Compare Signed Byte Values on C64 | Pattern Library

A velocity of −1 must sort below zero even though its stored byte is 255.

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

LEFT and RIGHT hold two’s-complement values −128..127. Return ORDER=0 for less, 1 for equal or 2 for greater. Preserve the input bytes.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: ORDER, LEFT, RIGHT. They occupy 3 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 signed_byte_order_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 signed_byte_order_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	LEFT and RIGHT
Read after service	ORDER

Flipping bit seven maps the signed sequence −128..127 onto the unsigned sequence 0..255. The normal unsigned comparison then gives the signed order without subtracting and overflowing.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Compare Signed Byte Values. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
signed_byte_order_init:
    lda #0
    sta ORDER
    sta LEFT
    sta RIGHT
    sta T0
    sta T1
    rts
signed_byte_order_step:
; T0 = LEFT ^ 128
    lda LEFT
    eor #128
    sta T0
; T1 = RIGHT ^ 128
    lda RIGHT
    eor #128
    sta T1
; ORDER = 1
    lda #1
    sta ORDER
; if T0 < T1:
    lda T0
    cmp T1
    bcc signed_byte_order_L4
signed_byte_order_L5:
    jmp signed_byte_order_L2
signed_byte_order_L4:
    jmp signed_byte_order_L1
signed_byte_order_L1:
; ORDER = 0
    lda #0
    sta ORDER
    jmp signed_byte_order_L3
signed_byte_order_L2:
signed_byte_order_L3:
; if T0 > T1:
    lda T0
    cmp T1
    beq signed_byte_order_L10
    bcs signed_byte_order_L9
signed_byte_order_L10:
    jmp signed_byte_order_L7
signed_byte_order_L9:
    jmp signed_byte_order_L6
signed_byte_order_L6:
; ORDER = 2
    lda #2
    sta ORDER
    jmp signed_byte_order_L8
signed_byte_order_L7:
signed_byte_order_L8:
    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

ORDER=$80
LEFT=$81
RIGHT=$82
T0=$83
T1=$84
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 {
; LEFT = 128
    lda #128
    sta LEFT
; RIGHT = 127
    lda #127
    sta RIGHT
!if BROKEN {
; LEFT = LEFT & 127
    lda LEFT
    and #127
    sta LEFT
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; LEFT = 255
    lda #255
    sta LEFT
; RIGHT = 0
    lda #0
    sta RIGHT
!if BROKEN {
; LEFT = LEFT & 127
    lda LEFT
    and #127
    sta LEFT
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Compare the two extremes of the signed byte range. 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

ORDER=$80
LEFT=$81
RIGHT=$82
T0=$83
T1=$84
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 00 80 7F 00 00 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller strips the sign bit, turning −1 into +127. Keep the encoded signed byte intact; masking a sign bit is not an absolute-value operation.

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

ORDER=$80
LEFT=$81
RIGHT=$82
T0=$83
T1=$84
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 02 7F 00 00 00 00 00 00. — The wrong client completes with 02 7F 00 00 00 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 70 code bytes; public state is 3 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.

Constant work and two scratch bytes. Native signed branches can shorten the 68000 version; MOS code must reason about overflow when using a subtraction instead.

This routine compares encoded signed bytes; it does not convert them to magnitudes or handle sixteen-bit velocities.

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

Fractional positions: Choose a representation before comparing a signed velocity with a threshold.

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