Select the Nearest Active Target on Amiga | Pattern Library

An enemy or homing projectile needs a target-selection rule that stays predictable when candidates tie.

Amiga shows the working diagnostic row 01 05 00 01 03 00 00 64. — A completed native caller displays 01 05 00 01 03 00 00 64. 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

ACTIVE uses its low four bits for four candidates. Select the smallest Manhattan distance from unsigned byte ORIGIN_X/Y, breaking ties by lowest slot. Return a little-endian distance 0–510 and FOUND=1; no active targets returns SLOT=255, distance zero and FOUND=0. High ACTIVE bits are ignored.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SLOT, DISTANCE_LO, DISTANCE_HI, FOUND, ACTIVE, ORIGIN_X, ORIGIN_Y, PX0, PX1, PX2, PX3, PY0, PY1, PY2, PY3. They occupy 15 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, T5, T6, T7 (8 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.

Call nearest_target_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 nearest_target_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	ACTIVE, origin and candidate coordinates
Read after service	SLOT, DISTANCE_LO/HI and FOUND

Compute two absolute coordinate differences and add them with a ninth bit. Replace the best candidate only for a strictly smaller distance, preserving the earlier slot on ties.

Reusable initialisation and service

component.inc

; SPDX-License-Identifier: MIT
; Select the Nearest Active Target. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
nearest_target_init:
    move.b #0,d0
    move.b d0,SLOT
    move.b d0,DISTANCE_LO
    move.b d0,DISTANCE_HI
    move.b d0,FOUND
    move.b d0,ACTIVE
    move.b d0,ORIGIN_X
    move.b d0,ORIGIN_Y
    move.b d0,PX0
    move.b d0,PX1
    move.b d0,PX2
    move.b d0,PX3
    move.b d0,PY0
    move.b d0,PY1
    move.b d0,PY2
    move.b d0,PY3
    move.b d0,T0
    move.b d0,T1
    move.b d0,T2
    move.b d0,T3
    move.b d0,T4
    move.b d0,T5
    move.b d0,T6
    move.b d0,T7
    rts
nearest_target_step:
; SLOT = 255
    move.b #255,d0
    move.b d0,SLOT
; DISTANCE_LO = 0
    move.b #0,d0
    move.b d0,DISTANCE_LO
; DISTANCE_HI = 0
    move.b #0,d0
    move.b d0,DISTANCE_HI
; FOUND = 0
    move.b #0,d0
    move.b d0,FOUND
; T0 = 0
    move.b #0,d0
    move.b d0,T0
; T1 = 1
    move.b #1,d0
    move.b d0,T1
; while T0 < 4:
nearest_target_L1:
    move.b T0,d0
    cmp.b #4,d0
    bcs nearest_target_L2
    jmp nearest_target_L3
nearest_target_L2:
; T2 = ACTIVE & T1
    move.b ACTIVE,d0
    move.b T1,d1
    and.b d1,d0
    move.b d0,T2
; if T2 != 0:
    move.b T2,d0
    cmp.b #0,d0
    bne nearest_target_L4
    jmp nearest_target_L5
nearest_target_L4:
; T2 = PX[T0]
    moveq #0,d1
    move.b T0,d1
    lea PX0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T2
; if T2 >= ORIGIN_X:
    move.b T2,d0
    move.b ORIGIN_X,d1
    cmp.b d1,d0
    bcc nearest_target_L7
    jmp nearest_target_L8
nearest_target_L7:
; T3 = T2 - ORIGIN_X
    move.b T2,d0
    move.b ORIGIN_X,d1
    sub.b d1,d0
    move.b d0,T3
    jmp nearest_target_L9
nearest_target_L8:
; T3 = ORIGIN_X - T2
    move.b ORIGIN_X,d0
    move.b T2,d1
    sub.b d1,d0
    move.b d0,T3
nearest_target_L9:
; T2 = PY[T0]
    moveq #0,d1
    move.b T0,d1
    lea PY0,a0
    move.b 0(a0,d1.w),d0
    move.b d0,T2
; if T2 >= ORIGIN_Y:
    move.b T2,d0
    move.b ORIGIN_Y,d1
    cmp.b d1,d0
    bcc nearest_target_L10
    jmp nearest_target_L11
nearest_target_L10:
; T4 = T2 - ORIGIN_Y
    move.b T2,d0
    move.b ORIGIN_Y,d1
    sub.b d1,d0
    move.b d0,T4
    jmp nearest_target_L12
nearest_target_L11:
; T4 = ORIGIN_Y - T2
    move.b ORIGIN_Y,d0
    move.b T2,d1
    sub.b d1,d0
    move.b d0,T4
nearest_target_L12:
; T5 = T3 + T4
    move.b T3,d0
    move.b T4,d1
    add.b d1,d0
    move.b d0,T5
; T6 = 0
    move.b #0,d0
    move.b d0,T6
; if T5 < T3:
    move.b T5,d0
    move.b T3,d1
    cmp.b d1,d0
    bcs nearest_target_L13
    jmp nearest_target_L14
nearest_target_L13:
; T6 = 1
    move.b #1,d0
    move.b d0,T6
    jmp nearest_target_L15
nearest_target_L14:
nearest_target_L15:
; T7 = 0
    move.b #0,d0
    move.b d0,T7
; if FOUND == 0:
    move.b FOUND,d0
    cmp.b #0,d0
    beq nearest_target_L16
    jmp nearest_target_L17
nearest_target_L16:
; T7 = 1
    move.b #1,d0
    move.b d0,T7
    jmp nearest_target_L18
nearest_target_L17:
nearest_target_L18:
; if T6 < DISTANCE_HI:
    move.b T6,d0
    move.b DISTANCE_HI,d1
    cmp.b d1,d0
    bcs nearest_target_L19
    jmp nearest_target_L20
nearest_target_L19:
; T7 = 1
    move.b #1,d0
    move.b d0,T7
    jmp nearest_target_L21
nearest_target_L20:
nearest_target_L21:
; if T6 == DISTANCE_HI:
    move.b T6,d0
    move.b DISTANCE_HI,d1
    cmp.b d1,d0
    beq nearest_target_L22
    jmp nearest_target_L23
nearest_target_L22:
; if T5 < DISTANCE_LO:
    move.b T5,d0
    move.b DISTANCE_LO,d1
    cmp.b d1,d0
    bcs nearest_target_L25
    jmp nearest_target_L26
nearest_target_L25:
; T7 = 1
    move.b #1,d0
    move.b d0,T7
    jmp nearest_target_L27
nearest_target_L26:
nearest_target_L27:
    jmp nearest_target_L24
nearest_target_L23:
nearest_target_L24:
; if T7 != 0:
    move.b T7,d0
    cmp.b #0,d0
    bne nearest_target_L28
    jmp nearest_target_L29
nearest_target_L28:
; SLOT = T0
    move.b T0,d0
    move.b d0,SLOT
; DISTANCE_LO = T5
    move.b T5,d0
    move.b d0,DISTANCE_LO
; DISTANCE_HI = T6
    move.b T6,d0
    move.b d0,DISTANCE_HI
; FOUND = 1
    move.b #1,d0
    move.b d0,FOUND
    jmp nearest_target_L30
nearest_target_L29:
nearest_target_L30:
    jmp nearest_target_L6
nearest_target_L5:
nearest_target_L6:
; T0 = T0 + 1
    move.b T0,d0
    add.b #1,d0
    move.b d0,T0
; T1 = T1 << 1
    move.b T1,d0
    lsl.b #1,d0
    move.b d0,T1
    jmp nearest_target_L1
nearest_target_L3:
    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. 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 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:
SLOT: dc.b 0
DISTANCE_LO: dc.b 0
DISTANCE_HI: dc.b 0
FOUND: dc.b 0
ACTIVE: dc.b 0
ORIGIN_X: dc.b 0
ORIGIN_Y: dc.b 0
PX0: dc.b 0
PX1: dc.b 0
PX2: dc.b 0
PX3: dc.b 0
PY0: dc.b 0
PY1: dc.b 0
PY2: dc.b 0
PY3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XTOOLKIT0000",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
; ACTIVE = 9
    move.b #9,d0
    move.b d0,ACTIVE
; ORIGIN_X = 255
    move.b #255,d0
    move.b d0,ORIGIN_X
; ORIGIN_Y = 255
    move.b #255,d0
    move.b d0,ORIGIN_Y
; PX0 = 0
    move.b #0,d0
    move.b d0,PX0
; PY0 = 0
    move.b #0,d0
    move.b d0,PY0
; PX3 = 254
    move.b #254,d0
    move.b d0,PX3
; PY3 = 255
    move.b #255,d0
    move.b d0,PY3
    ifne BROKEN
; ACTIVE = 1
    move.b #1,d0
    move.b d0,ACTIVE
    endif
    bsr checked_step
    bsr emit_state
    rts
    else
; ACTIVE = 3
    move.b #3,d0
    move.b d0,ACTIVE
; PX0 = 100
    move.b #100,d0
    move.b d0,PX0
; PY0 = 100
    move.b #100,d0
    move.b d0,PY0
; PX1 = 2
    move.b #2,d0
    move.b d0,PX1
; PY1 = 3
    move.b #3,d0
    move.b d0,PY1
    ifne BROKEN
; ACTIVE = 1
    move.b #1,d0
    move.b d0,ACTIVE
    endif
    bsr checked_step
    bsr emit_state
    endif
    rts

