Code Like It's 198x

Turn an Analogue Axis into Intent on C64

A joystick that rests a little off-centre should not make the player creep across the screen.

A joystick that rests a little off-centre should not make the player creep across the screen.

C64 shows the working diagnostic row 76 80 04 01 06 01 00 00.
A completed native caller displays 76 80 04 01 06 01 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

SAMPLE and CENTRE are unsigned bytes. RADIUS must be 0–127. The dead zone includes both boundaries: distance at most RADIUS returns still. DIRECTION is 0 still, 1 below centre or 2 above; MAGNITUDE is the distance beyond the zone, not rescaled to 255. Invalid radius returns zero results.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SAMPLE, CENTRE, RADIUS, DIRECTION, MAGNITUDE, VALID. They occupy 6 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call analogue_deadzone_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 analogue_deadzone_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 between independent samples
Supply before service SAMPLE, CENTRE and RADIUS
Read after service DIRECTION, MAGNITUDE and VALID

Subtract the centre to find direction and distance, then remove the dead-zone radius. There is no jump to full movement at the boundary: the first accepted sample has magnitude one.

Reusable initialisation and service

6502.inc
; SPDX-License-Identifier: MIT
; Turn an Analogue Axis into Intent. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
analogue_deadzone_init:
    lda #0
    sta SAMPLE
    sta CENTRE
    sta RADIUS
    sta DIRECTION
    sta MAGNITUDE
    sta VALID
    sta T0
    rts
analogue_deadzone_step:
; DIRECTION = 0
    lda #0
    sta DIRECTION
; MAGNITUDE = 0
    lda #0
    sta MAGNITUDE
; VALID = 0
    lda #0
    sta VALID
; if RADIUS >= 128:
    lda RADIUS
    cmp #128
    bcs analogue_deadzone_L4
analogue_deadzone_L5:
    jmp analogue_deadzone_L2
analogue_deadzone_L4:
    jmp analogue_deadzone_L1
analogue_deadzone_L1:
; return
    rts
    jmp analogue_deadzone_L3
analogue_deadzone_L2:
analogue_deadzone_L3:
; VALID = 1
    lda #1
    sta VALID
; if SAMPLE >= CENTRE:
    lda SAMPLE
    cmp CENTRE
    bcs analogue_deadzone_L9
analogue_deadzone_L10:
    jmp analogue_deadzone_L7
analogue_deadzone_L9:
    jmp analogue_deadzone_L6
analogue_deadzone_L6:
; T0 = SAMPLE - CENTRE
    lda SAMPLE
    sec
    sbc CENTRE
    sta T0
; if T0 > RADIUS:
    lda T0
    cmp RADIUS
    beq analogue_deadzone_L15
    bcs analogue_deadzone_L14
analogue_deadzone_L15:
    jmp analogue_deadzone_L12
analogue_deadzone_L14:
    jmp analogue_deadzone_L11
analogue_deadzone_L11:
; MAGNITUDE = T0 - RADIUS
    lda T0
    sec
    sbc RADIUS
    sta MAGNITUDE
; DIRECTION = 2
    lda #2
    sta DIRECTION
    jmp analogue_deadzone_L13
analogue_deadzone_L12:
analogue_deadzone_L13:
    jmp analogue_deadzone_L8
analogue_deadzone_L7:
; T0 = CENTRE - SAMPLE
    lda CENTRE
    sec
    sbc SAMPLE
    sta T0
; if T0 > RADIUS:
    lda T0
    cmp RADIUS
    beq analogue_deadzone_L20
    bcs analogue_deadzone_L19
analogue_deadzone_L20:
    jmp analogue_deadzone_L17
analogue_deadzone_L19:
    jmp analogue_deadzone_L16
analogue_deadzone_L16:
; MAGNITUDE = T0 - RADIUS
    lda T0
    sec
    sbc RADIUS
    sta MAGNITUDE
; DIRECTION = 1
    lda #1
    sta DIRECTION
    jmp analogue_deadzone_L18
analogue_deadzone_L17:
analogue_deadzone_L18:
analogue_deadzone_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. 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
SAMPLE=$80
CENTRE=$81
RADIUS=$82
DIRECTION=$83
MAGNITUDE=$84
VALID=$85
T0=$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 {
; SAMPLE = 130
    lda #130
    sta SAMPLE
; CENTRE = 128
    lda #128
    sta CENTRE
; RADIUS = 8
    lda #8
    sta RADIUS
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
; SAMPLE = 140
    lda #140
    sta SAMPLE
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
; SAMPLE = 118
    lda #118
    sta SAMPLE
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; SAMPLE = 130
    lda #130
    sta SAMPLE
; CENTRE = 128
    lda #128
    sta CENTRE
; RADIUS = 4
    lda #4
    sta RADIUS
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
; SAMPLE = 140
    lda #140
    sta SAMPLE
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
; SAMPLE = 118
    lda #118
    sta SAMPLE
!if BROKEN {
; RADIUS = 0
    lda #0
    sta RADIUS
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Widen the dead zone to eight sample units. 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
SAMPLE=$80
CENTRE=$81
RADIUS=$82
DIRECTION=$83
MAGNITUDE=$84
VALID=$85
T0=$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 76 80 08 01 02 01 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller replaces RADIUS with zero. Small sample deviations now produce movement. Use the calibrated dead zone rather than silently bypassing it at the call site.

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
SAMPLE=$80
CENTRE=$81
RADIUS=$82
DIRECTION=$83
MAGNITUDE=$84
VALID=$85
T0=$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 76 80 00 01 0A 01 00 00.
The wrong client completes with 76 80 00 01 0A 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 135 code bytes; public state is 6 bytes and private scratch is 1 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 division. Rescaling the remaining range to a fixed maximum needs additional arithmetic or a lookup table.

No analogue device is assumed on these machines. Feed normalised bytes from an available peripheral driver; digital-only controls do not become analogue by using this routine. Calibration, hysteresis and radial two-axis dead zones are separate techniques.

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

Four-way arbitration: After thresholding two sampled axes, a four-way game still needs a deliberate diagonal policy.

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