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:
move.b #0,d0
move.b d0,HELD
move.b d0,LAST_AXIS
move.b d0,DIRECTION
move.b d0,VALID
move.b d0,T0
move.b d0,T1
move.b d0,T2
rts
four_way_input_step:
; DIRECTION = 0
move.b #0,d0
move.b d0,DIRECTION
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if LAST_AXIS >= 2:
move.b LAST_AXIS,d0
cmp.b #2,d0
bcc four_way_input_L1
jmp four_way_input_L2
four_way_input_L1:
; return
rts
jmp four_way_input_L3
four_way_input_L2:
four_way_input_L3:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; T0 = HELD & 3
move.b HELD,d0
and.b #3,d0
move.b d0,T0
; T1 = 0
move.b #0,d0
move.b d0,T1
; if T0 == 1:
move.b T0,d0
cmp.b #1,d0
beq four_way_input_L4
jmp four_way_input_L5
four_way_input_L4:
; T1 = 1
move.b #1,d0
move.b d0,T1
jmp four_way_input_L6
four_way_input_L5:
four_way_input_L6:
; if T0 == 2:
move.b T0,d0
cmp.b #2,d0
beq four_way_input_L7
jmp four_way_input_L8
four_way_input_L7:
; T1 = 2
move.b #2,d0
move.b d0,T1
jmp four_way_input_L9
four_way_input_L8:
four_way_input_L9:
; T0 = HELD & 12
move.b HELD,d0
and.b #12,d0
move.b d0,T0
; T2 = 0
move.b #0,d0
move.b d0,T2
; if T0 == 4:
move.b T0,d0
cmp.b #4,d0
beq four_way_input_L10
jmp four_way_input_L11
four_way_input_L10:
; T2 = 3
move.b #3,d0
move.b d0,T2
jmp four_way_input_L12
four_way_input_L11:
four_way_input_L12:
; if T0 == 8:
move.b T0,d0
cmp.b #8,d0
beq four_way_input_L13
jmp four_way_input_L14
four_way_input_L13:
; T2 = 4
move.b #4,d0
move.b d0,T2
jmp four_way_input_L15
four_way_input_L14:
four_way_input_L15:
; if T1 != 0:
move.b T1,d0
cmp.b #0,d0
bne four_way_input_L16
jmp four_way_input_L17
four_way_input_L16:
; if T2 != 0:
move.b T2,d0
cmp.b #0,d0
bne four_way_input_L19
jmp four_way_input_L20
four_way_input_L19:
; if LAST_AXIS == 1:
move.b LAST_AXIS,d0
cmp.b #1,d0
beq four_way_input_L22
jmp four_way_input_L23
four_way_input_L22:
; DIRECTION = T2
move.b T2,d0
move.b d0,DIRECTION
; return
rts
jmp four_way_input_L24
four_way_input_L23:
four_way_input_L24:
jmp four_way_input_L21
four_way_input_L20:
four_way_input_L21:
; DIRECTION = T1
move.b T1,d0
move.b d0,DIRECTION
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
; return
rts
jmp four_way_input_L18
four_way_input_L17:
four_way_input_L18:
; if T2 != 0:
move.b T2,d0
cmp.b #0,d0
bne four_way_input_L25
jmp four_way_input_L26
four_way_input_L25:
; DIRECTION = T2
move.b T2,d0
move.b d0,DIRECTION
; LAST_AXIS = 1
move.b #1,d0
move.b d0,LAST_AXIS
jmp four_way_input_L27
four_way_input_L26:
four_way_input_L27:
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
HELD: dc.b 0
LAST_AXIS: dc.b 0
DIRECTION: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; HELD = 5
move.b #5,d0
move.b d0,HELD
; LAST_AXIS = 1
move.b #1,d0
move.b d0,LAST_AXIS
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 8
move.b #8,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 5
move.b #5,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 3
move.b #3,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
rts
else
; HELD = 5
move.b #5,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 8
move.b #8,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 5
move.b #5,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
; HELD = 3
move.b #3,d0
move.b d0,HELD
ifne BROKEN
; LAST_AXIS = 0
move.b #0,d0
move.b d0,LAST_AXIS
endif
bsr checked_step
bsr emit_state
endif
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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
HELD: dc.b 0
LAST_AXIS: dc.b 0
DIRECTION: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The 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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
HELD: dc.b 0
LAST_AXIS: dc.b 0
DIRECTION: dc.b 0
VALID: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 380 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 A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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.