A joystick that rests a little off-centre should not make the player creep across the screen.
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
; 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:
ld a,0
ld (SAMPLE),a
ld (CENTRE),a
ld (RADIUS),a
ld (DIRECTION),a
ld (MAGNITUDE),a
ld (VALID),a
ld (T0),a
ret
analogue_deadzone_step:
; DIRECTION = 0
ld a,0
ld (DIRECTION),a
; MAGNITUDE = 0
ld a,0
ld (MAGNITUDE),a
; VALID = 0
ld a,0
ld (VALID),a
; if RADIUS >= 128:
ld a,(RADIUS)
cp 128
jp nc,analogue_deadzone_L1
jp analogue_deadzone_L2
analogue_deadzone_L1:
; return
ret
jp analogue_deadzone_L3
analogue_deadzone_L2:
analogue_deadzone_L3:
; VALID = 1
ld a,1
ld (VALID),a
; if SAMPLE >= CENTRE:
ld a,(SAMPLE)
ld b,a
ld a,(CENTRE)
ld c,a
ld a,b
cp c
jp nc,analogue_deadzone_L4
jp analogue_deadzone_L5
analogue_deadzone_L4:
; T0 = SAMPLE - CENTRE
ld a,(SAMPLE)
ld b,a
ld a,(CENTRE)
ld c,a
ld a,b
sub c
ld (T0),a
; if T0 > RADIUS:
ld a,(T0)
ld b,a
ld a,(RADIUS)
ld c,a
ld a,b
cp c
jp z,analogue_deadzone_L8
jp nc,analogue_deadzone_L7
jp analogue_deadzone_L8
analogue_deadzone_L7:
; MAGNITUDE = T0 - RADIUS
ld a,(T0)
ld b,a
ld a,(RADIUS)
ld c,a
ld a,b
sub c
ld (MAGNITUDE),a
; DIRECTION = 2
ld a,2
ld (DIRECTION),a
jp analogue_deadzone_L9
analogue_deadzone_L8:
analogue_deadzone_L9:
jp analogue_deadzone_L6
analogue_deadzone_L5:
; T0 = CENTRE - SAMPLE
ld a,(CENTRE)
ld b,a
ld a,(SAMPLE)
ld c,a
ld a,b
sub c
ld (T0),a
; if T0 > RADIUS:
ld a,(T0)
ld b,a
ld a,(RADIUS)
ld c,a
ld a,b
cp c
jp z,analogue_deadzone_L11
jp nc,analogue_deadzone_L10
jp analogue_deadzone_L11
analogue_deadzone_L10:
; MAGNITUDE = T0 - RADIUS
ld a,(T0)
ld b,a
ld a,(RADIUS)
ld c,a
ld a,b
sub c
ld (MAGNITUDE),a
; DIRECTION = 1
ld a,1
ld (DIRECTION),a
jp analogue_deadzone_L12
analogue_deadzone_L11:
analogue_deadzone_L12:
analogue_deadzone_L6:
ret
component_end:Machine contract
AF/BC/DE/HL may change; IX/IY are preserved. A direct CALL 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 48K diagnostic boots through ROM tape loading and USR, disables interrupts and owns the screen. The helper performs no port or screen writes.
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
org $8000
CUSTOM equ 0
BROKEN equ 0
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SAMPLE: defb 0
CENTRE: defb 0
RADIUS: defb 0
DIRECTION: defb 0
MAGNITUDE: defb 0
VALID: defb 0
T0: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startA client retaining state between calls
preview_rules:
if CUSTOM
; SAMPLE = 130
ld a,130
ld (SAMPLE),a
; CENTRE = 128
ld a,128
ld (CENTRE),a
; RADIUS = 8
ld a,8
ld (RADIUS),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
; SAMPLE = 140
ld a,140
ld (SAMPLE),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
; SAMPLE = 118
ld a,118
ld (SAMPLE),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
ret
else
; SAMPLE = 130
ld a,130
ld (SAMPLE),a
; CENTRE = 128
ld a,128
ld (CENTRE),a
; RADIUS = 4
ld a,4
ld (RADIUS),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
; SAMPLE = 140
ld a,140
ld (SAMPLE),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
; SAMPLE = 118
ld a,118
ld (SAMPLE),a
if BROKEN
; RADIUS = 0
ld a,0
ld (RADIUS),a
endif
call checked_step
call emit_state
endif
retCustomise 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
org $8000
CUSTOM equ 1
BROKEN equ 0
COYOTE_STEPS equ 2
BUFFER_STEPS equ 2
TEXT_CAP equ 4
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 49
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SAMPLE: defb 0
CENTRE: defb 0
RADIUS: defb 0
DIRECTION: defb 0
MAGNITUDE: defb 0
VALID: defb 0
T0: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end startThe 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
org $8000
CUSTOM equ 0
BROKEN equ 1
COYOTE_STEPS equ 4
BUFFER_STEPS equ 4
TEXT_CAP equ 8
PROPERTY_COUNT equ 16
STREAM_BYTES equ 13
CRC_POLY equ 7
include "caller.inc"
include "exercise.inc"
component_start:
include "component.inc"
include "data.inc"
include "cases.inc"
include "../../physics/rectangle-overlap/display.inc"
code_end:
STATE:
SAMPLE: defb 0
CENTRE: defb 0
RADIUS: defb 0
DIRECTION: defb 0
MAGNITUDE: defb 0
VALID: defb 0
T0: defb 0
input_ptr: defw 0
output_ptr: defw 0
cases_left: defb 0
ready: defb 0
preview: defs 8,0
OUTPUT: defs 1300,0
end start
Cost and alternatives
The measured helper extent is 185 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 48K PAL ROM tape load and USR, 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.