Walk an Ordered Waypoint Route on NES | Pattern Library

A guard should reach a waypoint before choosing the next, even when one movement step would overshoot it.

NES shows the working diagnostic row 09 02 03 04 00 00 01 01. — A completed native caller displays 09 02 03 04 00 00 01 01. 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

Up to four caller-owned byte positions form a one-dimensional route. COUNT=1–4, TARGET_INDEX below COUNT and SPEED=1–255 are required. Move towards the current target by at most SPEED, snap on arrival, pulse ARRIVED and advance the target. At the final waypoint LOOP wraps; otherwise FINISHED latches. The caller owns starting position and route data.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: POSITION, TARGET_INDEX, COUNT, SPEED, LOOP, ARRIVED, FINISHED, VALID, P0, P1, P2, P3. They occupy 12 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 waypoint_route_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 waypoint_route_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	POSITION, TARGET_INDEX and FINISHED
Supply before service	COUNT, SPEED, LOOP and P0–P3
Read after service	ARRIVED and VALID

Approach one target without overshoot. Once it is reached, pulse arrival and select the next target for a later update. The helper never skips across several coincident waypoints in one call.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Walk an Ordered Waypoint Route. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
waypoint_route_init:
    lda #0
    sta POSITION
    sta TARGET_INDEX
    sta COUNT
    sta SPEED
    sta LOOP
    sta ARRIVED
    sta FINISHED
    sta VALID
    sta P0
    sta P1
    sta P2
    sta P3
    sta T0
    sta T1
    rts
waypoint_route_step:
; ARRIVED = 0
    lda #0
    sta ARRIVED
; VALID = 0
    lda #0
    sta VALID
; if COUNT == 0:
    lda COUNT
    cmp #0
    beq waypoint_route_L4
waypoint_route_L5:
    jmp waypoint_route_L2
waypoint_route_L4:
    jmp waypoint_route_L1
waypoint_route_L1:
; return
    rts
    jmp waypoint_route_L3
waypoint_route_L2:
waypoint_route_L3:
; if COUNT > 4:
    lda COUNT
    cmp #4
    beq waypoint_route_L10
    bcs waypoint_route_L9
waypoint_route_L10:
    jmp waypoint_route_L7
waypoint_route_L9:
    jmp waypoint_route_L6
waypoint_route_L6:
; return
    rts
    jmp waypoint_route_L8
waypoint_route_L7:
waypoint_route_L8:
; if TARGET_INDEX >= COUNT:
    lda TARGET_INDEX
    cmp COUNT
    bcs waypoint_route_L14
waypoint_route_L15:
    jmp waypoint_route_L12
waypoint_route_L14:
    jmp waypoint_route_L11
waypoint_route_L11:
; return
    rts
    jmp waypoint_route_L13
waypoint_route_L12:
waypoint_route_L13:
; if SPEED == 0:
    lda SPEED
    cmp #0
    beq waypoint_route_L19
waypoint_route_L20:
    jmp waypoint_route_L17
waypoint_route_L19:
    jmp waypoint_route_L16
waypoint_route_L16:
; return
    rts
    jmp waypoint_route_L18
waypoint_route_L17:
waypoint_route_L18:
; VALID = 1
    lda #1
    sta VALID
; if FINISHED != 0:
    lda FINISHED
    cmp #0
    bne waypoint_route_L24
waypoint_route_L25:
    jmp waypoint_route_L22
waypoint_route_L24:
    jmp waypoint_route_L21
waypoint_route_L21:
; return
    rts
    jmp waypoint_route_L23
waypoint_route_L22:
waypoint_route_L23:
; T0 = P[TARGET_INDEX]
    ldx TARGET_INDEX
    lda P0,x
    sta T0
; if POSITION < T0:
    lda POSITION
    cmp T0
    bcc waypoint_route_L29
waypoint_route_L30:
    jmp waypoint_route_L27
waypoint_route_L29:
    jmp waypoint_route_L26
waypoint_route_L26:
; T1 = T0 - POSITION
    lda T0
    sec
    sbc POSITION
    sta T1
; if T1 > SPEED:
    lda T1
    cmp SPEED
    beq waypoint_route_L35
    bcs waypoint_route_L34
waypoint_route_L35:
    jmp waypoint_route_L32
waypoint_route_L34:
    jmp waypoint_route_L31
waypoint_route_L31:
; POSITION = POSITION + SPEED
    lda POSITION
    clc
    adc SPEED
    sta POSITION
; return
    rts
    jmp waypoint_route_L33
waypoint_route_L32:
waypoint_route_L33:
    jmp waypoint_route_L28
waypoint_route_L27:
; T1 = POSITION - T0
    lda POSITION
    sec
    sbc T0
    sta T1
; if T1 > SPEED:
    lda T1
    cmp SPEED
    beq waypoint_route_L40
    bcs waypoint_route_L39
waypoint_route_L40:
    jmp waypoint_route_L37
waypoint_route_L39:
    jmp waypoint_route_L36
waypoint_route_L36:
; POSITION = POSITION - SPEED
    lda POSITION
    sec
    sbc SPEED
    sta POSITION
; return
    rts
    jmp waypoint_route_L38
waypoint_route_L37:
waypoint_route_L38:
waypoint_route_L28:
; POSITION = T0
    lda T0
    sta POSITION
; ARRIVED = 1
    lda #1
    sta ARRIVED
; T1 = COUNT - 1
    lda COUNT
    sec
    sbc #1
    sta T1
; if TARGET_INDEX < T1:
    lda TARGET_INDEX
    cmp T1
    bcc waypoint_route_L44
waypoint_route_L45:
    jmp waypoint_route_L42
waypoint_route_L44:
    jmp waypoint_route_L41
waypoint_route_L41:
; TARGET_INDEX = TARGET_INDEX + 1
    lda TARGET_INDEX
    clc
    adc #1
    sta TARGET_INDEX
    jmp waypoint_route_L43
waypoint_route_L42:
; if LOOP != 0:
    lda LOOP
    cmp #0
    bne waypoint_route_L49
waypoint_route_L50:
    jmp waypoint_route_L47
waypoint_route_L49:
    jmp waypoint_route_L46
waypoint_route_L46:
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
    jmp waypoint_route_L48
waypoint_route_L47:
; FINISHED = 1
    lda #1
    sta FINISHED
waypoint_route_L48:
waypoint_route_L43:
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. 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 NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
TARGET_INDEX=$81
COUNT=$82
SPEED=$83
LOOP=$84
ARRIVED=$85
FINISHED=$86
VALID=$87
P0=$88
P1=$89
P2=$8a
P3=$8b
T0=$8c
T1=$8d
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
.if CUSTOM
; COUNT = 3
    lda #3
    sta COUNT
; SPEED = 4
    lda #4
    sta SPEED
; P0 = 6
    lda #6
    sta P0
; P1 = 2
    lda #2
    sta P1
; P2 = 9
    lda #9
    sta P2
; LOOP = 1
    lda #1
    sta LOOP
.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; COUNT = 3
    lda #3
    sta COUNT
; SPEED = 4
    lda #4
    sta SPEED
; P0 = 6
    lda #6
    sta P0
; P1 = 2
    lda #2
    sta P1
; P2 = 9
    lda #9
    sta P2
.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state

