The same contact can appear in both objects’ candidate lists. Apply its response once, even when the pair arrives in the opposite order.
Use it in a project
Object identifiers are 0–7. MODE=0 clears the eight-byte cache; MODE=1 tests and remembers a distinct pair; MODE=2 tests without changing the cache. Return FIRST=1 if the unordered pair has not been remembered and VALID=1 for accepted operations. Invalid mode/identifier or a self-pair returns zero outputs without cache writes. Clear once before enumerating a simulation step.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: FIRST, VALID, MODE, OBJECT_A, OBJECT_B, B0, B1, B2, B3, B4, B5, B6, B7. They occupy 13 contiguous bytes. Also reserve the private workspace: T0, T1, T2, T3, T4, EXPR_SAVE (6 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call contact_pair_cache_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 contact_pair_cache_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 | B0..B7 |
| Supply before service | MODE and OBJECT_A/B |
| Read after service | FIRST and VALID |
Order the two identifiers, then use the smaller as a row and the larger as a bit. Both traversal orders address the same bit.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Remember Contact Pairs for a Step. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
contact_pair_cache_init:
ld a,0
ld (FIRST),a
ld (VALID),a
ld (MODE),a
ld (OBJECT_A),a
ld (OBJECT_B),a
ld (B0),a
ld (B1),a
ld (B2),a
ld (B3),a
ld (B4),a
ld (B5),a
ld (B6),a
ld (B7),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (T3),a
ld (T4),a
ld (EXPR_SAVE),a
ret
contact_pair_cache_step:
; FIRST = 0
ld a,0
ld (FIRST),a
; VALID = 0
ld a,0
ld (VALID),a
; if MODE > 2:
ld a,(MODE)
cp 2
jp z,contact_pair_cache_L2
jp nc,contact_pair_cache_L1
jp contact_pair_cache_L2
contact_pair_cache_L1:
; return
ret
jp contact_pair_cache_L3
contact_pair_cache_L2:
contact_pair_cache_L3:
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,contact_pair_cache_L4
jp contact_pair_cache_L5
contact_pair_cache_L4:
; T0 = 0
ld a,0
ld (T0),a
; while T0 < 8:
contact_pair_cache_L7:
ld a,(T0)
cp 8
jp c,contact_pair_cache_L8
jp contact_pair_cache_L9
contact_pair_cache_L8:
; B[T0] = 0
ld a,0
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,B0
add hl,de
ld (hl),c
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
jp contact_pair_cache_L7
contact_pair_cache_L9:
; VALID = 1
ld a,1
ld (VALID),a
; return
ret
jp contact_pair_cache_L6
contact_pair_cache_L5:
contact_pair_cache_L6:
; if OBJECT_A >= 8:
ld a,(OBJECT_A)
cp 8
jp nc,contact_pair_cache_L10
jp contact_pair_cache_L11
contact_pair_cache_L10:
; return
ret
jp contact_pair_cache_L12
contact_pair_cache_L11:
contact_pair_cache_L12:
; if OBJECT_B >= 8:
ld a,(OBJECT_B)
cp 8
jp nc,contact_pair_cache_L13
jp contact_pair_cache_L14
contact_pair_cache_L13:
; return
ret
jp contact_pair_cache_L15
contact_pair_cache_L14:
contact_pair_cache_L15:
; if OBJECT_A == OBJECT_B:
ld a,(OBJECT_A)
ld b,a
ld a,(OBJECT_B)
ld c,a
ld a,b
cp c
jp z,contact_pair_cache_L16
jp contact_pair_cache_L17
contact_pair_cache_L16:
; return
ret
jp contact_pair_cache_L18
contact_pair_cache_L17:
contact_pair_cache_L18:
; T0 = OBJECT_A
ld a,(OBJECT_A)
ld (T0),a
; T1 = OBJECT_B
ld a,(OBJECT_B)
ld (T1),a
; if T0 > T1:
ld a,(T0)
ld b,a
ld a,(T1)
ld c,a
ld a,b
cp c
jp z,contact_pair_cache_L20
jp nc,contact_pair_cache_L19
jp contact_pair_cache_L20
contact_pair_cache_L19:
; T0 = OBJECT_B
ld a,(OBJECT_B)
ld (T0),a
; T1 = OBJECT_A
ld a,(OBJECT_A)
ld (T1),a
jp contact_pair_cache_L21
contact_pair_cache_L20:
contact_pair_cache_L21:
; T2 = 1
ld a,1
ld (T2),a
; while T1 != 0:
contact_pair_cache_L22:
ld a,(T1)
cp 0
jp nz,contact_pair_cache_L23
jp contact_pair_cache_L24
