Format a Byte as Hexadecimal on C64 | Pattern Library

Two hexadecimal digits are a compact way to show a byte in a debugger, password or diagnostic panel.

C64 shows the working diagnostic row 41 42 AB 00 00 00 00 00. — A completed native caller displays 41 42 AB 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

VALUE is any byte. Return two ASCII codes, HIGH_CHAR then LOW_CHAR. LOWERCASE=0 selects A–F; any other value selects a–f. No terminator or display write is emitted.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: HIGH_CHAR, LOW_CHAR, VALUE, LOWERCASE. 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_format_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_format_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	VALUE and LOWERCASE
Read after service	HIGH_CHAR and LOW_CHAR

Each nibble selects a decimal digit or a letter offset. The output order is most-significant nibble first, regardless of the machine’s word byte order.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Format a Byte as Hexadecimal. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
hexadecimal_format_init:
    lda #0
    sta HIGH_CHAR
    sta LOW_CHAR
    sta VALUE
    sta LOWERCASE
    sta T0
    sta T1
    sta T2
    rts
hexadecimal_format_step:
; T0 = VALUE >> 4
    lda VALUE
    lsr
    lsr
    lsr
    lsr
    sta T0
; T1 = VALUE & 15
    lda VALUE
    and #15
    sta T1
; HIGH_CHAR = T0 + 48
    lda T0
    clc
    adc #48
    sta HIGH_CHAR
; LOW_CHAR = T1 + 48
    lda T1
    clc
    adc #48
    sta LOW_CHAR
; T2 = 55
    lda #55
    sta T2
; if LOWERCASE != 0:
    lda LOWERCASE
    cmp #0
    bne hexadecimal_format_L4
hexadecimal_format_L5:
    jmp hexadecimal_format_L2
hexadecimal_format_L4:
    jmp hexadecimal_format_L1
hexadecimal_format_L1:
; T2 = 87
    lda #87
    sta T2
    jmp hexadecimal_format_L3
hexadecimal_format_L2:
hexadecimal_format_L3:
; if T0 >= 10:
    lda T0
    cmp #10
    bcs hexadecimal_format_L9
hexadecimal_format_L10:
    jmp hexadecimal_format_L7
hexadecimal_format_L9:
    jmp hexadecimal_format_L6
hexadecimal_format_L6:
; HIGH_CHAR = T0 + T2
    lda T0
    clc
    adc T2
    sta HIGH_CHAR
    jmp hexadecimal_format_L8
hexadecimal_format_L7:
hexadecimal_format_L8:
; if T1 >= 10:
    lda T1
    cmp #10
    bcs hexadecimal_format_L14
hexadecimal_format_L15:
    jmp hexadecimal_format_L12
hexadecimal_format_L14:
    jmp hexadecimal_format_L11
hexadecimal_format_L11:
; LOW_CHAR = T1 + T2
    lda T1
    clc
    adc T2
    sta LOW_CHAR
    jmp hexadecimal_format_L13
hexadecimal_format_L12:
hexadecimal_format_L13:
    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

HIGH_CHAR=$80
LOW_CHAR=$81
VALUE=$82
LOWERCASE=$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 {
; VALUE = 254
    lda #254
    sta VALUE
; LOWERCASE = 1
    lda #1
    sta LOWERCASE
!if BROKEN {
; VALUE = VALUE & 15
    lda VALUE
    and #15
    sta VALUE
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; VALUE = 171
    lda #171
    sta VALUE
!if BROKEN {
; VALUE = VALUE & 15
    lda VALUE
    and #15
    sta VALUE
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Select lower-case digits for FE. 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

HIGH_CHAR=$80
LOW_CHAR=$81
VALUE=$82
LOWERCASE=$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 66 65 FE 01 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller keeps only the low nibble. AB becomes 0B. Pass the whole byte and let the formatter split the two digits.

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

HIGH_CHAR=$80
LOW_CHAR=$81
VALUE=$82
LOWERCASE=$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 30 42 0B 00 00 00 00 00. — The wrong client completes with 30 42 0B 00 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 113 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.

Constant work with no lookup table. A sixteen-byte digit table saves branches at the cost of ROM space.

The output is ASCII, not PETSCII screen codes or a NES tile index. Translate through the game’s font mapping before drawing.

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

Decimal formatting: Hexadecimal needs two nibbles; decimal needs division into base-ten digits.

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