Step Towards a Packed Colour on NES | Pattern Library

A fade can approach its target one channel step at a time instead of treating a packed colour as a single number.

NES shows the working diagnostic row 93 FF 00 00 00 00 00 00. — A completed native caller displays 93 FF 00 00 00 00 00 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

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

6502.inc

; 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:
    lda #0
    sta COLOUR
    sta TARGET
    sta T0
    sta T1
    sta T2
    sta T3
    sta T4
    sta T5
    rts
packed_colour_step_step:
; T0 = COLOUR >> 5
    lda COLOUR
    lsr
    lsr
    lsr
    lsr
    lsr
    sta T0
; T1 = TARGET >> 5
    lda TARGET
    lsr
    lsr
    lsr
    lsr
    lsr
    sta T1
; if T0 < T1:
    lda T0
    cmp T1
    bcc packed_colour_step_L4
packed_colour_step_L5:
    jmp packed_colour_step_L2
packed_colour_step_L4:
    jmp packed_colour_step_L1
packed_colour_step_L1:
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp packed_colour_step_L3
packed_colour_step_L2:
packed_colour_step_L3:
; if T0 > T1:
    lda T0
    cmp T1
    beq packed_colour_step_L10
    bcs packed_colour_step_L9
packed_colour_step_L10:
    jmp packed_colour_step_L7
packed_colour_step_L9:
    jmp packed_colour_step_L6
packed_colour_step_L6:
; T0 = T0 - 1
    lda T0
    sec
    sbc #1
    sta T0
    jmp packed_colour_step_L8
packed_colour_step_L7:
packed_colour_step_L8:
; T2 = COLOUR >> 2
    lda COLOUR
    lsr
    lsr
    sta T2
; T2 = T2 & 7
    lda T2
    and #7
    sta T2
; T3 = TARGET >> 2
    lda TARGET
    lsr
    lsr
    sta T3
; T3 = T3 & 7
    lda T3
    and #7
    sta T3
; if T2 < T3:
    lda T2
    cmp T3
    bcc packed_colour_step_L14
packed_colour_step_L15:
    jmp packed_colour_step_L12
packed_colour_step_L14:
    jmp packed_colour_step_L11
packed_colour_step_L11:
; T2 = T2 + 1
    lda T2
    clc
    adc #1
    sta T2
    jmp packed_colour_step_L13
packed_colour_step_L12:
packed_colour_step_L13:
; if T2 > T3:
    lda T2
    cmp T3
    beq packed_colour_step_L20
    bcs packed_colour_step_L19
packed_colour_step_L20:
    jmp packed_colour_step_L17
packed_colour_step_L19:
    jmp packed_colour_step_L16
packed_colour_step_L16:
; T2 = T2 - 1
    lda T2
    sec
    sbc #1
    sta T2
    jmp packed_colour_step_L18
packed_colour_step_L17:
packed_colour_step_L18:
; T4 = COLOUR & 3
    lda COLOUR
    and #3
    sta T4
; T5 = TARGET & 3
    lda TARGET
    and #3
    sta T5
; if T4 < T5:
    lda T4
    cmp T5
    bcc packed_colour_step_L24
packed_colour_step_L25:
    jmp packed_colour_step_L22
packed_colour_step_L24:
    jmp packed_colour_step_L21
packed_colour_step_L21:
; T4 = T4 + 1
    lda T4
    clc
    adc #1
    sta T4
    jmp packed_colour_step_L23
packed_colour_step_L22:
packed_colour_step_L23:
; if T4 > T5:
    lda T4
    cmp T5
    beq packed_colour_step_L30
    bcs packed_colour_step_L29
packed_colour_step_L30:
    jmp packed_colour_step_L27
packed_colour_step_L29:
    jmp packed_colour_step_L26
packed_colour_step_L26:
; T4 = T4 - 1
    lda T4
    sec
    sbc #1
    sta T4
    jmp packed_colour_step_L28
packed_colour_step_L27:
packed_colour_step_L28:
; COLOUR = T0 << 5
    lda T0
    asl
    asl
    asl
    asl
    asl
    sta COLOUR
; T2 = T2 << 2
    lda T2
    asl
    asl
    sta T2
; COLOUR = COLOUR | T2
    lda COLOUR
    ora T2
    sta COLOUR
; COLOUR = COLOUR | T4
    lda COLOUR
    ora T4
    sta COLOUR
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
COLOUR=$80
TARGET=$81
T0=$82
T1=$83
T2=$84
T3=$85
T4=$86
T5=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
.if CUSTOM
; COLOUR = 255
    lda #255
    sta COLOUR
; TARGET = 0
    lda #0
    sta TARGET
.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; TARGET = 255
    lda #255
    sta TARGET
.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; COLOUR = 0
    lda #0
    sta COLOUR
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise 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

custom.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
COLOUR=$80
TARGET=$81
T0=$82
T1=$83
T2=$84
T3=$85
T4=$86
T5=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

The 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

wrong.asm

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
COLOUR=$80
TARGET=$81
T0=$82
T1=$83
T2=$84
T3=$85
T4=$86
T5=$87
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

NES shows the deliberate error row 25 FF 00 00 00 00 00 00. — The wrong client completes with 25 FF 00 00 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 239 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 NTSC NROM, background only, DMC/NMI disabled, 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.

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