Buffer Turns Through a Grid on C64 | Pattern Library

A maze feels kinder when a turn pressed just before a junction happens as soon as the route opens.

C64 shows the working diagnostic row 01 03 00 01 01 00 00 00. — A completed native caller displays 01 03 00 01 01 00 00 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

CELL indexes a 4×4 row-major map; zero cells are walkable and nonzero cells block. Directions are 0 still, 1 left, 2 right, 3 up, 4 down. A nonzero REQUEST replaces PENDING. Try that turn first; if blocked, retain it and try the current DIRECTION. A successful turn clears PENDING. Each call moves at most one cell. Invalid indices/directions or a blocked current cell return VALID=0 without changing movement state.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: CELL, DIRECTION, PENDING, MOVED, VALID, REQUEST, 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 (5 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call grid_turn_buffer_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_turn_buffer_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	CELL, DIRECTION and PENDING; map bytes
Supply before service	REQUEST
Read after service	CELL, DIRECTION, PENDING, MOVED and VALID

Try at most two neighbouring cells: the buffered request and the existing direction. Check column boundaries before adding or subtracting one so movement cannot wrap into the next row.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Buffer Turns Through a Grid. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
grid_turn_buffer_init:
    lda #0
    sta CELL
    sta DIRECTION
    sta PENDING
    sta MOVED
    sta VALID
    sta REQUEST
    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
    rts
grid_turn_buffer_step:
; MOVED = 0
    lda #0
    sta MOVED
; VALID = 0
    lda #0
    sta VALID
; if CELL >= 16:
    lda CELL
    cmp #16
    bcs grid_turn_buffer_L4
grid_turn_buffer_L5:
    jmp grid_turn_buffer_L2
grid_turn_buffer_L4:
    jmp grid_turn_buffer_L1
grid_turn_buffer_L1:
; return
    rts
    jmp grid_turn_buffer_L3
grid_turn_buffer_L2:
grid_turn_buffer_L3:
; if DIRECTION > 4:
    lda DIRECTION
    cmp #4
    beq grid_turn_buffer_L10
    bcs grid_turn_buffer_L9
grid_turn_buffer_L10:
    jmp grid_turn_buffer_L7
grid_turn_buffer_L9:
    jmp grid_turn_buffer_L6
grid_turn_buffer_L6:
; return
    rts
    jmp grid_turn_buffer_L8
grid_turn_buffer_L7:
grid_turn_buffer_L8:
; if PENDING > 4:
    lda PENDING
    cmp #4
    beq grid_turn_buffer_L15
    bcs grid_turn_buffer_L14
grid_turn_buffer_L15:
    jmp grid_turn_buffer_L12
grid_turn_buffer_L14:
    jmp grid_turn_buffer_L11
grid_turn_buffer_L11:
; return
    rts
    jmp grid_turn_buffer_L13
grid_turn_buffer_L12:
grid_turn_buffer_L13:
; if REQUEST > 4:
    lda REQUEST
    cmp #4
    beq grid_turn_buffer_L20
    bcs grid_turn_buffer_L19
grid_turn_buffer_L20:
    jmp grid_turn_buffer_L17
grid_turn_buffer_L19:
    jmp grid_turn_buffer_L16
grid_turn_buffer_L16:
; return
    rts
    jmp grid_turn_buffer_L18
grid_turn_buffer_L17:
grid_turn_buffer_L18:
; T0 = C[CELL]
    ldx CELL
    lda C0,x
    sta T0
; if T0 != 0:
    lda T0
    cmp #0
    bne grid_turn_buffer_L24
grid_turn_buffer_L25:
    jmp grid_turn_buffer_L22
grid_turn_buffer_L24:
    jmp grid_turn_buffer_L21
grid_turn_buffer_L21:
; return
    rts
    jmp grid_turn_buffer_L23
grid_turn_buffer_L22:
grid_turn_buffer_L23:
; VALID = 1
    lda #1
    sta VALID
; if REQUEST != 0:
    lda REQUEST
    cmp #0
    bne grid_turn_buffer_L29
grid_turn_buffer_L30:
    jmp grid_turn_buffer_L27
grid_turn_buffer_L29:
    jmp grid_turn_buffer_L26
grid_turn_buffer_L26:
; PENDING = REQUEST
    lda REQUEST
    sta PENDING
    jmp grid_turn_buffer_L28
grid_turn_buffer_L27:
grid_turn_buffer_L28:
; T4 = 2
    lda #2
    sta T4
; T0 = PENDING
    lda PENDING
    sta T0
; while T4 != 0:
