A held pose and a running animation need different clocks. Give each frame time to be seen.
Use it in a project
Set FRAME_COUNT to 1–255 and DURATION to 1–255. FRAME must be below FRAME_COUNT and TICKS below DURATION. Each call consumes one tick. LOOP is a boolean: nonzero wraps; zero holds the final frame and sets FINISHED when its last interval expires. FINISHED remains set until reset. Invalid state sets VALID to zero and leaves the clock alone.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: FRAME, TICKS, FRAME_COUNT, DURATION, LOOP, FINISHED, VALID. They occupy 7 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 animation_timeline_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 animation_timeline_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 | FRAME, TICKS and FINISHED |
| Supply before service | FRAME_COUNT, DURATION and LOOP |
| Read after service | VALID |
The timer is local to an animation instance. Its frame interval expires before the frame index changes. Keeping that distinction makes a looping walk and a held final death pose use the same helper.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Advance a Timed Animation. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
animation_timeline_init:
lda #0
sta FRAME
sta TICKS
sta FRAME_COUNT
sta DURATION
sta LOOP
sta FINISHED
sta VALID
sta T0
rts
animation_timeline_step:
; VALID = 0
lda #0
sta VALID
; if FRAME_COUNT == 0:
lda FRAME_COUNT
cmp #0
beq animation_timeline_L4
animation_timeline_L5:
jmp animation_timeline_L2
animation_timeline_L4:
jmp animation_timeline_L1
animation_timeline_L1:
; return
rts
jmp animation_timeline_L3
animation_timeline_L2:
animation_timeline_L3:
; if DURATION == 0:
lda DURATION
cmp #0
beq animation_timeline_L9
animation_timeline_L10:
jmp animation_timeline_L7
animation_timeline_L9:
jmp animation_timeline_L6
animation_timeline_L6:
; return
rts
jmp animation_timeline_L8
animation_timeline_L7:
animation_timeline_L8:
; if FRAME >= FRAME_COUNT:
lda FRAME
cmp FRAME_COUNT
bcs animation_timeline_L14
animation_timeline_L15:
jmp animation_timeline_L12
animation_timeline_L14:
jmp animation_timeline_L11
animation_timeline_L11:
; return
rts
jmp animation_timeline_L13
animation_timeline_L12:
animation_timeline_L13:
; if TICKS >= DURATION:
lda TICKS
cmp DURATION
bcs animation_timeline_L19
animation_timeline_L20:
jmp animation_timeline_L17
animation_timeline_L19:
jmp animation_timeline_L16
animation_timeline_L16:
; return
rts
jmp animation_timeline_L18
animation_timeline_L17:
animation_timeline_L18:
; VALID = 1
lda #1
sta VALID
; if FINISHED != 0:
lda FINISHED
cmp #0
bne animation_timeline_L24
animation_timeline_L25:
jmp animation_timeline_L22
animation_timeline_L24:
jmp animation_timeline_L21
animation_timeline_L21:
; return
rts
jmp animation_timeline_L23
animation_timeline_L22:
animation_timeline_L23:
; TICKS = TICKS + 1
lda TICKS
clc
adc #1
sta TICKS
; if TICKS < DURATION:
lda TICKS
cmp DURATION
bcc animation_timeline_L29
animation_timeline_L30:
jmp animation_timeline_L27
animation_timeline_L29:
jmp animation_timeline_L26
animation_timeline_L26:
; return
rts
jmp animation_timeline_L28
animation_timeline_L27:
animation_timeline_L28:
; TICKS = 0
lda #0
sta TICKS
; T0 = FRAME_COUNT - 1
lda FRAME_COUNT
sec
sbc #1
sta T0
; if FRAME < T0:
lda FRAME
cmp T0
bcc animation_timeline_L34
animation_timeline_L35:
jmp animation_timeline_L32
animation_timeline_L34:
jmp animation_timeline_L31
animation_timeline_L31:
; FRAME = FRAME + 1
lda FRAME
clc
adc #1
sta FRAME
; return
rts
jmp animation_timeline_L33
animation_timeline_L32:
animation_timeline_L33:
; if LOOP != 0:
lda LOOP
cmp #0
bne animation_timeline_L39
animation_timeline_L40:
jmp animation_timeline_L37
animation_timeline_L39:
jmp animation_timeline_L36
animation_timeline_L36:
; FRAME = 0
lda #0
sta FRAME
jmp animation_timeline_L38
animation_timeline_L37:
; FINISHED = 1
lda #1
sta FINISHED
animation_timeline_L38:
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
FRAME=$80
TICKS=$81
FRAME_COUNT=$82
DURATION=$83
LOOP=$84
FINISHED=$85
VALID=$86
T0=$87
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
preview_rules:
!if CUSTOM {
; FRAME_COUNT = 3
lda #3
sta FRAME_COUNT
; DURATION = 2
lda #2
sta DURATION
; LOOP = 1
lda #1
sta LOOP
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
rts
} else {
; FRAME_COUNT = 3
lda #3
sta FRAME_COUNT
; DURATION = 3
lda #3
sta DURATION
; LOOP = 1
lda #1
sta LOOP
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
!if BROKEN {
; TICKS = 0
lda #0
sta TICKS
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Use two ticks per frame instead of three. 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
FRAME=$80
TICKS=$81
FRAME_COUNT=$82
DURATION=$83
LOOP=$84
FINISHED=$85
VALID=$86
T0=$87
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 03 02 01 00 01 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller resets TICKS before every update. The frame never receives its full duration. Reset the clock at an animation boundary, then retain it between updates.
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
FRAME=$80
TICKS=$81
FRAME_COUNT=$82
DURATION=$83
LOOP=$84
FINISHED=$85
VALID=$86
T0=$87
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:
Cost and alternatives
The measured helper extent is 188 code bytes; public state is 7 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 per service. A table of per-frame durations costs data space but supports holds and accents within one animation.
This selects a logical frame; upload or draw it through the display backend. A duration table or animation event track is a useful later extension. The helper never measures wall time.
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
Action cooldowns: Both retain elapsed ticks, but an animation owns a frame cursor as well as a timer.