.if BROKEN
; TARGET_INDEX = 0
    lda #0
    sta TARGET_INDEX
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise it

Loop the route instead of latching at its end. 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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
TARGET_INDEX=$81
COUNT=$82
SPEED=$83
LOOP=$84
ARRIVED=$85
FINISHED=$86
VALID=$87
P0=$88
P1=$89
P2=$8a
P3=$8b
T0=$8c
T1=$8d
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

The custom diagnostic row is 06 02 03 04 01 00 00 01. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller rewinds TARGET_INDEX before each update. The actor keeps targeting the first waypoint. Retain the route cursor until a real restart or route replacement.

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

.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
POSITION=$80
TARGET_INDEX=$81
COUNT=$82
SPEED=$83
LOOP=$84
ARRIVED=$85
FINISHED=$86
VALID=$87
P0=$88
P1=$89
P2=$8a
P3=$8b
T0=$8c
T1=$8d
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

NES shows the deliberate error row 06 01 03 04 00 01 00 01. — The wrong client completes with 06 01 03 04 00 01 00 01; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 271 code bytes; public state is 12 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 per update. Only one target can be completed per call, bounding work even when several route points coincide.

This extends target approach into a route, but it is not pathfinding or collision avoidance. Two-dimensional paths need paired coordinates and an explicit movement metric. A waypoint equal to the current position still consumes one call.

Execution evidence

Four variants run on NTSC NROM, background only, DMC/NMI disabled, 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

Approach a target: The route adds an ordered target cursor and completion policy to the simpler movement operation.

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