grid_turn_buffer_L31:
    lda T4
    cmp #0
    bne grid_turn_buffer_L34
grid_turn_buffer_L35:
    jmp grid_turn_buffer_L33
grid_turn_buffer_L34:
    jmp grid_turn_buffer_L32
grid_turn_buffer_L32:
; T1 = 255
    lda #255
    sta T1
; T2 = CELL & 3
    lda CELL
    and #3
    sta T2
; if T0 == 1:
    lda T0
    cmp #1
    beq grid_turn_buffer_L39
grid_turn_buffer_L40:
    jmp grid_turn_buffer_L37
grid_turn_buffer_L39:
    jmp grid_turn_buffer_L36
grid_turn_buffer_L36:
; if T2 != 0:
    lda T2
    cmp #0
    bne grid_turn_buffer_L44
grid_turn_buffer_L45:
    jmp grid_turn_buffer_L42
grid_turn_buffer_L44:
    jmp grid_turn_buffer_L41
grid_turn_buffer_L41:
; T1 = CELL - 1
    lda CELL
    sec
    sbc #1
    sta T1
    jmp grid_turn_buffer_L43
grid_turn_buffer_L42:
grid_turn_buffer_L43:
    jmp grid_turn_buffer_L38
grid_turn_buffer_L37:
grid_turn_buffer_L38:
; if T0 == 2:
    lda T0
    cmp #2
    beq grid_turn_buffer_L49
grid_turn_buffer_L50:
    jmp grid_turn_buffer_L47
grid_turn_buffer_L49:
    jmp grid_turn_buffer_L46
grid_turn_buffer_L46:
; if T2 != 3:
    lda T2
    cmp #3
    bne grid_turn_buffer_L54
grid_turn_buffer_L55:
    jmp grid_turn_buffer_L52
grid_turn_buffer_L54:
    jmp grid_turn_buffer_L51
grid_turn_buffer_L51:
; T1 = CELL + 1
    lda CELL
    clc
    adc #1
    sta T1
    jmp grid_turn_buffer_L53
grid_turn_buffer_L52:
grid_turn_buffer_L53:
    jmp grid_turn_buffer_L48
grid_turn_buffer_L47:
grid_turn_buffer_L48:
; if T0 == 3:
    lda T0
    cmp #3
    beq grid_turn_buffer_L59
grid_turn_buffer_L60:
    jmp grid_turn_buffer_L57
grid_turn_buffer_L59:
    jmp grid_turn_buffer_L56
grid_turn_buffer_L56:
; if CELL >= 4:
    lda CELL
    cmp #4
    bcs grid_turn_buffer_L64
grid_turn_buffer_L65:
    jmp grid_turn_buffer_L62
grid_turn_buffer_L64:
    jmp grid_turn_buffer_L61
grid_turn_buffer_L61:
; T1 = CELL - 4
    lda CELL
    sec
    sbc #4
    sta T1
    jmp grid_turn_buffer_L63
grid_turn_buffer_L62:
grid_turn_buffer_L63:
    jmp grid_turn_buffer_L58
grid_turn_buffer_L57:
grid_turn_buffer_L58:
; if T0 == 4:
    lda T0
    cmp #4
    beq grid_turn_buffer_L69
grid_turn_buffer_L70:
    jmp grid_turn_buffer_L67
grid_turn_buffer_L69:
    jmp grid_turn_buffer_L66
grid_turn_buffer_L66:
; if CELL < 12:
    lda CELL
    cmp #12
    bcc grid_turn_buffer_L74
grid_turn_buffer_L75:
    jmp grid_turn_buffer_L72
grid_turn_buffer_L74:
    jmp grid_turn_buffer_L71
grid_turn_buffer_L71:
; T1 = CELL + 4
    lda CELL
    clc
    adc #4
    sta T1
    jmp grid_turn_buffer_L73
grid_turn_buffer_L72:
grid_turn_buffer_L73:
    jmp grid_turn_buffer_L68
grid_turn_buffer_L67:
grid_turn_buffer_L68:
; if T1 < 16:
    lda T1
    cmp #16
    bcc grid_turn_buffer_L79
grid_turn_buffer_L80:
    jmp grid_turn_buffer_L77
grid_turn_buffer_L79:
    jmp grid_turn_buffer_L76
grid_turn_buffer_L76:
; T3 = C[T1]
    ldx T1
    lda C0,x
    sta T3
; if T3 == 0:
    lda T3
    cmp #0
    beq grid_turn_buffer_L84
grid_turn_buffer_L85:
    jmp grid_turn_buffer_L82
grid_turn_buffer_L84:
    jmp grid_turn_buffer_L81
grid_turn_buffer_L81:
; CELL = T1
    lda T1
    sta CELL
; DIRECTION = T0
    lda T0
    sta DIRECTION
; if T4 == 2:
    lda T4
    cmp #2
    beq grid_turn_buffer_L89
grid_turn_buffer_L90:
    jmp grid_turn_buffer_L87
grid_turn_buffer_L89:
    jmp grid_turn_buffer_L86
grid_turn_buffer_L86:
; PENDING = 0
    lda #0
    sta PENDING
    jmp grid_turn_buffer_L88
grid_turn_buffer_L87:
grid_turn_buffer_L88:
; MOVED = 1
    lda #1
    sta MOVED
; return
    rts
    jmp grid_turn_buffer_L83
grid_turn_buffer_L82:
grid_turn_buffer_L83:
    jmp grid_turn_buffer_L78
grid_turn_buffer_L77:
grid_turn_buffer_L78:
; T4 = T4 - 1
    lda T4
    sec
    sbc #1
    sta T4
; T0 = DIRECTION
    lda DIRECTION
    sta T0
    jmp grid_turn_buffer_L31
grid_turn_buffer_L33:
    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