contact_pair_cache_L23:
; T2 = T2 << 1
ld a,(T2)
sla a
ld (T2),a
; T1 = T1 - 1
ld a,(T1)
sub 1
ld (T1),a
jp contact_pair_cache_L22
contact_pair_cache_L24:
; T3 = B[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,B0
add hl,de
ld a,(hl)
ld (T3),a
; T4 = T3 & T2
ld a,(T3)
ld b,a
ld a,(T2)
ld c,a
ld a,b
and c
ld (T4),a
; VALID = 1
ld a,1
ld (VALID),a
; if T4 == 0:
ld a,(T4)
cp 0
jp z,contact_pair_cache_L25
jp contact_pair_cache_L26
contact_pair_cache_L25:
; FIRST = 1
ld a,1
ld (FIRST),a
; if MODE == 1:
ld a,(MODE)
cp 1
jp z,contact_pair_cache_L28
jp contact_pair_cache_L29
contact_pair_cache_L28:
; T3 = T3 | T2
ld a,(T3)
ld b,a
ld a,(T2)
ld c,a
ld a,b
or c
ld (T3),a
; B[T0] = T3
ld a,(T3)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,B0
add hl,de
ld (hl),c
jp contact_pair_cache_L30
contact_pair_cache_L29:
contact_pair_cache_L30:
jp contact_pair_cache_L27
contact_pair_cache_L26:
contact_pair_cache_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. 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 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:
FIRST: defb 0
VALID: defb 0
MODE: defb 0
OBJECT_A: defb 0
OBJECT_B: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: defb 0
B4: defb 0
B5: defb 0
B6: defb 0
B7: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
EXPR_SAVE: 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
; MODE = 1
ld a,1
ld (MODE),a
; OBJECT_A = 2
ld a,2
ld (OBJECT_A),a
; OBJECT_B = 7
ld a,7
ld (OBJECT_B),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
; MODE = 2
ld a,2
ld (MODE),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
; MODE = 0
ld a,0
ld (MODE),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
ret
else
; MODE = 1
ld a,1
ld (MODE),a
; OBJECT_A = 0
ld a,0
ld (OBJECT_A),a
; OBJECT_B = 3
ld a,3
ld (OBJECT_B),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
; OBJECT_A = 3
ld a,3
ld (OBJECT_A),a
; OBJECT_B = 0
ld a,0
ld (OBJECT_B),a
if BROKEN
; B0 = 0
ld a,0
ld (B0),a
endif
call checked_step
call emit_state
endif
retCustomise it
Query without inserting, then reset and remember a pair in a new step. 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:
FIRST: defb 0
VALID: defb 0
MODE: defb 0
OBJECT_A: defb 0
OBJECT_B: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: defb 0
B4: defb 0
B5: defb 0
B6: defb 0
B7: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
EXPR_SAVE: 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 01 01 01 02 07 00 00 80. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears cache row zero before every pair. Repeated (0,3) and (3,0) contacts are each reported as first contacts. Reset once before the whole traversal, not between callbacks.
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:
FIRST: defb 0
VALID: defb 0
MODE: defb 0
OBJECT_A: defb 0
OBJECT_B: defb 0
B0: defb 0
B1: defb 0
B2: defb 0
B3: defb 0
B4: defb 0
B5: defb 0
B6: defb 0
B7: defb 0
T0: defb 0
T1: defb 0
T2: defb 0
T3: defb 0
T4: defb 0
EXPR_SAVE: 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 374 code bytes; public state is 13 bytes and private scratch is 6 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.
Eight reset writes, or bounded mask construction per pair. A triangular packed table saves unused bits but complicates indexing; larger object counts need a different storage budget.
Identifiers must remain stable for the step; do not recycle a slot halfway through enumeration. This is step-local deduplication, not persistent enter/stay/exit tracking or an interrupt-safe shared container.
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
Trigger transitions: Persistent contact transitions need previous/current sets, beyond a once-per-step pair cache.