A guard looking across a room needs more than a same-row check.
Use it in a project
FROM and TO are row-major indices 0–15 in a 4×4 grid. Zero cells are transparent and nonzero cells block. Trace a thin Bresenham line, excluding the observer and including the destination; a cell sees itself. Invalid coordinates return VALID=0 and VISIBLE=0 without indexing the grid.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VISIBLE, VALID, FROM, TO, 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, T3, T4, T5, T6, T7, T8, T9, T10, T11 (12 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call grid_line_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 grid_line_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 |
| Supply before service | FROM, TO and C0–C15 |
| Read after service | VISIBLE and VALID |
The error accumulator decides when to advance each axis. A bias of 128 represents the small signed error using byte comparisons; doubled error stays within range on this fixed 4×4 domain.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Trace a Thin Grid Line of Sight. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
grid_line_sight_init:
lda #0
sta VISIBLE
sta VALID
sta FROM
sta TO
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
sta T3
sta T4
sta T5
sta T6
sta T7
sta T8
sta T9
sta T10
sta T11
rts
grid_line_sight_step:
; VISIBLE = 0
lda #0
sta VISIBLE
; VALID = 0
lda #0
sta VALID
; if FROM >= 16:
lda FROM
cmp #16
bcs grid_line_sight_L4
grid_line_sight_L5:
jmp grid_line_sight_L2
grid_line_sight_L4:
jmp grid_line_sight_L1
grid_line_sight_L1:
; return
rts
jmp grid_line_sight_L3
grid_line_sight_L2:
grid_line_sight_L3:
; if TO >= 16:
lda TO
cmp #16
bcs grid_line_sight_L9
grid_line_sight_L10:
jmp grid_line_sight_L7
grid_line_sight_L9:
jmp grid_line_sight_L6
grid_line_sight_L6:
; return
rts
jmp grid_line_sight_L8
grid_line_sight_L7:
grid_line_sight_L8:
; VALID = 1
lda #1
sta VALID
; T0 = FROM & 3
lda FROM
and #3
sta T0
; T1 = FROM >> 2
lda FROM
lsr
lsr
sta T1
; T2 = TO & 3
lda TO
and #3
sta T2
; T3 = TO >> 2
lda TO
lsr
lsr
sta T3
; T6 = 1
lda #1
sta T6
; if T2 >= T0:
lda T2
cmp T0
bcs grid_line_sight_L14
grid_line_sight_L15:
jmp grid_line_sight_L12
grid_line_sight_L14:
jmp grid_line_sight_L11
grid_line_sight_L11:
; T4 = T2 - T0
lda T2
sec
sbc T0
sta T4
jmp grid_line_sight_L13
grid_line_sight_L12:
; T4 = T0 - T2
lda T0
sec
sbc T2
sta T4
; T6 = 255
lda #255
sta T6
grid_line_sight_L13:
; T7 = 1
lda #1
sta T7
; if T3 >= T1:
lda T3
cmp T1
bcs grid_line_sight_L19
grid_line_sight_L20:
jmp grid_line_sight_L17
grid_line_sight_L19:
jmp grid_line_sight_L16
grid_line_sight_L16:
; T5 = T3 - T1
lda T3
sec
sbc T1
sta T5
jmp grid_line_sight_L18
grid_line_sight_L17:
; T5 = T1 - T3
lda T1
sec
sbc T3
sta T5
; T7 = 255
lda #255
sta T7
grid_line_sight_L18:
; T8 = 128 + T4
lda #128
clc
adc T4
sta T8
; T8 = T8 - T5
lda T8
sec
sbc T5
sta T8
; T9 = FROM
lda FROM
sta T9
; while T9 != TO:
grid_line_sight_L21:
lda T9
cmp TO
bne grid_line_sight_L24
grid_line_sight_L25:
jmp grid_line_sight_L23
grid_line_sight_L24:
jmp grid_line_sight_L22
grid_line_sight_L22:
; T10 = T8 - 128
lda T8
sec
sbc #128
sta T10
; T10 = T10 << 1
lda T10
asl
sta T10
; T10 = T10 + 128
lda T10
clc
adc #128
sta T10
; T11 = 128 - T5
lda #128
sec
sbc T5
sta T11
; if T10 > T11:
lda T10
cmp T11
beq grid_line_sight_L30
bcs grid_line_sight_L29
grid_line_sight_L30:
jmp grid_line_sight_L27
grid_line_sight_L29:
jmp grid_line_sight_L26
grid_line_sight_L26:
; T8 = T8 - T5
lda T8
sec
sbc T5
sta T8
; T0 = T0 + T6
lda T0
clc
adc T6
sta T0
jmp grid_line_sight_L28
