Recognise a Short Input Sequence on C64 | Pattern Library

A sequence is about order. Four correct buttons in the wrong order should not open the door.

C64 shows the working diagnostic row 02 02 01 01 00 00 00 00. — A completed native caller displays 02 02 01 01 00 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

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

6502.inc

; 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:
    lda #0
    sta PROGRESS
    sta SYMBOL
    sta MATCHED
    sta VALID
    sta T0
    rts
input_sequence_step:
; MATCHED = 0
    lda #0
    sta MATCHED
; VALID = 0
    lda #0
    sta VALID
; if PROGRESS >= 4:
    lda PROGRESS
    cmp #4
    bcs input_sequence_L4
input_sequence_L5:
    jmp input_sequence_L2
input_sequence_L4:
    jmp input_sequence_L1
input_sequence_L1:
; return
    rts
    jmp input_sequence_L3
input_sequence_L2:
input_sequence_L3:
; if SYMBOL >= 3:
    lda SYMBOL
    cmp #3
    bcs input_sequence_L9
input_sequence_L10:
    jmp input_sequence_L7
input_sequence_L9:
    jmp input_sequence_L6
input_sequence_L6:
; return
    rts
    jmp input_sequence_L8
input_sequence_L7:
input_sequence_L8:
; VALID = 1
    lda #1
    sta VALID
; if SYMBOL == 0:
    lda SYMBOL
    cmp #0
    beq input_sequence_L14
input_sequence_L15:
    jmp input_sequence_L12
input_sequence_L14:
    jmp input_sequence_L11
input_sequence_L11:
; return
    rts
    jmp input_sequence_L13
input_sequence_L12:
input_sequence_L13:
; T0 = PROGRESS & 1
    lda PROGRESS
    and #1
    sta T0
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
; if SYMBOL == T0:
    lda SYMBOL
    cmp T0
    beq input_sequence_L19
input_sequence_L20:
    jmp input_sequence_L17
input_sequence_L19:
    jmp input_sequence_L16
input_sequence_L16:
; PROGRESS = PROGRESS + 1
    lda PROGRESS
    clc
    adc #1
    sta PROGRESS
; if PROGRESS == 4:
    lda PROGRESS
    cmp #4
    beq input_sequence_L24
input_sequence_L25:
    jmp input_sequence_L22
input_sequence_L24:
    jmp input_sequence_L21
input_sequence_L21:
; MATCHED = 1
    lda #1
    sta MATCHED
; PROGRESS = 2
    lda #2
    sta PROGRESS
    jmp input_sequence_L23
input_sequence_L22:
input_sequence_L23:
    jmp input_sequence_L18
input_sequence_L17:
; PROGRESS = 0
    lda #0
    sta PROGRESS
; if SYMBOL == 1:
    lda SYMBOL
    cmp #1
    beq input_sequence_L29
input_sequence_L30:
    jmp input_sequence_L27
input_sequence_L29:
    jmp input_sequence_L26
input_sequence_L26:
; PROGRESS = 1
    lda #1
    sta PROGRESS
    jmp input_sequence_L28
input_sequence_L27:
input_sequence_L28:
input_sequence_L18:
    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

demo.asm

PROGRESS=$80
SYMBOL=$81
MATCHED=$82
VALID=$83
T0=$84
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 {
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 2
    lda #2
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 2
    lda #2
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 2
    lda #2
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 2
    lda #2
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 1
    lda #1
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
; SYMBOL = 2
    lda #2
    sta SYMBOL
!if BROKEN {
; PROGRESS = 0
    lda #0
    sta PROGRESS
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise 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

custom.asm

PROGRESS=$80
SYMBOL=$81
MATCHED=$82
VALID=$83
T0=$84
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 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

wrong.asm

PROGRESS=$80
SYMBOL=$81
MATCHED=$82
VALID=$83
T0=$84
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 00 02 00 01 00 00 00 00. — The wrong client completes with 00 02 00 01 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 155 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 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

Input repeat: Choose whether a sequence receives initial presses or repeated events; a held snapshot is not itself a stream of new actions.

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