A fade can approach its target one channel step at a time instead of treating a packed colour as a single number.
Use it in a project
COLOUR and TARGET use logical RGB332: red bits 7–5, green 4–2, blue 1–0. Move each channel by at most one towards its target and write COLOUR. A call at the target is idempotent. No hardware palette register is written.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: COLOUR, TARGET. They occupy 2 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5 (6 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call packed_colour_step_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 packed_colour_step_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 | COLOUR |
| Supply before service | TARGET |
| Read after service | updated COLOUR |
Unpack each channel, approach its target independently, then repack. The completion time is the greatest initial channel difference.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Step Towards a Packed Colour. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
packed_colour_step_init:
move.b #0,d0
move.b d0,COLOUR
move.b d0,TARGET
move.b d0,T0
move.b d0,T1
move.b d0,T2
move.b d0,T3
move.b d0,T4
move.b d0,T5
rts
packed_colour_step_step:
; T0 = COLOUR >> 5
move.b COLOUR,d0
lsr.b #5,d0
move.b d0,T0
; T1 = TARGET >> 5
move.b TARGET,d0
lsr.b #5,d0
move.b d0,T1
; if T0 < T1:
move.b T0,d0
move.b T1,d1
cmp.b d1,d0
bcs packed_colour_step_L1
jmp packed_colour_step_L2
packed_colour_step_L1:
; T0 = T0 + 1
move.b T0,d0
add.b #1,d0
move.b d0,T0
jmp packed_colour_step_L3
packed_colour_step_L2:
packed_colour_step_L3:
; if T0 > T1:
move.b T0,d0
move.b T1,d1
cmp.b d1,d0
bhi packed_colour_step_L4
jmp packed_colour_step_L5
packed_colour_step_L4:
; T0 = T0 - 1
move.b T0,d0
sub.b #1,d0
move.b d0,T0
jmp packed_colour_step_L6
packed_colour_step_L5:
packed_colour_step_L6:
; T2 = COLOUR >> 2
move.b COLOUR,d0
lsr.b #2,d0
move.b d0,T2
; T2 = T2 & 7
move.b T2,d0
and.b #7,d0
move.b d0,T2
; T3 = TARGET >> 2
move.b TARGET,d0
lsr.b #2,d0
move.b d0,T3
; T3 = T3 & 7
move.b T3,d0
and.b #7,d0
move.b d0,T3
; if T2 < T3:
move.b T2,d0
move.b T3,d1
cmp.b d1,d0
bcs packed_colour_step_L7
jmp packed_colour_step_L8
packed_colour_step_L7:
; T2 = T2 + 1
move.b T2,d0
add.b #1,d0
move.b d0,T2
jmp packed_colour_step_L9
packed_colour_step_L8:
packed_colour_step_L9:
; if T2 > T3:
move.b T2,d0
move.b T3,d1
cmp.b d1,d0
bhi packed_colour_step_L10
jmp packed_colour_step_L11
packed_colour_step_L10:
; T2 = T2 - 1
move.b T2,d0
sub.b #1,d0
move.b d0,T2
jmp packed_colour_step_L12
packed_colour_step_L11:
packed_colour_step_L12:
; T4 = COLOUR & 3
move.b COLOUR,d0
and.b #3,d0
move.b d0,T4
; T5 = TARGET & 3
move.b TARGET,d0
and.b #3,d0
move.b d0,T5
; if T4 < T5:
move.b T4,d0
move.b T5,d1
cmp.b d1,d0
bcs packed_colour_step_L13
jmp packed_colour_step_L14
packed_colour_step_L13:
; T4 = T4 + 1
move.b T4,d0
add.b #1,d0
move.b d0,T4
jmp packed_colour_step_L15
packed_colour_step_L14:
packed_colour_step_L15:
; if T4 > T5:
move.b T4,d0
move.b T5,d1
cmp.b d1,d0
bhi packed_colour_step_L16
jmp packed_colour_step_L17
packed_colour_step_L16:
; T4 = T4 - 1
move.b T4,d0
sub.b #1,d0
move.b d0,T4
jmp packed_colour_step_L18
packed_colour_step_L17:
packed_colour_step_L18:
; COLOUR = T0 << 5
move.b T0,d0
lsl.b #5,d0
move.b d0,COLOUR
; T2 = T2 << 2
move.b T2,d0
lsl.b #2,d0
move.b d0,T2
; COLOUR = COLOUR | T2
move.b COLOUR,d0
move.b T2,d1
or.b d1,d0
move.b d0,COLOUR
; COLOUR = COLOUR | T4
move.b COLOUR,d0
move.b T4,d1
or.b d1,d0
move.b d0,COLOUR
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
COLOUR: dc.b 0
TARGET: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; COLOUR = 255
move.b #255,d0
move.b d0,COLOUR
; TARGET = 0
move.b #0,d0
move.b d0,TARGET
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
rts
else
; TARGET = 255
move.b #255,d0
move.b d0,TARGET
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; COLOUR = 0
move.b #0,d0
move.b d0,COLOUR
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Fade all channels down instead of up. 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
section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
COLOUR: dc.b 0
TARGET: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The custom diagnostic row is 6C 00 00 00 00 00 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller resets the current colour before every step, so the fade never progresses beyond its first increment. Retain COLOUR between ticks.
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
section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
COLOUR: dc.b 0
TARGET: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 408 code bytes; public state is 2 bytes and private scratch is 6 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 arithmetic. Precomputed hardware palette sequences can give better-looking fades and reduce runtime at the cost of data.
RGB332 is a logical teaching format, not the native palette encoding of these machines. C64, Spectrum and NES need a chosen palette-index mapping; Amiga RGB444 conversion needs channel scaling. Hardware colours are not perceptually uniform.
Execution evidence
Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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
Approach a target: Apply a bounded approach independently to meaningful components rather than to a packed byte.