A maze character needs one direction even when the player holds a diagonal.
Use it in a project
HELD uses bits 0 left, 1 right, 2 up and 3 down. Opposite directions cancel within each axis. DIRECTION is 0 still, 1 left, 2 right, 3 up or 4 down. LAST_AXIS=0 prefers horizontal for a diagonal; 1 prefers vertical. It records the last nonzero chosen axis. Other bits in HELD are ignored; invalid LAST_AXIS rejects.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: HELD, LAST_AXIS, DIRECTION, VALID. They occupy 4 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 four_way_input_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 four_way_input_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 | LAST_AXIS |
| Supply before service | HELD |
| Read after service | DIRECTION and VALID |
Cancellation happens before arbitration. Left plus right cancels horizontal intent; it does not accidentally beat a valid vertical direction. The remembered axis resolves only the remaining diagonal.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Resolve Input to Four Directions. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
four_way_input_init:
lda #0
sta HELD
sta LAST_AXIS
sta DIRECTION
sta VALID
sta T0
sta T1
sta T2
rts
four_way_input_step:
; DIRECTION = 0
lda #0
sta DIRECTION
; VALID = 0
lda #0
sta VALID
; if LAST_AXIS >= 2:
lda LAST_AXIS
cmp #2
bcs four_way_input_L4
four_way_input_L5:
jmp four_way_input_L2
four_way_input_L4:
jmp four_way_input_L1
four_way_input_L1:
; return
rts
jmp four_way_input_L3
four_way_input_L2:
four_way_input_L3:
; VALID = 1
lda #1
sta VALID
; T0 = HELD & 3
lda HELD
and #3
sta T0
; T1 = 0
lda #0
sta T1
; if T0 == 1:
lda T0
cmp #1
beq four_way_input_L9
four_way_input_L10:
jmp four_way_input_L7
four_way_input_L9:
jmp four_way_input_L6
four_way_input_L6:
; T1 = 1
lda #1
sta T1
jmp four_way_input_L8
four_way_input_L7:
four_way_input_L8:
; if T0 == 2:
lda T0
cmp #2
beq four_way_input_L14
four_way_input_L15:
jmp four_way_input_L12
four_way_input_L14:
jmp four_way_input_L11
four_way_input_L11:
; T1 = 2
lda #2
sta T1
jmp four_way_input_L13
four_way_input_L12:
four_way_input_L13:
; T0 = HELD & 12
lda HELD
and #12
sta T0
; T2 = 0
lda #0
sta T2
; if T0 == 4:
lda T0
cmp #4
beq four_way_input_L19
four_way_input_L20:
jmp four_way_input_L17
four_way_input_L19:
jmp four_way_input_L16
four_way_input_L16:
; T2 = 3
lda #3
sta T2
jmp four_way_input_L18
four_way_input_L17:
four_way_input_L18:
; if T0 == 8:
lda T0
cmp #8
beq four_way_input_L24
four_way_input_L25:
jmp four_way_input_L22
four_way_input_L24:
jmp four_way_input_L21
four_way_input_L21:
; T2 = 4
lda #4
sta T2
jmp four_way_input_L23
four_way_input_L22:
four_way_input_L23:
; if T1 != 0:
lda T1
cmp #0
bne four_way_input_L29
four_way_input_L30:
jmp four_way_input_L27
four_way_input_L29:
jmp four_way_input_L26
four_way_input_L26:
; if T2 != 0:
lda T2
cmp #0
bne four_way_input_L34
four_way_input_L35:
jmp four_way_input_L32
four_way_input_L34:
jmp four_way_input_L31
four_way_input_L31:
; if LAST_AXIS == 1:
lda LAST_AXIS
cmp #1
beq four_way_input_L39
four_way_input_L40:
jmp four_way_input_L37
four_way_input_L39:
jmp four_way_input_L36
four_way_input_L36:
; DIRECTION = T2
lda T2
sta DIRECTION
; return
rts
jmp four_way_input_L38
four_way_input_L37:
four_way_input_L38:
jmp four_way_input_L33
four_way_input_L32:
four_way_input_L33:
; DIRECTION = T1
lda T1
sta DIRECTION
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
; return
rts
jmp four_way_input_L28
four_way_input_L27:
four_way_input_L28:
; if T2 != 0:
lda T2
cmp #0
bne four_way_input_L44
four_way_input_L45:
jmp four_way_input_L42
four_way_input_L44:
jmp four_way_input_L41
four_way_input_L41:
; DIRECTION = T2
lda T2
sta DIRECTION
; LAST_AXIS = 1
lda #1
sta LAST_AXIS
jmp four_way_input_L43
four_way_input_L42:
four_way_input_L43:
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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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
HELD=$80
LAST_AXIS=$81
DIRECTION=$82
VALID=$83
T0=$84
T1=$85
T2=$86
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 {
; HELD = 5
lda #5
sta HELD
; LAST_AXIS = 1
lda #1
sta LAST_AXIS
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 8
lda #8
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 5
lda #5
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 3
lda #3
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
rts
} else {
; HELD = 5
lda #5
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 8
lda #8
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 5
lda #5
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
; HELD = 3
lda #3
sta HELD
!if BROKEN {
; LAST_AXIS = 0
lda #0
sta LAST_AXIS
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Begin with vertical preference instead of horizontal. 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
HELD=$80
LAST_AXIS=$81
DIRECTION=$82
VALID=$83
T0=$84
T1=$85
T2=$86
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 03 01 00 01 00 00 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller overwrites LAST_AXIS with horizontal preference every time. A diagonal loses its established vertical preference. Keep this policy state between snapshots; change it deliberately when changing the control scheme.
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
HELD=$80
LAST_AXIS=$81
DIRECTION=$82
VALID=$83
T0=$84
T1=$85
T2=$86
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 224 code bytes; public state is 4 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.
Constant work. A timestamp-based newest-direction policy needs more state and input events but can give a different control feel.
This is a deliberate direction policy, not diagonal movement. Newest-key wins needs per-key events or timestamps. Digital pads can use this directly; analogue hardware needs sampling and thresholds first.
Execution evidence
Four variants run on PAL stock-ROM PRG, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 64 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
Chord recognition: Both start from a held snapshot, but a chord recognises a combination while this policy resolves it to one direction.