Code Like It's 198x

Read and Update Packed Four-bit Cells on NES

Sixteen tile values do not need sixteen whole bytes when each value fits in four bits.

Sixteen tile values do not need sixteen whole bytes when each value fits in four bits.

NES shows the working diagnostic row 0A 01 02 00 05 00 A5 00.
A completed native caller displays 0A 01 02 00 05 00 A5 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

Sixteen cells occupy eight caller-owned bytes: an even cell uses the high nibble and the following odd cell the low nibble. CELL must be 0–15. WRITE=0 reads; nonzero writes NEW_VALUE 0–15 and returns that value. Invalid requests return VALUE=0 and VALID=0 without changing packed bytes. An unused NEW_VALUE is ignored on reads.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, VALID, CELL, WRITE, NEW_VALUE, B0, B1, B2, B3, B4, B5, B6, B7. They occupy 13 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, EXPR_SAVE (5 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call packed_nibble_map_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_nibble_map_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 B0–B7
Supply before service CELL, WRITE and NEW_VALUE
Read after service VALUE and VALID

Shift the cell index once to select its byte, then use the low index bit to select a nibble. A write masks only the chosen half and preserves its neighbour.

Reusable initialisation and service

6502.inc
; SPDX-License-Identifier: MIT
; Read and Update Packed Four-bit Cells. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
packed_nibble_map_init:
    lda #0
    sta VALUE
    sta VALID
    sta CELL
    sta WRITE
    sta NEW_VALUE
    sta B0
    sta B1
    sta B2
    sta B3
    sta B4
    sta B5
    sta B6
    sta B7
    sta T0
    sta T1
    sta T2
    sta T3
    sta EXPR_SAVE
    rts
packed_nibble_map_step:
; VALUE = 0
    lda #0
    sta VALUE
; VALID = 0
    lda #0
    sta VALID
; if CELL >= 16:
    lda CELL
    cmp #16
    bcs packed_nibble_map_L4
packed_nibble_map_L5:
    jmp packed_nibble_map_L2
packed_nibble_map_L4:
    jmp packed_nibble_map_L1
packed_nibble_map_L1:
; return
    rts
    jmp packed_nibble_map_L3
packed_nibble_map_L2:
packed_nibble_map_L3:
; if WRITE != 0:
    lda WRITE
    cmp #0
    bne packed_nibble_map_L9
packed_nibble_map_L10:
    jmp packed_nibble_map_L7
packed_nibble_map_L9:
    jmp packed_nibble_map_L6
packed_nibble_map_L6:
; if NEW_VALUE >= 16:
    lda NEW_VALUE
    cmp #16
    bcs packed_nibble_map_L14
packed_nibble_map_L15:
    jmp packed_nibble_map_L12
packed_nibble_map_L14:
    jmp packed_nibble_map_L11
packed_nibble_map_L11:
; return
    rts
    jmp packed_nibble_map_L13
packed_nibble_map_L12:
packed_nibble_map_L13:
    jmp packed_nibble_map_L8
packed_nibble_map_L7:
packed_nibble_map_L8:
; T0 = CELL >> 1
    lda CELL
    lsr
    sta T0
; T1 = CELL & 1
    lda CELL
    and #1
    sta T1
; T2 = B[T0]
    ldx T0
    lda B0,x
    sta T2
; if WRITE != 0:
    lda WRITE
    cmp #0
    bne packed_nibble_map_L19
packed_nibble_map_L20:
    jmp packed_nibble_map_L17
packed_nibble_map_L19:
    jmp packed_nibble_map_L16
packed_nibble_map_L16:
; if T1 == 0:
    lda T1
    cmp #0
    beq packed_nibble_map_L24
packed_nibble_map_L25:
    jmp packed_nibble_map_L22
packed_nibble_map_L24:
    jmp packed_nibble_map_L21
packed_nibble_map_L21:
; T2 = T2 & 15
    lda T2
    and #15
    sta T2
; T3 = NEW_VALUE << 4
    lda NEW_VALUE
    asl
    asl
    asl
    asl
    sta T3
; T2 = T2 | T3
    lda T2
    ora T3
    sta T2
    jmp packed_nibble_map_L23
packed_nibble_map_L22:
; T2 = T2 & 240
