Parse Two Hexadecimal Digits on C64 | Pattern Library

A short hexadecimal input still needs validation before it becomes an address, colour or seed.

C64 shows the working diagnostic row AB 01 41 62 00 00 00 00. — A completed native caller displays AB 01 41 62 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

FIRST and SECOND are ASCII bytes. Accept exactly two digits 0–9, A–F or a–f. Return VALUE and VALID=1; invalid input returns VALUE=0 and VALID=0. Inputs are preserved.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, VALID, FIRST, SECOND. They occupy 4 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call hexadecimal_parser_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 hexadecimal_parser_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	none
Supply before service	FIRST and SECOND
Read after service	VALUE and VALID

Validate each ASCII interval before converting it to a nibble. Shifting the accumulated high nibble by four leaves room for the second digit.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Parse Two Hexadecimal Digits. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
hexadecimal_parser_init:
    lda #0
    sta VALUE
    sta VALID
    sta FIRST
    sta SECOND
    sta T0
    sta T1
    sta T2
    rts
hexadecimal_parser_step:
; VALUE = 0
    lda #0
    sta VALUE
; VALID = 0
    lda #0
    sta VALID
; T0 = 0
    lda #0
    sta T0
; while T0 < 2:
hexadecimal_parser_L1:
    lda T0
    cmp #2
    bcc hexadecimal_parser_L4
hexadecimal_parser_L5:
    jmp hexadecimal_parser_L3
hexadecimal_parser_L4:
    jmp hexadecimal_parser_L2
hexadecimal_parser_L2:
; T1 = FIRST
    lda FIRST
    sta T1
; if T0 != 0:
    lda T0
    cmp #0
    bne hexadecimal_parser_L9
hexadecimal_parser_L10:
    jmp hexadecimal_parser_L7
hexadecimal_parser_L9:
    jmp hexadecimal_parser_L6
hexadecimal_parser_L6:
; T1 = SECOND
    lda SECOND
    sta T1
    jmp hexadecimal_parser_L8
hexadecimal_parser_L7:
hexadecimal_parser_L8:
; T2 = 255
    lda #255
    sta T2
; if T1 >= 48:
    lda T1
    cmp #48
    bcs hexadecimal_parser_L14
hexadecimal_parser_L15:
    jmp hexadecimal_parser_L12
hexadecimal_parser_L14:
    jmp hexadecimal_parser_L11
hexadecimal_parser_L11:
; if T1 <= 57:
    lda T1
    cmp #57
    bcc hexadecimal_parser_L19
    beq hexadecimal_parser_L19
hexadecimal_parser_L20:
    jmp hexadecimal_parser_L17
hexadecimal_parser_L19:
    jmp hexadecimal_parser_L16
hexadecimal_parser_L16:
; T2 = T1 - 48
    lda T1
    sec
    sbc #48
    sta T2
    jmp hexadecimal_parser_L18
hexadecimal_parser_L17:
hexadecimal_parser_L18:
    jmp hexadecimal_parser_L13
hexadecimal_parser_L12:
hexadecimal_parser_L13:
; if T1 >= 65:
    lda T1
    cmp #65
    bcs hexadecimal_parser_L24
hexadecimal_parser_L25:
    jmp hexadecimal_parser_L22
hexadecimal_parser_L24:
    jmp hexadecimal_parser_L21
hexadecimal_parser_L21:
; if T1 <= 70:
    lda T1
    cmp #70
    bcc hexadecimal_parser_L29
    beq hexadecimal_parser_L29
hexadecimal_parser_L30:
    jmp hexadecimal_parser_L27
hexadecimal_parser_L29:
    jmp hexadecimal_parser_L26
hexadecimal_parser_L26:
; T2 = T1 - 55
    lda T1
    sec
    sbc #55
    sta T2
    jmp hexadecimal_parser_L28
hexadecimal_parser_L27:
hexadecimal_parser_L28:
    jmp hexadecimal_parser_L23
hexadecimal_parser_L22:
hexadecimal_parser_L23:
; if T1 >= 97:
    lda T1
    cmp #97
    bcs hexadecimal_parser_L34
hexadecimal_parser_L35:
    jmp hexadecimal_parser_L32
hexadecimal_parser_L34:
    jmp hexadecimal_parser_L31
hexadecimal_parser_L31:
; if T1 <= 102:
    lda T1
    cmp #102
    bcc hexadecimal_parser_L39
    beq hexadecimal_parser_L39
hexadecimal_parser_L40:
    jmp hexadecimal_parser_L37
hexadecimal_parser_L39:
    jmp hexadecimal_parser_L36
hexadecimal_parser_L36:
; T2 = T1 - 87
    lda T1
    sec
    sbc #87
    sta T2
    jmp hexadecimal_parser_L38
hexadecimal_parser_L37:
hexadecimal_parser_L38:
    jmp hexadecimal_parser_L33
hexadecimal_parser_L32:
hexadecimal_parser_L33:
; if T2 == 255:
    lda T2
    cmp #255
    beq hexadecimal_parser_L44
hexadecimal_parser_L45:
    jmp hexadecimal_parser_L42
hexadecimal_parser_L44:
    jmp hexadecimal_parser_L41
hexadecimal_parser_L41:
; VALUE = 0
    lda #0
    sta VALUE
; return
    rts
    jmp hexadecimal_parser_L43
hexadecimal_parser_L42:
hexadecimal_parser_L43:
; VALUE = VALUE << 4
    lda VALUE
    asl
    asl
    asl
    asl
    sta VALUE
; VALUE = VALUE | T2
    lda VALUE
    ora T2
    sta VALUE
; T0 = T0 + 1
    lda T0
    clc
    adc #1
    sta T0
    jmp hexadecimal_parser_L1
hexadecimal_parser_L3:
; VALID = 1
    lda #1
    sta VALID
    rts
component_end:

Machine contract

A/X/status may change; Y and decimal mode are preserved. Use CLD for binary arithmetic. 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 diagnostic PRG owns screen, font and colour RAM and disables interrupts; it does not return to BASIC. The reusable helper does none of those things.

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

VALUE=$80
VALID=$81
FIRST=$82
SECOND=$83
T0=$84
T1=$85
T2=$86
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
!if CUSTOM {
; FIRST = 102
    lda #102
    sta FIRST
; SECOND = 70
    lda #70
    sta SECOND
!if BROKEN {
; SECOND = 48
    lda #48
    sta SECOND
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; FIRST = 65
    lda #65
    sta FIRST
; SECOND = 98
    lda #98
    sta SECOND
!if BROKEN {
; SECOND = 48
    lda #48
    sta SECOND
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Accept mixed-case fF as the maximum byte. 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

VALUE=$80
VALID=$81
FIRST=$82
SECOND=$83
T0=$84
T1=$85
T2=$86
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

The custom diagnostic row is FF 01 66 46 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller replaces the second typed character with zero. Ab becomes A0. Keep both input bytes and inspect VALID before using 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

VALUE=$80
VALID=$81
FIRST=$82
SECOND=$83
T0=$84
T1=$85
T2=$86
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
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
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

C64 shows the deliberate error row A0 01 41 30 00 00 00 00. — The wrong client completes with A0 01 41 30 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 234 code bytes; public state is 4 bytes and private scratch is 3 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.

Two character stages and no table. A 256-byte classification table trades ROM for predictable lookup work.

This is a fixed two-character parser. Prefixes, whitespace, editing, variable lengths and memory-address permissions belong to the caller.

Execution evidence

Four variants run on PAL stock-ROM PRG, 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

Hexadecimal formatting: Formatting and parsing can round-trip every byte when the caller preserves the two characters.

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