sta VALID
    sta C0
    sta C1
    sta C2
    sta C3
    sta C4
    sta C5
    sta C6
    sta C7
    sta C8
    sta C9
    sta C10
    sta C11
    sta C12
    sta C13
    sta C14
    sta C15
    sta T0
    sta T1
    sta T2
    rts
cardinal_sight_step:
; VISIBLE = 0
    lda #0
    sta VISIBLE
; VALID = 0
    lda #0
    sta VALID
; if FROM >= 16:
    lda FROM
    cmp #16
    bcs cardinal_sight_L4
cardinal_sight_L5:
    jmp cardinal_sight_L2
cardinal_sight_L4:
    jmp cardinal_sight_L1
cardinal_sight_L1:
; return
    rts
    jmp cardinal_sight_L3
cardinal_sight_L2:
cardinal_sight_L3:
; if TO >= 16:
    lda TO
    cmp #16
    bcs cardinal_sight_L9
cardinal_sight_L10:
    jmp cardinal_sight_L7
cardinal_sight_L9:
    jmp cardinal_sight_L6
cardinal_sight_L6:
; return
    rts
    jmp cardinal_sight_L8
cardinal_sight_L7:
cardinal_sight_L8:
; if FROM == TO:
    lda FROM
    cmp TO
    beq cardinal_sight_L14
cardinal_sight_L15:
    jmp cardinal_sight_L12
cardinal_sight_L14:
    jmp cardinal_sight_L11
cardinal_sight_L11:
; VALID = 1
    lda #1
    sta VALID
; VISIBLE = 1
    lda #1
    sta VISIBLE
; return
    rts
    jmp cardinal_sight_L13
cardinal_sight_L12:
cardinal_sight_L13:
; T0 = FROM >> 2
    lda FROM
    lsr
    lsr
    sta T0
; T1 = TO >> 2
    lda TO
    lsr
    lsr
    sta T1
; T2 = 1
    lda #1
    sta T2
; if T0 != T1:
    lda T0
    cmp T1
    bne cardinal_sight_L19
cardinal_sight_L20:
    jmp cardinal_sight_L17
cardinal_sight_L19:
    jmp cardinal_sight_L16
cardinal_sight_L16:
; T0 = FROM & 3
    lda FROM
    and #3
    sta T0
; T1 = TO & 3
    lda TO
    and #3
    sta T1
; if T0 != T1:
    lda T0
    cmp T1
    bne cardinal_sight_L24
cardinal_sight_L25:
    jmp cardinal_sight_L22
cardinal_sight_L24:
    jmp cardinal_sight_L21
cardinal_sight_L21:
; return
    rts
    jmp cardinal_sight_L23
cardinal_sight_L22:
cardinal_sight_L23:
; T2 = 4
    lda #4
    sta T2
    jmp cardinal_sight_L18
cardinal_sight_L17:
cardinal_sight_L18:
; VALID = 1
    lda #1
    sta VALID
; T0 = FROM
    lda FROM
    sta T0
; while T0 != TO:
cardinal_sight_L26:
    lda T0
    cmp TO
    bne cardinal_sight_L29
cardinal_sight_L30:
    jmp cardinal_sight_L28
cardinal_sight_L29:
    jmp cardinal_sight_L27
cardinal_sight_L27:
; if T0 < TO:
    lda T0
    cmp TO
    bcc cardinal_sight_L34
cardinal_sight_L35:
    jmp cardinal_sight_L32
cardinal_sight_L34:
    jmp cardinal_sight_L31
cardinal_sight_L31:
; T0 = T0 + T2
    lda T0
    clc
    adc T2
    sta T0
    jmp cardinal_sight_L33
cardinal_sight_L32:
; T0 = T0 - T2
    lda T0
    sec
    sbc T2
    sta T0
cardinal_sight_L33:
; T1 = C[T0]
    ldx T0
    lda C0,x
    sta T1
; if T1 != 0:
    lda T1
    cmp #0
    bne cardinal_sight_L39
cardinal_sight_L40:
    jmp cardinal_sight_L37
cardinal_sight_L39:
    jmp cardinal_sight_L36
cardinal_sight_L36:
; return
    rts
    jmp cardinal_sight_L38
cardinal_sight_L37:
cardinal_sight_L38:
    jmp cardinal_sight_L26
cardinal_sight_L28:
; VISIBLE = 1
    lda #1
    sta VISIBLE
    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

demo.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
FROM=$80
TO=$81
VISIBLE=$82
VALID=$83
C0=$84
C1=$85
C2=$86
C3=$87
C4=$88
C5=$89
C6=$8a
C7=$8b
C8=$8c
C9=$8d
C10=$8e
C11=$8f
C12=$90
C13=$91
C14=$92
C15=$93
T0=$94
T1=$95
T2=$96
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 8192-272,0

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
.if CUSTOM
; FROM = 0
    lda #0
    sta FROM
; TO = 12
    lda #12
    sta TO
; C2 = 1
    lda #1
    sta C2
; C8 = 1
    lda #1
    sta C8
.if BROKEN
; C2 = 0
    lda #0
    sta C2
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; FROM = 0
    lda #0
    sta FROM
; TO = 3
    lda #3
    sta TO
; C2 = 1
    lda #1
    sta C2
.if BROKEN
; C2 = 0
    lda #0
    sta C2
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise 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

custom.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
FROM=$80
TO=$81
VISIBLE=$82
VALID=$83
C0=$84
C1=$85
C2=$86
C3=$87
C4=$88
C5=$89
C6=$8a
C7=$8b
C8=$8c
C9=$8d
C10=$8e
C11=$8f
C12=$90
C13=$91
C14=$92
C15=$93
T0=$94
T1=$95
T2=$96
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 8192-272,0

The 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

wrong.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
FROM=$80
TO=$81
VISIBLE=$82
VALID=$83
C0=$84
C1=$85
C2=$86
C3=$87
C4=$88
C5=$89
C6=$8a
C7=$8b
C8=$8c
C9=$8d
C10=$8e
C11=$8f
C12=$90
C13=$91
C14=$92
C15=$93
T0=$94
T1=$95
T2=$96
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

NES shows the deliberate error row 00 03 01 01 00 00 00 00. — The wrong client completes with 00 03 01 01 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 255 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 NTSC NROM, background only, DMC/NMI disabled, using Asm198x 0.0.72, Emu198x 0.44.2. 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.

The Craft · Maths for Games · Source and native evidence.