A guard should reach a waypoint before choosing the next, even when one movement step would overshoot it.
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
; 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:
ld a,0
ld (POSITION),a
ld (TARGET_INDEX),a
ld (COUNT),a
ld (SPEED),a
ld (LOOP),a
ld (ARRIVED),a
ld (FINISHED),a
ld (VALID),a
ld (P0),a
ld (P1),a
ld (P2),a
ld (P3),a
ld (T0),a
ld (T1),a
ret
waypoint_route_step:
; ARRIVED = 0
ld a,0
ld (ARRIVED),a
; VALID = 0
ld a,0
ld (VALID),a
; if COUNT == 0:
ld a,(COUNT)
cp 0
jp z,waypoint_route_L1
jp waypoint_route_L2
waypoint_route_L1:
; return
ret
jp waypoint_route_L3
waypoint_route_L2:
waypoint_route_L3:
; if COUNT > 4:
ld a,(COUNT)
cp 4
jp z,waypoint_route_L5
jp nc,waypoint_route_L4
jp waypoint_route_L5
waypoint_route_L4:
; return
ret
jp waypoint_route_L6
waypoint_route_L5:
waypoint_route_L6:
; if TARGET_INDEX >= COUNT:
ld a,(TARGET_INDEX)
ld b,a
ld a,(COUNT)
ld c,a
ld a,b
cp c
jp nc,waypoint_route_L7
jp waypoint_route_L8
waypoint_route_L7:
; return
ret
jp waypoint_route_L9
waypoint_route_L8:
waypoint_route_L9:
; if SPEED == 0:
ld a,(SPEED)
cp 0
jp z,waypoint_route_L10
jp waypoint_route_L11
waypoint_route_L10:
; return
ret
jp waypoint_route_L12
waypoint_route_L11:
waypoint_route_L12:
; VALID = 1
ld a,1
ld (VALID),a
; if FINISHED != 0:
ld a,(FINISHED)
cp 0
jp nz,waypoint_route_L13
jp waypoint_route_L14
waypoint_route_L13:
; return
ret
jp waypoint_route_L15
waypoint_route_L14:
waypoint_route_L15:
; T0 = P[TARGET_INDEX]
ld a,(TARGET_INDEX)
ld e,a
ld d,0
ld hl,P0
add hl,de
ld a,(hl)
ld (T0),a
; if POSITION < T0:
ld a,(POSITION)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp c,waypoint_route_L16
jp waypoint_route_L17
waypoint_route_L16:
; T1 = T0 - POSITION
ld a,(T0)
ld b,a
ld a,(POSITION)
ld c,a
ld a,b
sub c
ld (T1),a
; if T1 > SPEED:
ld a,(T1)
ld b,a
ld a,(SPEED)
ld c,a
ld a,b
cp c
jp z,waypoint_route_L20
jp nc,waypoint_route_L19
jp waypoint_route_L20
waypoint_route_L19:
; POSITION = POSITION + SPEED
ld a,(POSITION)
ld b,a
ld a,(SPEED)
ld c,a
ld a,b
add a,c
ld (POSITION),a
; return
ret
jp waypoint_route_L21
waypoint_route_L20:
waypoint_route_L21:
jp waypoint_route_L18
waypoint_route_L17:
; T1 = POSITION - T0
ld a,(POSITION)
ld b,a
ld a,(T0)
ld c,a
ld a,b
sub c
ld (T1),a
; if T1 > SPEED:
ld a,(T1)
ld b,a
ld a,(SPEED)
ld c,a
ld a,b
cp c
jp z,waypoint_route_L23
jp nc,waypoint_route_L22
jp waypoint_route_L23
waypoint_route_L22:
; POSITION = POSITION - SPEED
ld a,(POSITION)
ld b,a
ld a,(SPEED)
ld c,a
ld a,b
sub c
ld (POSITION),a
; return
ret
jp waypoint_route_L24
waypoint_route_L23:
waypoint_route_L24:
waypoint_route_L18:
; POSITION = T0
ld a,(T0)
ld (POSITION),a
; ARRIVED = 1
ld a,1
ld (ARRIVED),a
; T1 = COUNT - 1
ld a,(COUNT)
sub 1
ld (T1),a
; if TARGET_INDEX < T1:
ld a,(TARGET_INDEX)
ld b,a
ld a,(T1)
ld c,a
ld a,b
cp c
jp c,waypoint_route_L25
jp waypoint_route_L26
waypoint_route_L25:
; TARGET_INDEX = TARGET_INDEX + 1
ld a,(TARGET_INDEX)
add a,1
ld (TARGET_INDEX),a
jp waypoint_route_L27
waypoint_route_L26:
; if LOOP != 0:
ld a,(LOOP)
cp 0
jp nz,waypoint_route_L28
jp waypoint_route_L29
waypoint_route_L28:
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
jp waypoint_route_L30
waypoint_route_L29:
; FINISHED = 1
ld a,1
ld (FINISHED),a
waypoint_route_L30:
waypoint_route_L27:
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:
POSITION: defb 0
TARGET_INDEX: defb 0
COUNT: defb 0
SPEED: defb 0
LOOP: defb 0
ARRIVED: defb 0
FINISHED: defb 0
VALID: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: 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
; COUNT = 3
ld a,3
ld (COUNT),a
; SPEED = 4
ld a,4
ld (SPEED),a
; P0 = 6
ld a,6
ld (P0),a
; P1 = 2
ld a,2
ld (P1),a
; P2 = 9
ld a,9
ld (P2),a
; LOOP = 1
ld a,1
ld (LOOP),a
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
ret
else
; COUNT = 3
ld a,3
ld (COUNT),a
; SPEED = 4
ld a,4
ld (SPEED),a
; P0 = 6
ld a,6
ld (P0),a
; P1 = 2
ld a,2
ld (P1),a
; P2 = 9
ld a,9
ld (P2),a
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
if BROKEN
; TARGET_INDEX = 0
ld a,0
ld (TARGET_INDEX),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
POSITION: defb 0
TARGET_INDEX: defb 0
COUNT: defb 0
SPEED: defb 0
LOOP: defb 0
ARRIVED: defb 0
FINISHED: defb 0
VALID: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: 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 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
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:
POSITION: defb 0
TARGET_INDEX: defb 0
COUNT: defb 0
SPEED: defb 0
LOOP: defb 0
ARRIVED: defb 0
FINISHED: defb 0
VALID: defb 0
P0: defb 0
P1: defb 0
P2: defb 0
P3: defb 0
T0: defb 0
T1: 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 345 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 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
Approach a target: The route adds an ordered target cursor and completion policy to the simpler movement operation.