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:
ld a,0
ld (FRAME),a
ld (TICKS),a
ld (FRAME_COUNT),a
ld (DURATION),a
ld (LOOP),a
ld (FINISHED),a
ld (VALID),a
ld (T0),a
ret
animation_timeline_step:
; VALID = 0
ld a,0
ld (VALID),a
; if FRAME_COUNT == 0:
ld a,(FRAME_COUNT)
cp 0
jp z,animation_timeline_L1
jp animation_timeline_L2
animation_timeline_L1:
; return
ret
jp animation_timeline_L3
animation_timeline_L2:
animation_timeline_L3:
; if DURATION == 0:
ld a,(DURATION)
cp 0
jp z,animation_timeline_L4
jp animation_timeline_L5
animation_timeline_L4:
; return
ret
jp animation_timeline_L6
animation_timeline_L5:
animation_timeline_L6:
; if FRAME >= FRAME_COUNT:
ld a,(FRAME)
ld b,a
ld a,(FRAME_COUNT)
ld c,a
ld a,b
cp c
jp nc,animation_timeline_L7
jp animation_timeline_L8
animation_timeline_L7:
; return
ret
jp animation_timeline_L9
animation_timeline_L8:
animation_timeline_L9:
; if TICKS >= DURATION:
ld a,(TICKS)
ld b,a
ld a,(DURATION)
ld c,a
ld a,b
cp c
jp nc,animation_timeline_L10
jp animation_timeline_L11
animation_timeline_L10:
; return
ret
jp animation_timeline_L12
animation_timeline_L11:
animation_timeline_L12:
; VALID = 1
ld a,1
ld (VALID),a
; if FINISHED != 0:
ld a,(FINISHED)
cp 0
jp nz,animation_timeline_L13
jp animation_timeline_L14
animation_timeline_L13:
; return
ret
jp animation_timeline_L15
animation_timeline_L14:
animation_timeline_L15:
; TICKS = TICKS + 1
ld a,(TICKS)
add a,1
ld (TICKS),a
; if TICKS < DURATION:
ld a,(TICKS)
ld b,a
ld a,(DURATION)
ld c,a
ld a,b
cp c
jp c,animation_timeline_L16
jp animation_timeline_L17
animation_timeline_L16:
; return
ret
jp animation_timeline_L18
animation_timeline_L17:
animation_timeline_L18:
; TICKS = 0
ld a,0
ld (TICKS),a
; T0 = FRAME_COUNT - 1
ld a,(FRAME_COUNT)
sub 1
ld (T0),a
; if FRAME < T0:
ld a,(FRAME)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp c,animation_timeline_L19
jp animation_timeline_L20
animation_timeline_L19:
; FRAME = FRAME + 1
ld a,(FRAME)
add a,1
ld (FRAME),a
; return
ret
jp animation_timeline_L21
animation_timeline_L20:
animation_timeline_L21:
; if LOOP != 0:
ld a,(LOOP)
cp 0
jp nz,animation_timeline_L22
jp animation_timeline_L23
animation_timeline_L22:
; FRAME = 0
ld a,0
ld (FRAME),a
jp animation_timeline_L24
animation_timeline_L23:
; FINISHED = 1
ld a,1
ld (FINISHED),a
animation_timeline_L24:
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:
FRAME: defb 0
TICKS: defb 0
FRAME_COUNT: defb 0
DURATION: defb 0
LOOP: defb 0
FINISHED: 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
; FRAME_COUNT = 3
ld a,3
ld (FRAME_COUNT),a
; DURATION = 2
ld a,2
ld (DURATION),a
; LOOP = 1
ld a,1
ld (LOOP),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
ret
else
; FRAME_COUNT = 3
ld a,3
ld (FRAME_COUNT),a
; DURATION = 3
ld a,3
ld (DURATION),a
; LOOP = 1
ld a,1
ld (LOOP),a
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
if BROKEN
; TICKS = 0
ld a,0
ld (TICKS),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
FRAME: defb 0
TICKS: defb 0
FRAME_COUNT: defb 0
DURATION: defb 0
LOOP: defb 0
FINISHED: 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 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
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:
FRAME: defb 0
TICKS: defb 0
FRAME_COUNT: defb 0
DURATION: defb 0
LOOP: defb 0
FINISHED: 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 216 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 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
Action cooldowns: Both retain elapsed ticks, but an animation owns a frame cursor as well as a timer.