Look Up a Bounded Byte Dictionary on NES | Pattern Library

An item code, tile identifier or command token often needs a small associated value.

NES shows the working diagnostic row 07 01 01 14 03 0A 14 14. — A completed native caller displays 07 01 01 14 03 0A 14 14. 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

Search COUNT 0–8 key/value pairs stored in K0..K7 and Q0..Q7 respectively. Return the first matching VALUE and FOUND=1; a miss returns zero with FOUND=0. COUNT>8 returns VALID=0. Preserve the dictionary and key. Duplicate keys deliberately use the earliest pair.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, FOUND, VALID, KEY, COUNT, K0, K1, K2, K3, K4, K5, K6, K7, Q0, Q1, Q2, Q3, Q4, Q5, Q6, Q7. They occupy 21 contiguous bytes. Also reserve the private workspace: T0, T1 (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call dictionary_lookup_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 dictionary_lookup_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	COUNT and key/value arrays
Supply before service	KEY
Read after service	VALUE, FOUND and VALID

Scan the active prefix and stop at the first equal key. FOUND carries presence separately from VALUE, allowing zero to be valid data.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Look Up a Bounded Byte Dictionary. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
dictionary_lookup_init:
    lda #0
    sta VALUE
    sta FOUND
    sta VALID
    sta KEY
    sta COUNT
    sta K0
    sta K1
    sta K2
    sta K3
    sta K4
    sta K5
    sta K6
    sta K7
    sta Q0
    sta Q1
    sta Q2
    sta Q3
    sta Q4
    sta Q5
    sta Q6
    sta Q7
    sta T0
    sta T1
    rts
dictionary_lookup_step:
; VALUE = 0
    lda #0
    sta VALUE
; FOUND = 0
    lda #0
    sta FOUND
; VALID = 0
    lda #0
    sta VALID
; if COUNT > 8:
    lda COUNT
    cmp #8
    beq dictionary_lookup_L5
    bcs dictionary_lookup_L4
dictionary_lookup_L5:
    jmp dictionary_lookup_L2
dictionary_lookup_L4:
    jmp dictionary_lookup_L1
dictionary_lookup_L1:
; return
    rts
    jmp dictionary_lookup_L3
dictionary_lookup_L2:
dictionary_lookup_L3:
; VALID = 1
    lda #1
    sta VALID
; T0 = 0
    lda #0
    sta T0
; while T0 < COUNT:
dictionary_lookup_L6:
    lda T0
    cmp COUNT
    bcc dictionary_lookup_L9
dictionary_lookup_L10:
    jmp dictionary_lookup_L8
dictionary_lookup_L9:
    jmp dictionary_lookup_L7
dictionary_lookup_L7:
; T1 = K[T0]
    ldx T0
    lda K0,x
    sta T1
; if KEY == T1:
    lda KEY
    cmp T1
    beq dictionary_lookup_L14
dictionary_lookup_L15:
    jmp dictionary_lookup_L12
dictionary_lookup_L14:
    jmp dictionary_lookup_L11
dictionary_lookup_L11:
; T1 = Q[T0]
    ldx T0
    lda Q0,x
    sta T1
; VALUE = T1
    lda T1
    sta VALUE
; FOUND = 1
    lda #1
    sta FOUND
; return
    rts
    jmp dictionary_lookup_L13
dictionary_lookup_L12:
dictionary_lookup_L13:
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp dictionary_lookup_L6
dictionary_lookup_L8:
    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
FOUND=$81
VALID=$82
KEY=$83
COUNT=$84
K0=$85
K1=$86
K2=$87
K3=$88
K4=$89
K5=$8a
K6=$8b
K7=$8c
Q0=$8d
Q1=$8e
Q2=$8f
Q3=$90
Q4=$91
Q5=$92
Q6=$93
Q7=$94
T0=$95
T1=$96
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
; COUNT = 1
    lda #1
    sta COUNT
; K0 = 255
    lda #255
    sta K0
; Q0 = 0
    lda #0
    sta Q0
; KEY = 255
    lda #255
    sta KEY
.if BROKEN
; KEY = 0
    lda #0
    sta KEY
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; COUNT = 3
    lda #3
    sta COUNT
; K0 = 10
    lda #10
    sta K0
; K1 = 20
    lda #20
    sta K1
; K2 = 20
    lda #20
    sta K2
; Q0 = 4
    lda #4
    sta Q0
; Q1 = 7
    lda #7
    sta Q1
; Q2 = 9
    lda #9
    sta Q2
; KEY = 20
    lda #20
    sta KEY
.if BROKEN
; KEY = 0
    lda #0
    sta KEY
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Look up key 255 whose stored value is zero. 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
FOUND=$81
VALID=$82
KEY=$83
COUNT=$84
K0=$85
K1=$86
K2=$87
K3=$88
K4=$89
K5=$8a
K6=$8b
K7=$8c
Q0=$8d
Q1=$8e
Q2=$8f
Q3=$90
Q4=$91
Q5=$92
Q6=$93
Q7=$94
T0=$95
T1=$96
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 00 01 01 FF 01 FF 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller clears the lookup key. It can turn a valid item into a miss or accidentally resolve key zero. Preserve KEY and distinguish FOUND=0 from a stored zero value.

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
FOUND=$81
VALID=$82
KEY=$83
COUNT=$84
K0=$85
K1=$86
K2=$87
K3=$88
K4=$89
K5=$8a
K6=$8b
K7=$8c
Q0=$8d
Q1=$8e
Q2=$8f
Q3=$90
Q4=$91
Q5=$92
Q6=$93
Q7=$94
T0=$95
T1=$96
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 00 00 01 00 03 0A 14 14. — The wrong client completes with 00 00 01 00 03 0A 14 14; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 146 code bytes; public state is 21 bytes and private scratch is 2 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.

At most eight comparisons. Sorted binary search lowers comparisons for larger immutable tables; direct 256-entry indexing trades ROM for constant lookup.

This is a linear byte-key dictionary, not a text parser or a hash table. The dictionary is read-only during service.

Execution evidence

Four variants run on NTSC NROM, background only, DMC/NMI disabled, using Asm198x 0.0.72, Emu198x 0.44.2. Each executes 47 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

Decimal parsing: Parsing converts text to a token; lookup associates that token with game data.

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