Code Like It's 198x

Turn an Analogue Axis into Intent on Amiga

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.

Amiga 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

component.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:
    move.b #0,d0
    move.b d0,SAMPLE
    move.b d0,CENTRE
    move.b d0,RADIUS
    move.b d0,DIRECTION
    move.b d0,MAGNITUDE
    move.b d0,VALID
    move.b d0,T0
    rts
analogue_deadzone_step:
; DIRECTION = 0
    move.b #0,d0
    move.b d0,DIRECTION
; MAGNITUDE = 0
    move.b #0,d0
    move.b d0,MAGNITUDE
; VALID = 0
    move.b #0,d0
    move.b d0,VALID
; if RADIUS >= 128:
    move.b RADIUS,d0
    cmp.b #128,d0
    bcc analogue_deadzone_L1
    jmp analogue_deadzone_L2
analogue_deadzone_L1:
; return
    rts
    jmp analogue_deadzone_L3
analogue_deadzone_L2:
analogue_deadzone_L3:
; VALID = 1
    move.b #1,d0
    move.b d0,VALID
; if SAMPLE >= CENTRE:
    move.b SAMPLE,d0
    move.b CENTRE,d1
    cmp.b d1,d0
    bcc analogue_deadzone_L4
    jmp analogue_deadzone_L5
analogue_deadzone_L4:
; T0 = SAMPLE - CENTRE
    move.b SAMPLE,d0
    move.b CENTRE,d1
    sub.b d1,d0
    move.b d0,T0
; if T0 > RADIUS:
    move.b T0,d0
    move.b RADIUS,d1
    cmp.b d1,d0
    bhi analogue_deadzone_L7
    jmp analogue_deadzone_L8
analogue_deadzone_L7:
; MAGNITUDE = T0 - RADIUS
    move.b T0,d0
    move.b RADIUS,d1
    sub.b d1,d0
    move.b d0,MAGNITUDE
; DIRECTION = 2
    move.b #2,d0
    move.b d0,DIRECTION
    jmp analogue_deadzone_L9
analogue_deadzone_L8:
analogue_deadzone_L9:
    jmp analogue_deadzone_L6
analogue_deadzone_L5:
; T0 = CENTRE - SAMPLE
    move.b CENTRE,d0
    move.b SAMPLE,d1
    sub.b d1,d0
    move.b d0,T0
; if T0 > RADIUS:
    move.b T0,d0
    move.b RADIUS,d1
    cmp.b d1,d0
    bhi analogue_deadzone_L10
    jmp analogue_deadzone_L11
analogue_deadzone_L10:
; MAGNITUDE = T0 - RADIUS
    move.b T0,d0
    move.b RADIUS,d1
    sub.b d1,d0
    move.b d0,MAGNITUDE
; DIRECTION = 1
    move.b #1,d0
    move.b d0,DIRECTION
    jmp analogue_deadzone_L12
analogue_deadzone_L11:
analogue_deadzone_L12:
analogue_deadzone_L6:
    rts
component_end:

Machine contract

D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.

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 and BUILD198X=/path/to/build198x. 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
    section code,code_c
CUSTOM = 0
BROKEN = 0
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SAMPLE: dc.b 0
CENTRE: dc.b 0
RADIUS: dc.b 0
DIRECTION: dc.b 0
MAGNITUDE: dc.b 0
VALID: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XGAMEKIT0000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
    ifne CUSTOM
; SAMPLE = 130
    move.b #130,d0
    move.b d0,SAMPLE
; CENTRE = 128
    move.b #128,d0
    move.b d0,CENTRE
; RADIUS = 8
    move.b #8,d0
    move.b d0,RADIUS
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
; SAMPLE = 140
    move.b #140,d0
    move.b d0,SAMPLE
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
; SAMPLE = 118
    move.b #118,d0
    move.b d0,SAMPLE
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; SAMPLE = 130
    move.b #130,d0
    move.b d0,SAMPLE
; CENTRE = 128
    move.b #128,d0
    move.b d0,CENTRE
; RADIUS = 4
    move.b #4,d0
    move.b d0,RADIUS
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
; SAMPLE = 140
    move.b #140,d0
    move.b d0,SAMPLE
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
; SAMPLE = 118
    move.b #118,d0
    move.b d0,SAMPLE
    ifne BROKEN
; RADIUS = 0
    move.b #0,d0
    move.b d0,RADIUS
    endif
    bsr checked_step
    bsr emit_state
    endif
    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
    section code,code_c
CUSTOM = 1
BROKEN = 0
COYOTE_STEPS = 2
BUFFER_STEPS = 2
TEXT_CAP = 4
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 49
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SAMPLE: dc.b 0
CENTRE: dc.b 0
RADIUS: dc.b 0
DIRECTION: dc.b 0
MAGNITUDE: dc.b 0
VALID: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XGAMEKIT0000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0

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
    section code,code_c
CUSTOM = 0
BROKEN = 1
COYOTE_STEPS = 4
BUFFER_STEPS = 4
TEXT_CAP = 8
PROPERTY_COUNT = 16
STREAM_BYTES = 13
CRC_POLY = 7
    include "caller.inc"
    include "exercise.inc"
component_start:
    include "component.inc"
    even
    include "data.inc"
    include "cases.inc"
    even
    include "display.inc"
    include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SAMPLE: dc.b 0
CENTRE: dc.b 0
RADIUS: dc.b 0
DIRECTION: dc.b 0
MAGNITUDE: dc.b 0
VALID: dc.b 0
T0: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XGAMEKIT0000",0
    even
copperlist:
    dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
    dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
    dc.w $180,$012,$182,$fff
copbpl:
    dc.w $0e0,0,$0e2,0
    dc.w $ffff,$fffe
plane: dcb.b 10240,0
Amiga 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 254 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 A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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.