Code Like It's 198x

Test Sight Along a Grid Row or Column on Spectrum

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

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

Spectrum 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

component.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:
    ld a,0
    ld (FROM),a
    ld (TO),a
    ld (VISIBLE),a
    ld (VALID),a
    ld (C0),a
    ld (C1),a
    ld (C2),a
    ld (C3),a
    ld (C4),a
    ld (C5),a
    ld (C6),a
    ld (C7),a
    ld (C8),a
    ld (C9),a
    ld (C10),a
    ld (C11),a
    ld (C12),a
    ld (C13),a
    ld (C14),a
    ld (C15),a
    ld (T0),a
    ld (T1),a
    ld (T2),a
    ret
cardinal_sight_step:
; VISIBLE = 0
    ld a,0
    ld (VISIBLE),a
; VALID = 0
    ld a,0
    ld (VALID),a
; if FROM >= 16:
    ld a,(FROM)
    cp 16
    jp nc,cardinal_sight_L1
    jp cardinal_sight_L2
cardinal_sight_L1:
; return
    ret
    jp cardinal_sight_L3
cardinal_sight_L2:
cardinal_sight_L3:
; if TO >= 16:
    ld a,(TO)
    cp 16
    jp nc,cardinal_sight_L4
    jp cardinal_sight_L5
cardinal_sight_L4:
; return
    ret
    jp cardinal_sight_L6
cardinal_sight_L5:
cardinal_sight_L6:
; if FROM == TO:
    ld a,(FROM)
    ld b,a
    ld a,(TO)
    ld c,a
    ld a,b
    cp c
    jp z,cardinal_sight_L7
    jp cardinal_sight_L8
cardinal_sight_L7:
; VALID = 1
    ld a,1
    ld (VALID),a
; VISIBLE = 1
    ld a,1
    ld (VISIBLE),a
; return
    ret
    jp cardinal_sight_L9
cardinal_sight_L8:
cardinal_sight_L9:
; T0 = FROM >> 2
    ld a,(FROM)
    srl a
    srl a
    ld (T0),a
; T1 = TO >> 2
    ld a,(TO)
    srl a
    srl a
    ld (T1),a
; T2 = 1
    ld a,1
    ld (T2),a
; if T0 != T1:
    ld a,(T0)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    cp c
    jp nz,cardinal_sight_L10
    jp cardinal_sight_L11
cardinal_sight_L10:
; T0 = FROM & 3
    ld a,(FROM)
    and 3
    ld (T0),a
; T1 = TO & 3
    ld a,(TO)
    and 3
    ld (T1),a
; if T0 != T1:
    ld a,(T0)
    ld b,a
    ld a,(T1)
    ld c,a
    ld a,b
    cp c
    jp nz,cardinal_sight_L13
    jp cardinal_sight_L14
cardinal_sight_L13:
; return
    ret
    jp cardinal_sight_L15
cardinal_sight_L14:
cardinal_sight_L15:
; T2 = 4
    ld a,4
    ld (T2),a
    jp cardinal_sight_L12
cardinal_sight_L11:
cardinal_sight_L12:
; VALID = 1
    ld a,1
    ld (VALID),a
; T0 = FROM
    ld a,(FROM)
    ld (T0),a
; while T0 != TO:
cardinal_sight_L16:
    ld a,(T0)
    ld b,a
    ld a,(TO)
    ld c,a
    ld a,b
    cp c
    jp nz,cardinal_sight_L17
    jp cardinal_sight_L18
cardinal_sight_L17:
; if T0 < TO:
    ld a,(T0)
    ld b,a
    ld a,(TO)
    ld c,a
    ld a,b
    cp c
    jp c,cardinal_sight_L19
    jp cardinal_sight_L20
cardinal_sight_L19:
; T0 = T0 + T2
    ld a,(T0)
    ld b,a
    ld a,(T2)
    ld c,a
    ld a,b
    add a,c
    ld (T0),a
    jp cardinal_sight_L21
cardinal_sight_L20:
; T0 = T0 - T2
    ld a,(T0)
    ld b,a
    ld a,(T2)
    ld c,a
    ld a,b
    sub c
    ld (T0),a
cardinal_sight_L21:
; T1 = C[T0]
    ld a,(T0)
    ld e,a
    ld d,0
    ld hl,C0
    add hl,de
    ld a,(hl)
    ld (T1),a
; if T1 != 0:
    ld a,(T1)
    cp 0
    jp nz,cardinal_sight_L22
    jp cardinal_sight_L23
cardinal_sight_L22:
; return
    ret
    jp cardinal_sight_L24
cardinal_sight_L23:
cardinal_sight_L24:
    jp cardinal_sight_L16
cardinal_sight_L18:
; VISIBLE = 1
    ld a,1
    ld (VISIBLE),a
    ret
component_end:

Machine contract

AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL 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 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.

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
    org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    include "data.inc"
    include "cases.inc"
    include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
FROM: defb 0
TO: defb 0
VISIBLE: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
    end start

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
    if CUSTOM
; FROM = 0
    ld a,0
    ld (FROM),a
; TO = 12
    ld a,12
    ld (TO),a
; C2 = 1
    ld a,1
    ld (C2),a
; C8 = 1
    ld a,1
    ld (C8),a
    if BROKEN
; C2 = 0
    ld a,0
    ld (C2),a
    endif
    call checked_step
    call emit_state
    ret
    else
; FROM = 0
    ld a,0
    ld (FROM),a
; TO = 3
    ld a,3
    ld (TO),a
; C2 = 1
    ld a,1
    ld (C2),a
    if BROKEN
; C2 = 0
    ld a,0
    ld (C2),a
    endif
    call checked_step
    call emit_state
    endif
    ret

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
    org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    include "data.inc"
    include "cases.inc"
    include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
FROM: defb 0
TO: defb 0
VISIBLE: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
    end start

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
    org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    include "data.inc"
    include "cases.inc"
    include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
FROM: defb 0
TO: defb 0
VISIBLE: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
C4: defb 0
C5: defb 0
C6: defb 0
C7: defb 0
C8: defb 0
C9: defb 0
C10: defb 0
C11: defb 0
C12: defb 0
C13: defb 0
C14: defb 0
C15: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
    end start
Spectrum 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 337 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 48K PAL ROM tape load and USR, 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.