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:
ld a,0
ld (COLOUR),a
ld (TARGET),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ld (T4),a
ld (T5),a
ret
packed_colour_step_step:
; T0 = COLOUR >> 5
ld a,(COLOUR)
srl a
srl a
srl a
srl a
srl a
ld (T0),a
; T1 = TARGET >> 5
ld a,(TARGET)
srl a
srl a
srl a
srl a
srl a
ld (T1),a
; if T0 < T1:
ld a,(T0)
ld b,a
ld a,(T1)
ld c,a
ld a,b
cp c
jp c,packed_colour_step_L1
jp packed_colour_step_L2
packed_colour_step_L1:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp packed_colour_step_L3
packed_colour_step_L2:
packed_colour_step_L3:
; if T0 > T1:
ld a,(T0)
ld b,a
ld a,(T1)
ld c,a
ld a,b
cp c
jp z,packed_colour_step_L5
jp nc,packed_colour_step_L4
jp packed_colour_step_L5
packed_colour_step_L4:
; T0 = T0 - 1
ld a,(T0)
sub 1
ld (T0),a
jp packed_colour_step_L6
packed_colour_step_L5:
packed_colour_step_L6:
; T2 = COLOUR >> 2
ld a,(COLOUR)
srl a
srl a
ld (T2),a
; T2 = T2 & 7
ld a,(T2)
and 7
ld (T2),a
; T3 = TARGET >> 2
ld a,(TARGET)
srl a
srl a
ld (T3),a
; T3 = T3 & 7
ld a,(T3)
and 7
ld (T3),a
; if T2 < T3:
ld a,(T2)
ld b,a
ld a,(T3)
ld c,a
ld a,b
cp c
jp c,packed_colour_step_L7
jp packed_colour_step_L8
packed_colour_step_L7:
; T2 = T2 + 1
ld a,(T2)
add a,1
ld (T2),a
jp packed_colour_step_L9
packed_colour_step_L8:
packed_colour_step_L9:
; if T2 > T3:
ld a,(T2)
ld b,a
ld a,(T3)
ld c,a
ld a,b
cp c
jp z,packed_colour_step_L11
jp nc,packed_colour_step_L10
jp packed_colour_step_L11
packed_colour_step_L10:
; T2 = T2 - 1
ld a,(T2)
sub 1
ld (T2),a
jp packed_colour_step_L12
packed_colour_step_L11:
packed_colour_step_L12:
; T4 = COLOUR & 3
ld a,(COLOUR)
and 3
ld (T4),a
; T5 = TARGET & 3
ld a,(TARGET)
and 3
ld (T5),a
; if T4 < T5:
ld a,(T4)
ld b,a
ld a,(T5)
ld c,a
ld a,b
cp c
jp c,packed_colour_step_L13
jp packed_colour_step_L14
packed_colour_step_L13:
; T4 = T4 + 1
ld a,(T4)
add a,1
ld (T4),a
jp packed_colour_step_L15
packed_colour_step_L14:
packed_colour_step_L15:
; if T4 > T5:
ld a,(T4)
ld b,a
ld a,(T5)
ld c,a
ld a,b
cp c
jp z,packed_colour_step_L17
jp nc,packed_colour_step_L16
jp packed_colour_step_L17
packed_colour_step_L16:
; T4 = T4 - 1
ld a,(T4)
sub 1
ld (T4),a
jp packed_colour_step_L18
packed_colour_step_L17:
packed_colour_step_L18:
; COLOUR = T0 << 5
ld a,(T0)
sla a
sla a
sla a
sla a
sla a
ld (COLOUR),a
; T2 = T2 << 2
ld a,(T2)
sla a
sla a
ld (T2),a
; COLOUR = COLOUR | T2
ld a,(COLOUR)
ld b,a
ld a,(T2)
ld c,a
ld a,b
or c
ld (COLOUR),a
; COLOUR = COLOUR | T4
ld a,(COLOUR)
ld b,a
ld a,(T4)
ld c,a
ld a,b
or c
ld (COLOUR),a
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. 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 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:
COLOUR: defb 0
TARGET: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: 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
; COLOUR = 255
ld a,255
ld (COLOUR),a
; TARGET = 0
ld a,0
ld (TARGET),a
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
ret
else
; TARGET = 255
ld a,255
ld (TARGET),a
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
if BROKEN
; COLOUR = 0
ld a,0
ld (COLOUR),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
COLOUR: defb 0
TARGET: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: 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 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
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:
COLOUR: defb 0
TARGET: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
T5: 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 335 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 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
Approach a target: Apply a bounded approach independently to meaningful components rather than to a packed byte.