Parse a Bounded Decimal Number on C64 | Pattern Library

A score name box and a numeric prompt need different input rules. Reject a bad digit without damaging the value already typed.

C64 shows the working diagnostic row FF 35 01 00 00 00 00 00. — A completed native caller displays FF 35 01 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

Feed one ASCII CHARACTER. Digits 0–9 extend VALUE if the mathematical result remains 0–255. STATUS=1 accepts, 2 rejects a non-digit, 3 rejects overflow. On either rejection VALUE is unchanged. Call init once to begin a number and keep the value between characters.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: VALUE, CHARACTER, STATUS. They occupy 3 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 decimal_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 decimal_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	VALUE
Supply before service	CHARACTER
Read after service	STATUS

Before multiplying by ten, compare the accepted value with 25. At exactly 25 only a final digit up to five fits. That rejects overflow before it can wrap into an apparently valid small number.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Parse a Bounded Decimal Number. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
decimal_parser_init:
    lda #0
    sta VALUE
    sta CHARACTER
    sta STATUS
    sta T0
    sta T1
    sta T2
    rts
decimal_parser_step:
; STATUS = 2
    lda #2
    sta STATUS
; if CHARACTER < 48:
    lda CHARACTER
    cmp #48
    bcc decimal_parser_L4
decimal_parser_L5:
    jmp decimal_parser_L2
decimal_parser_L4:
    jmp decimal_parser_L1
decimal_parser_L1:
; return
    rts
    jmp decimal_parser_L3
decimal_parser_L2:
decimal_parser_L3:
; if CHARACTER > 57:
    lda CHARACTER
    cmp #57
    beq decimal_parser_L10
    bcs decimal_parser_L9
decimal_parser_L10:
    jmp decimal_parser_L7
decimal_parser_L9:
    jmp decimal_parser_L6
decimal_parser_L6:
; return
    rts
    jmp decimal_parser_L8
decimal_parser_L7:
decimal_parser_L8:
; T0 = CHARACTER - 48
    lda CHARACTER
    sec
    sbc #48
    sta T0
; STATUS = 3
    lda #3
    sta STATUS
; if VALUE > 25:
    lda VALUE
    cmp #25
    beq decimal_parser_L15
    bcs decimal_parser_L14
decimal_parser_L15:
    jmp decimal_parser_L12
decimal_parser_L14:
    jmp decimal_parser_L11
decimal_parser_L11:
; return
    rts
    jmp decimal_parser_L13
decimal_parser_L12:
decimal_parser_L13:
; if VALUE == 25:
    lda VALUE
    cmp #25
    beq decimal_parser_L19
decimal_parser_L20:
    jmp decimal_parser_L17
decimal_parser_L19:
    jmp decimal_parser_L16
decimal_parser_L16:
; if T0 > 5:
    lda T0
    cmp #5
    beq decimal_parser_L25
    bcs decimal_parser_L24
decimal_parser_L25:
    jmp decimal_parser_L22
decimal_parser_L24:
    jmp decimal_parser_L21
decimal_parser_L21:
; return
    rts
    jmp decimal_parser_L23
decimal_parser_L22:
decimal_parser_L23:
    jmp decimal_parser_L18
decimal_parser_L17:
decimal_parser_L18:
; T1 = VALUE << 1
    lda VALUE
    asl
    sta T1
; T2 = VALUE << 3
    lda VALUE
    asl
    asl
    asl
    sta T2
; T1 = T1 + T2
    lda T1
    clc
    adc T2
    sta T1
; VALUE = T1 + T0
    lda T1
    clc
    adc T0
    sta VALUE
; STATUS = 1
    lda #1
    sta STATUS
    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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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
CHARACTER=$81
STATUS=$82
T0=$83
T1=$84
T2=$85
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 {
; CHARACTER = 49
    lda #49
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
; CHARACTER = 48
    lda #48
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
; CHARACTER = 52
    lda #52
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; CHARACTER = 50
    lda #50
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
; CHARACTER = 53
    lda #53
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
; CHARACTER = 54
    lda #54
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
; CHARACTER = 53
    lda #53
    sta CHARACTER
!if BROKEN {
; VALUE = 0
    lda #0
    sta VALUE
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Parse 104 rather than the normal 2,5,6,5 sequence. 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
CHARACTER=$81
STATUS=$82
T0=$83
T1=$84
T2=$85
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 68 34 01 00 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller resets VALUE before every character. The final digit replaces the whole number. Initialise once at the start of input, then retain the accepted prefix through later characters and errors.

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
CHARACTER=$81
STATUS=$82
T0=$83
T1=$84
T2=$85
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 05 35 01 00 00 00 00 00. — The wrong client completes with 05 35 01 00 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 146 code bytes; public state is 3 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 per character. Shifts and additions replace multiplication by ten; they are justified only after the overflow precheck.

No sign, whitespace, backspace, decimal point or end-of-line handling is implicit. A prompt owns those actions. Wider integer parsing uses the same precheck idea with a wider quotient and remainder, rather than accepting wrapped results.

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

Text-entry buffers: A text buffer keeps characters for editing; the parser turns accepted digit events into a bounded numeric value.

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