grid_line_sight_L27:
grid_line_sight_L28:
; T11 = 128 + T4
lda #128
clc
adc T4
sta T11
; if T10 < T11:
lda T10
cmp T11
bcc grid_line_sight_L34
grid_line_sight_L35:
jmp grid_line_sight_L32
grid_line_sight_L34:
jmp grid_line_sight_L31
grid_line_sight_L31:
; T8 = T8 + T4
lda T8
clc
adc T4
sta T8
; T1 = T1 + T7
lda T1
clc
adc T7
sta T1
jmp grid_line_sight_L33
grid_line_sight_L32:
grid_line_sight_L33:
; T9 = T1 << 2
lda T1
asl
asl
sta T9
; T9 = T9 + T0
lda T9
clc
adc T0
sta T9
; T11 = C[T9]
ldx T9
lda C0,x
sta T11
; if T11 != 0:
lda T11
cmp #0
bne grid_line_sight_L39
grid_line_sight_L40:
jmp grid_line_sight_L37
grid_line_sight_L39:
jmp grid_line_sight_L36
grid_line_sight_L36:
; return
rts
jmp grid_line_sight_L38
grid_line_sight_L37:
grid_line_sight_L38:
jmp grid_line_sight_L21
grid_line_sight_L23:
; VISIBLE = 1
lda #1
sta VISIBLE
rts
component_end:Machine contract
A/X/status may change; Y and decimal mode are preserved. Use CLD for binary arithmetic. 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. 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 diagnostic PRG owns screen, font and colour RAM and disables interrupts; it does not return to BASIC. The reusable helper does none of those things.
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
VISIBLE=$80
VALID=$81
FROM=$82
TO=$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
T3=$97
T4=$98
T5=$99
T6=$9a
T7=$9b
T8=$9c
T9=$9d
T10=$9e
T11=$9f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:A client retaining state between calls
preview_rules:
!if CUSTOM {
; FROM = 0
lda #0
sta FROM
; TO = 14
lda #14
sta TO
; C5 = 0
lda #0
sta C5
!if BROKEN {
; C5 = 0
lda #0
sta C5
}
jsr checked_step
jsr emit_state
rts
} else {
; FROM = 0
lda #0
sta FROM
; TO = 15
lda #15
sta TO
; C5 = 1
lda #1
sta C5
!if BROKEN {
; C5 = 0
lda #0
sta C5
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Trace a different diagonal through a clear grid. 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
VISIBLE=$80
VALID=$81
FROM=$82
TO=$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
T3=$97
T4=$98
T5=$99
T6=$9a
T7=$9b
T8=$9c
T9=$9d
T10=$9e
T11=$9f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:The custom diagnostic row is 01 01 00 0E 00 00 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears a blocking cell before the query. The ray becomes clear because the world data was altered, not because the sight algorithm should see through a wall. Keep visibility queries read-only.
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
VISIBLE=$80
VALID=$81
FROM=$82
TO=$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
T3=$97
T4=$98
T5=$99
T6=$9a
T7=$9b
T8=$9c
T9=$9d
T10=$9e
T11=$9f
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
!byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:
Cost and alternatives
The measured helper extent is 375 code bytes; public state is 20 bytes and private scratch is 12 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 visited cells after the observer. A cardinal corridor query is cheaper. Larger grids need wider coordinate/error arithmetic and a freshly checked bound.
This is a thin line with a stated tie rule, not a conservative supercover or a complete field-of-view system. Reversing a half-cell tie can choose a different raster cell. Decide corner visibility deliberately rather than assuming all line algorithms agree.
Execution evidence
Four variants run on PAL stock-ROM PRG, 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
Cardinal sight: Keep the simpler row/column path when a game’s rules do not need diagonal rays.