A sequence is about order. Four correct buttons in the wrong order should not open the door.
Use it in a project
Feed one event SYMBOL per call: 0 means no event, 1 and 2 are the accepted symbols. The normal sequence is 1,2,1,2; PROGRESS is 0–3. MATCHED is a one-call pulse. On mismatch, a 1 restarts at progress 1, preserving the useful overlapping prefix. Invalid symbols or progress reject without advancing.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: PROGRESS, SYMBOL, MATCHED, VALID. They occupy 4 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call input_sequence_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 input_sequence_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 | PROGRESS |
| Supply before service | SYMBOL |
| Read after service | MATCHED and VALID |
The progress value is the length of the recognised prefix. A completed 1,2,1,2 also ends in 1,2, so progress returns to two and overlapping matches remain possible.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Recognise a Short Input Sequence. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
input_sequence_init:
ld a,0
ld (PROGRESS),a
ld (SYMBOL),a
ld (MATCHED),a
ld (VALID),a
ld (T0),a
ret
input_sequence_step:
; MATCHED = 0
ld a,0
ld (MATCHED),a
; VALID = 0
ld a,0
ld (VALID),a
; if PROGRESS >= 4:
ld a,(PROGRESS)
cp 4
jp nc,input_sequence_L1
jp input_sequence_L2
input_sequence_L1:
; return
ret
jp input_sequence_L3
input_sequence_L2:
input_sequence_L3:
; if SYMBOL >= 3:
ld a,(SYMBOL)
cp 3
jp nc,input_sequence_L4
jp input_sequence_L5
input_sequence_L4:
; return
ret
jp input_sequence_L6
input_sequence_L5:
input_sequence_L6:
; VALID = 1
ld a,1
ld (VALID),a
; if SYMBOL == 0:
ld a,(SYMBOL)
cp 0
jp z,input_sequence_L7
jp input_sequence_L8
input_sequence_L7:
; return
ret
jp input_sequence_L9
input_sequence_L8:
input_sequence_L9:
; T0 = PROGRESS & 1
ld a,(PROGRESS)
and 1
ld (T0),a
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
; if SYMBOL == T0:
ld a,(SYMBOL)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp z,input_sequence_L10
jp input_sequence_L11
input_sequence_L10:
; PROGRESS = PROGRESS + 1
ld a,(PROGRESS)
add a,1
ld (PROGRESS),a
; if PROGRESS == 4:
ld a,(PROGRESS)
cp 4
jp z,input_sequence_L13
jp input_sequence_L14
input_sequence_L13:
; MATCHED = 1
ld a,1
ld (MATCHED),a
; PROGRESS = 2
ld a,2
ld (PROGRESS),a
jp input_sequence_L15
input_sequence_L14:
input_sequence_L15:
jp input_sequence_L12
input_sequence_L11:
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
; if SYMBOL == 1:
ld a,(SYMBOL)
cp 1
jp z,input_sequence_L16
jp input_sequence_L17
input_sequence_L16:
; PROGRESS = 1
ld a,1
ld (PROGRESS),a
jp input_sequence_L18
input_sequence_L17:
input_sequence_L18:
input_sequence_L12:
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:
PROGRESS: defb 0
SYMBOL: defb 0
MATCHED: defb 0
VALID: defb 0
T0: 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
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 2
ld a,2
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 2
ld a,2
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
ret
else
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 2
ld a,2
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 2
ld a,2
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 1
ld a,1
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
; SYMBOL = 2
ld a,2
ld (SYMBOL),a
if BROKEN
; PROGRESS = 0
ld a,0
ld (PROGRESS),a
endif
call checked_step
call emit_state
endif
retCustomise it
Insert an incorrect event, then restart the sequence. 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:
PROGRESS: defb 0
SYMBOL: defb 0
MATCHED: defb 0
VALID: defb 0
T0: 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 02 02 01 01 00 00 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears PROGRESS before every event, so no multi-event sequence can complete. Keep recognition state between input events and reset it only when that is the intended policy.
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:
PROGRESS: defb 0
SYMBOL: defb 0
MATCHED: defb 0
VALID: defb 0
T0: 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 170 code bytes; public state is 4 bytes and private scratch is 1 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 for this fixed sequence. A general prefix-function matcher uses a data table but avoids writing a new recogniser for each longer sequence.
This deliberately implements one overlapping four-event recogniser, not a general parser. It has no time limit and ignores simultaneous button ambiguity; an event producer owns those policies. Longer arbitrary patterns need a prefix table or state machine.
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
Input repeat: Choose whether a sequence receives initial presses or repeated events; a held snapshot is not itself a stream of new actions.