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:
ld a,0
ld (HELD),a
ld (LAST_AXIS),a
ld (DIRECTION),a
ld (VALID),a
ld (T0),a
ld (T1),a
ld (T2),a
ret
four_way_input_step:
; DIRECTION = 0
ld a,0
ld (DIRECTION),a
; VALID = 0
ld a,0
ld (VALID),a
; if LAST_AXIS >= 2:
ld a,(LAST_AXIS)
cp 2
jp nc,four_way_input_L1
jp four_way_input_L2
four_way_input_L1:
; return
ret
jp four_way_input_L3
four_way_input_L2:
four_way_input_L3:
; VALID = 1
ld a,1
ld (VALID),a
; T0 = HELD & 3
ld a,(HELD)
and 3
ld (T0),a
; T1 = 0
ld a,0
ld (T1),a
; if T0 == 1:
ld a,(T0)
cp 1
jp z,four_way_input_L4
jp four_way_input_L5
four_way_input_L4:
; T1 = 1
ld a,1
ld (T1),a
jp four_way_input_L6
four_way_input_L5:
four_way_input_L6:
; if T0 == 2:
ld a,(T0)
cp 2
jp z,four_way_input_L7
jp four_way_input_L8
four_way_input_L7:
; T1 = 2
ld a,2
ld (T1),a
jp four_way_input_L9
four_way_input_L8:
four_way_input_L9:
; T0 = HELD & 12
ld a,(HELD)
and 12
ld (T0),a
; T2 = 0
ld a,0
ld (T2),a
; if T0 == 4:
ld a,(T0)
cp 4
jp z,four_way_input_L10
jp four_way_input_L11
four_way_input_L10:
; T2 = 3
ld a,3
ld (T2),a
jp four_way_input_L12
four_way_input_L11:
four_way_input_L12:
; if T0 == 8:
ld a,(T0)
cp 8
jp z,four_way_input_L13
jp four_way_input_L14
four_way_input_L13:
; T2 = 4
ld a,4
ld (T2),a
jp four_way_input_L15
four_way_input_L14:
four_way_input_L15:
; if T1 != 0:
ld a,(T1)
cp 0
jp nz,four_way_input_L16
jp four_way_input_L17
four_way_input_L16:
; if T2 != 0:
ld a,(T2)
cp 0
jp nz,four_way_input_L19
jp four_way_input_L20
four_way_input_L19:
; if LAST_AXIS == 1:
ld a,(LAST_AXIS)
cp 1
jp z,four_way_input_L22
jp four_way_input_L23
four_way_input_L22:
; DIRECTION = T2
ld a,(T2)
ld (DIRECTION),a
; return
ret
jp four_way_input_L24
four_way_input_L23:
four_way_input_L24:
jp four_way_input_L21
four_way_input_L20:
four_way_input_L21:
; DIRECTION = T1
ld a,(T1)
ld (DIRECTION),a
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
; return
ret
jp four_way_input_L18
four_way_input_L17:
four_way_input_L18:
; if T2 != 0:
ld a,(T2)
cp 0
jp nz,four_way_input_L25
jp four_way_input_L26
four_way_input_L25:
; DIRECTION = T2
ld a,(T2)
ld (DIRECTION),a
; LAST_AXIS = 1
ld a,1
ld (LAST_AXIS),a
jp four_way_input_L27
four_way_input_L26:
four_way_input_L27:
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
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:
HELD: defb 0
LAST_AXIS: defb 0
DIRECTION: defb 0
VALID: 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 startA client retaining state between calls
preview_rules:
if CUSTOM
; HELD = 5
ld a,5
ld (HELD),a
; LAST_AXIS = 1
ld a,1
ld (LAST_AXIS),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 8
ld a,8
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 5
ld a,5
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 3
ld a,3
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
ret
else
; HELD = 5
ld a,5
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 8
ld a,8
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 5
ld a,5
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
; HELD = 3
ld a,3
ld (HELD),a
if BROKEN
; LAST_AXIS = 0
ld a,0
ld (LAST_AXIS),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
HELD: defb 0
LAST_AXIS: defb 0
DIRECTION: defb 0
VALID: 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 startThe 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
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:
HELD: defb 0
LAST_AXIS: defb 0
DIRECTION: defb 0
VALID: 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
Cost and alternatives
The measured helper extent is 243 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 48K PAL ROM tape load and USR, 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.