Customise it

Select near the far corner of the coordinate 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

    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:
SLOT: dc.b 0
DISTANCE_LO: dc.b 0
DISTANCE_HI: dc.b 0
FOUND: dc.b 0
ACTIVE: dc.b 0
ORIGIN_X: dc.b 0
ORIGIN_Y: dc.b 0
PX0: dc.b 0
PX1: dc.b 0
PX2: dc.b 0
PX3: dc.b 0
PY0: dc.b 0
PY1: dc.b 0
PY2: dc.b 0
PY3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XTOOLKIT0000",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 03 01 00 01 09 FF FF 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller only marks slot zero active, excluding a nearer candidate. Pass the actual occupancy mask and check FOUND before using SLOT.

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:
SLOT: dc.b 0
DISTANCE_LO: dc.b 0
DISTANCE_HI: dc.b 0
FOUND: dc.b 0
ACTIVE: dc.b 0
ORIGIN_X: dc.b 0
ORIGIN_Y: dc.b 0
PX0: dc.b 0
PX1: dc.b 0
PX2: dc.b 0
PX3: dc.b 0
PY0: dc.b 0
PY1: dc.b 0
PY2: dc.b 0
PY3: dc.b 0
T0: dc.b 0
T1: dc.b 0
T2: dc.b 0
T3: dc.b 0
T4: dc.b 0
T5: dc.b 0
T6: dc.b 0
T7: dc.b 0
    even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
    even
signature: dc.b "198XTOOLKIT0000",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 00 C8 00 01 01 00 00 64. — The wrong client completes with 00 C8 00 01 01 00 00 64; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 652 code bytes; public state is 15 bytes and private scratch is 8 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.

Four candidates, no multiplication or square root. Squared Euclidean distance changes the selection and needs a wider sum.

Manhattan distance favours an axis-aligned metric, not circular Euclidean distance. It ignores walls and reachability.

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

Manhattan distance: The metric is part of the targeting behaviour, not merely an optimisation.

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