Service a Timed Status Effect on C64 | Pattern Library

A poison effect needs a duration and a pulse interval, not a fresh timer every time the actor is updated.

C64 shows the working diagnostic row 00 00 00 05 02 00 01 00. — A completed native caller displays 00 00 00 05 02 00 01 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

PERIOD must be nonzero and PHASE below it. APPLY nonzero refreshes REMAINING from DURATION and restarts phase. The same service call consumes the first active tick. Each active tick advances PHASE, emitting PULSE=1 on a period boundary. Expiration clears PHASE; a pulse on the final active tick is allowed. Invalid configuration returns VALID=0 and PULSE=0 without changing timer state.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: PULSE, REMAINING, PHASE, DURATION, PERIOD, APPLY, VALID. They occupy 7 contiguous bytes. Also reserve the private workspace: none (0 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call timed_status_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 timed_status_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	REMAINING and PHASE
Supply before service	DURATION, PERIOD and APPLY
Read after service	PULSE and VALID

Duration bounds the lifetime while phase measures progress within a pulse interval. Keeping those clocks separate avoids making the last partial interval extend the effect forever.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Service a Timed Status Effect. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
timed_status_init:
    lda #0
    sta PULSE
    sta REMAINING
    sta PHASE
    sta DURATION
    sta PERIOD
    sta APPLY
    sta VALID
    rts
timed_status_step:
; PULSE = 0
    lda #0
    sta PULSE
; VALID = 0
    lda #0
    sta VALID
; if PERIOD == 0:
    lda PERIOD
    cmp #0
    beq timed_status_L4
timed_status_L5:
    jmp timed_status_L2
timed_status_L4:
    jmp timed_status_L1
timed_status_L1:
; return
    rts
    jmp timed_status_L3
timed_status_L2:
timed_status_L3:
; if PHASE >= PERIOD:
    lda PHASE
    cmp PERIOD
    bcs timed_status_L9
timed_status_L10:
    jmp timed_status_L7
timed_status_L9:
    jmp timed_status_L6
timed_status_L6:
; return
    rts
    jmp timed_status_L8
timed_status_L7:
timed_status_L8:
; VALID = 1
    lda #1
    sta VALID
; if APPLY != 0:
    lda APPLY
    cmp #0
    bne timed_status_L14
timed_status_L15:
    jmp timed_status_L12
timed_status_L14:
    jmp timed_status_L11
timed_status_L11:
; REMAINING = DURATION
    lda DURATION
    sta REMAINING
; PHASE = 0
    lda #0
    sta PHASE
    jmp timed_status_L13
timed_status_L12:
timed_status_L13:
; if REMAINING == 0:
    lda REMAINING
    cmp #0
    beq timed_status_L19
timed_status_L20:
    jmp timed_status_L17
timed_status_L19:
    jmp timed_status_L16
timed_status_L16:
; PHASE = 0
    lda #0
    sta PHASE
; return
    rts
    jmp timed_status_L18
timed_status_L17:
timed_status_L18:
; REMAINING = REMAINING - 1
    lda REMAINING
    sec
    sbc #1
    sta REMAINING
; PHASE = PHASE + 1
    lda PHASE
    clc
    adc #1
    sta PHASE
; if PHASE == PERIOD:
    lda PHASE
    cmp PERIOD
    beq timed_status_L24
timed_status_L25:
    jmp timed_status_L22
timed_status_L24:
    jmp timed_status_L21
timed_status_L21:
; PHASE = 0
    lda #0
    sta PHASE
; PULSE = 1
    lda #1
    sta PULSE
    jmp timed_status_L23
timed_status_L22:
timed_status_L23:
; if REMAINING == 0:
    lda REMAINING
    cmp #0
    beq timed_status_L29
timed_status_L30:
    jmp timed_status_L27
timed_status_L29:
    jmp timed_status_L26
timed_status_L26:
; PHASE = 0
    lda #0
    sta PHASE
    jmp timed_status_L28
timed_status_L27:
timed_status_L28:
    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. The samples use fixed symbol bindings. Scope or rename every label and binding, including the component_end measurement marker, and allocate a separate block for each independent instance. Different helpers can also use common field names, so combining them needs separate namespaces or explicitly renamed bindings. 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

PULSE=$80
REMAINING=$81
PHASE=$82
DURATION=$83
PERIOD=$84
APPLY=$85
VALID=$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

exercise.inc, excerpt

preview_rules:
!if CUSTOM {
; DURATION = 6
    lda #6
    sta DURATION
; PERIOD = 3
    lda #3
    sta PERIOD
; APPLY = 1
    lda #1
    sta APPLY
!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state
; APPLY = 0
    lda #0
    sta APPLY
!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; DURATION = 5
    lda #5
    sta DURATION
; PERIOD = 2
    lda #2
    sta PERIOD
; APPLY = 1
    lda #1
    sta APPLY
!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state
; APPLY = 0
    lda #0
    sta APPLY
!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; APPLY = 1
    lda #1
    sta APPLY
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Use a six-tick duration with one pulse every three ticks. 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

PULSE=$80
REMAINING=$81
PHASE=$82
DURATION=$83
PERIOD=$84
APPLY=$85
VALID=$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 01 00 00 06 03 00 01 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller keeps APPLY set. Every call refreshes the effect and restarts its interval, preventing the expected pulses and expiry. Feed an application event once, then service retained state.

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

PULSE=$80
REMAINING=$81
PHASE=$82
DURATION=$83
PERIOD=$84
APPLY=$85
VALID=$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:

C64 shows the deliberate error row 00 04 01 05 02 01 01 00. — The wrong client completes with 00 04 01 05 02 01 01 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 161 code bytes; public state is 7 bytes and private scratch is 0 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 per service. Multiple independent effects need separate state or a bounded effect table; sharing one timer silently merges their lifetimes.

PULSE is an event; a damage routine decides what it means for health and invulnerability. This stores only one effect instance and refreshes rather than stacks it. A paused game must choose whether to service the effect.

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

Damage rules: A periodic pulse can request damage without taking ownership of health or its protection policy.

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