demo.asm

CELL=$80
DIRECTION=$81
PENDING=$82
MOVED=$83
VALID=$84
REQUEST=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
T4=$9a
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

exercise.inc, excerpt

preview_rules:
!if CUSTOM {
; CELL = 0
    lda #0
    sta CELL
; DIRECTION = 2
    lda #2
    sta DIRECTION
; REQUEST = 3
    lda #3
    sta REQUEST
!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state
; REQUEST = 0
    lda #0
    sta REQUEST
!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state
; REQUEST = 4
    lda #4
    sta REQUEST
!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; CELL = 8
    lda #8
    sta CELL
; DIRECTION = 2
    lda #2
    sta DIRECTION
; REQUEST = 3
    lda #3
    sta REQUEST
; C4 = 1
    lda #1
    sta C4
!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state
; REQUEST = 0
    lda #0
    sta REQUEST
!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PENDING = 0
    lda #0
    sta PENDING
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Queue a blocked upward turn at the top edge, then replace it with a downward request. 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

CELL=$80
DIRECTION=$81
PENDING=$82
MOVED=$83
VALID=$84
REQUEST=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
T4=$9a
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 06 04 00 01 01 04 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller clears the queued turn between cells. The player continues along the row instead of turning upwards when the next junction opens. Retain PENDING until a turn succeeds or a new request replaces it.

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

CELL=$80
DIRECTION=$81
PENDING=$82
MOVED=$83
VALID=$84
REQUEST=$85
C0=$86
C1=$87
C2=$88
C3=$89
C4=$8a
C5=$8b
C6=$8c
C7=$8d
C8=$8e
C9=$8f
C10=$90
C11=$91
C12=$92
C13=$93
C14=$94
C15=$95
T0=$96
T1=$97
T2=$98
T3=$99
T4=$9a
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:

C64 shows the deliberate error row 0B 02 00 01 01 00 00 00. — The wrong client completes with 0B 02 00 01 01 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 441 code bytes; public state is 22 bytes and private scratch is 5 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 two map reads after validation. A larger map needs a wider index and declared dimensions; a pixel mover needs centre-line snapping and collision checks between cell boundaries.

This is logical cell movement, not pixel interpolation or a complete maze controller. Call only at a cell-step boundary; animate between those positions separately. The map is read-only during service.

Execution evidence

Four variants run on PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. 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

Four-way input: Resolve contradictory buttons before turning the result into a buffered movement request.

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