    lda T2
    and #240
    sta T2
; T2 = T2 | NEW_VALUE
    lda T2
    ora NEW_VALUE
    sta T2
packed_nibble_map_L23:
; B[T0] = T2
    lda T2
    sta EXPR_SAVE
    ldx T0
    lda EXPR_SAVE
    sta B0,x
    jmp packed_nibble_map_L18
packed_nibble_map_L17:
packed_nibble_map_L18:
; if T1 == 0:
    lda T1
    cmp #0
    beq packed_nibble_map_L29
packed_nibble_map_L30:
    jmp packed_nibble_map_L27
packed_nibble_map_L29:
    jmp packed_nibble_map_L26
packed_nibble_map_L26:
; VALUE = T2 >> 4
    lda T2
    lsr
    lsr
    lsr
    lsr
    sta VALUE
    jmp packed_nibble_map_L28
packed_nibble_map_L27:
; VALUE = T2 & 15
    lda T2
    and #15
    sta VALUE
packed_nibble_map_L28:
; VALID = 1
    lda #1
    sta VALID
    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
VALUE=$80
VALID=$81
CELL=$82
WRITE=$83
NEW_VALUE=$84
B0=$85
B1=$86
B2=$87
B3=$88
B4=$89
B5=$8a
B6=$8b
B7=$8c
T0=$8d
T1=$8e
T2=$8f
T3=$90
EXPR_SAVE=$91
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
; CELL = 15
    lda #15
    sta CELL
; WRITE = 1
    lda #1
    sta WRITE
; NEW_VALUE = 15
    lda #15
    sta NEW_VALUE
.if BROKEN
; CELL = 0
    lda #0
    sta CELL
.endif
    jsr checked_step
    jsr emit_state
; WRITE = 0
    lda #0
    sta WRITE
.if BROKEN
; CELL = 0
    lda #0
    sta CELL
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; B1 = 171
    lda #171
    sta B1
; CELL = 3
    lda #3
    sta CELL
.if BROKEN
; CELL = 0
    lda #0
    sta CELL
.endif
    jsr checked_step
    jsr emit_state
; WRITE = 1
    lda #1
    sta WRITE
; NEW_VALUE = 5
    lda #5
    sta NEW_VALUE
.if BROKEN
; CELL = 0
    lda #0
    sta CELL
.endif
    jsr checked_step
    jsr emit_state
; WRITE = 0
    lda #0
    sta WRITE
; CELL = 2
    lda #2
    sta CELL
.if BROKEN
; CELL = 0
    lda #0
    sta CELL
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Write and read the final low nibble. 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
VALUE=$80
VALID=$81
CELL=$82
WRITE=$83
NEW_VALUE=$84
B0=$85
B1=$86
B2=$87
B3=$88
B4=$89
B5=$8a
B6=$8b
B7=$8c
T0=$8d
T1=$8e
T2=$8f
T3=$90
EXPR_SAVE=$91
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 0F 01 0F 00 0F 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller passes cell zero instead of the requested cell. Reads look plausible and writes succeed, but the wrong map location changes. Keep the logical cell index separate from its packed-byte offset.

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
VALUE=$80
VALID=$81
CELL=$82
WRITE=$83
NEW_VALUE=$84
B0=$85
B1=$86
B2=$87
B3=$88
B4=$89
B5=$8a
B6=$8b
B7=$8c
T0=$8d
T1=$8e
T2=$8f
T3=$90
EXPR_SAVE=$91
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 05 01 00 00 05 50 AB 00.
The wrong client completes with 05 01 00 00 05 50 AB 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 217 code bytes; public state is 13 bytes and private scratch is 5 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, with masks and shifts in place of direct byte indexing. Byte-per-cell maps are easier to edit and can hold more values; packing pays when memory is tighter than update time.

This halves storage only when values fit 0–15. Packed writes are read-modify-write operations; an interrupt must not update the same byte concurrently without an ownership policy. Rendering and collision properties remain separate.

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

Tile maps: The same logical map queries can sit over different storage representations, provided their value range stays explicit.

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