Report Trigger Enter, Stay and Exit Events on NES | Pattern Library

Standing in a doorway should not count as entering it on every update.

NES shows the working diagnostic row 00 06 00 06 06 00 00 00. — A completed native caller displays 00 06 00 06 06 00 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

CURRENT is an eight-bit overlap snapshot, one bit per trigger. Compare it with retained PREVIOUS: ENTERED is current-only, EXITED is previous-only and STAYED is their intersection. Then retain CURRENT as PREVIOUS. Every bit is handled independently; the helper does not decide overlap geometry.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: ENTERED, STAYED, EXITED, PREVIOUS, CURRENT. They occupy 5 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 trigger_transitions_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 trigger_transitions_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	PREVIOUS
Supply before service	CURRENT
Read after service	ENTERED, STAYED and EXITED

An intersection reports shared occupancy, while complement-and-intersection picks the two differences. The stored mask turns static collision answers into temporal events.

Reusable initialisation and service

6502.inc

; SPDX-License-Identifier: MIT
; Report Trigger Enter, Stay and Exit Events. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
trigger_transitions_init:
    lda #0
    sta ENTERED
    sta STAYED
    sta EXITED
    sta PREVIOUS
    sta CURRENT
    sta T0
    rts
trigger_transitions_step:
; T0 = PREVIOUS ^ 255
    lda PREVIOUS
    eor #255
    sta T0
; ENTERED = CURRENT & T0
    lda CURRENT
    and T0
    sta ENTERED
; T0 = CURRENT ^ 255
    lda CURRENT
    eor #255
    sta T0
; EXITED = PREVIOUS & T0
    lda PREVIOUS
    and T0
    sta EXITED
; STAYED = CURRENT & PREVIOUS
    lda CURRENT
    and PREVIOUS
    sta STAYED
; PREVIOUS = CURRENT
    lda CURRENT
    sta PREVIOUS
    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. 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 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
ENTERED=$80
STAYED=$81
EXITED=$82
PREVIOUS=$83
CURRENT=$84
T0=$85
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
; CURRENT = 128
    lda #128
    sta CURRENT
.if BROKEN
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
.endif
    jsr checked_step
    jsr emit_state
; CURRENT = 0
    lda #0
    sta CURRENT
.if BROKEN
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
.endif
    jsr checked_step
    jsr emit_state
; CURRENT = 128
    lda #128
    sta CURRENT
.if BROKEN
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; CURRENT = 3
    lda #3
    sta CURRENT
.if BROKEN
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
.endif
    jsr checked_step
    jsr emit_state
; CURRENT = 6
    lda #6
    sta CURRENT
.if BROKEN
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
.endif
    jsr checked_step
    jsr emit_state

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

Customise it

Use the high trigger bit and an exit/re-entry sequence. 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
ENTERED=$80
STAYED=$81
EXITED=$82
PREVIOUS=$83
CURRENT=$84
T0=$85
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 80 00 00 80 80 00 00 00. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller clears PREVIOUS each time. Occupied triggers keep re-entering and exit events disappear. Retain the last complete snapshot for this same actor.

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
ENTERED=$80
STAYED=$81
EXITED=$82
PREVIOUS=$83
CURRENT=$84
T0=$85
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 00 00 06 06 00 00 00. — The wrong client completes with 06 00 00 06 06 00 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 50 code bytes; public state is 5 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 for eight zones in one byte. More triggers can use more mask bytes or a sparse event list; neither removes the need to own the previous state per actor.

Snapshots can miss a fast pass through a narrow zone between updates. Swept geometry or substeps may be needed. Separate actors need separate previous masks; combining their snapshots changes the meaning of an event.

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

Chord edges: Both techniques derive events from state, but a trigger snapshot describes world overlap rather than held controls.

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