A sword can overlap the same enemy for several frames without dealing damage on every frame.
Use it in a project
START_FRAME..END_FRAME is a half-open active interval, with START_FRAME strictly below END_FRAME. FRAME is the caller’s current attack frame. TARGET is 0–7. A nonzero NEW_ATTACK clears the retained SEEN bitset before testing contact. In the active interval, nonzero CONTACT emits HIT=1 once per target and records its bit. Outside the interval HIT=0. Invalid interval or target clears HIT/VALID and preserves SEEN.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: HIT, VALID, SEEN, FRAME, START_FRAME, END_FRAME, TARGET, CONTACT, NEW_ATTACK. They occupy 9 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 attack_hit_window_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 attack_hit_window_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 | SEEN |
| Supply before service | frame interval, TARGET, CONTACT and NEW_ATTACK |
| Read after service | HIT, SEEN and VALID |
The active interval decides when damage is permitted; the bitset decides whether this attack already affected that target. They answer different questions and are checked independently.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Limit Hits to an Attack Window. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
attack_hit_window_init:
ld a,0
ld (HIT),a
ld (VALID),a
ld (SEEN),a
ld (FRAME),a
ld (START_FRAME),a
ld (END_FRAME),a
ld (TARGET),a
ld (CONTACT),a
ld (NEW_ATTACK),a
ld (T0),a
ld (T1),a
ret
attack_hit_window_step:
; HIT = 0
ld a,0
ld (HIT),a
; VALID = 0
ld a,0
ld (VALID),a
; if START_FRAME >= END_FRAME:
ld a,(START_FRAME)
ld b,a
ld a,(END_FRAME)
ld c,a
ld a,b
cp c
jp nc,attack_hit_window_L1
jp attack_hit_window_L2
attack_hit_window_L1:
; return
ret
jp attack_hit_window_L3
attack_hit_window_L2:
attack_hit_window_L3:
; if TARGET >= 8:
ld a,(TARGET)
cp 8
jp nc,attack_hit_window_L4
jp attack_hit_window_L5
attack_hit_window_L4:
; return
ret
jp attack_hit_window_L6
attack_hit_window_L5:
attack_hit_window_L6:
; VALID = 1
ld a,1
ld (VALID),a
; if NEW_ATTACK != 0:
ld a,(NEW_ATTACK)
cp 0
jp nz,attack_hit_window_L7
jp attack_hit_window_L8
attack_hit_window_L7:
; SEEN = 0
ld a,0
ld (SEEN),a
jp attack_hit_window_L9
attack_hit_window_L8:
attack_hit_window_L9:
; if FRAME < START_FRAME:
ld a,(FRAME)
ld b,a
ld a,(START_FRAME)
ld c,a
ld a,b
cp c
jp c,attack_hit_window_L10
jp attack_hit_window_L11
attack_hit_window_L10:
; return
ret
jp attack_hit_window_L12
attack_hit_window_L11:
attack_hit_window_L12:
; if FRAME >= END_FRAME:
ld a,(FRAME)
ld b,a
ld a,(END_FRAME)
ld c,a
ld a,b
cp c
jp nc,attack_hit_window_L13
jp attack_hit_window_L14
attack_hit_window_L13:
; return
ret
jp attack_hit_window_L15
attack_hit_window_L14:
attack_hit_window_L15:
; if CONTACT == 0:
ld a,(CONTACT)
cp 0
jp z,attack_hit_window_L16
jp attack_hit_window_L17
attack_hit_window_L16:
; return
ret
jp attack_hit_window_L18
attack_hit_window_L17:
attack_hit_window_L18:
; T0 = 1
ld a,1
ld (T0),a
; T1 = TARGET
ld a,(TARGET)
ld (T1),a
; while T1 != 0:
attack_hit_window_L19:
ld a,(T1)
cp 0
jp nz,attack_hit_window_L20
jp attack_hit_window_L21
attack_hit_window_L20:
; T0 = T0 << 1
ld a,(T0)
sla a
ld (T0),a
; T1 = T1 - 1
ld a,(T1)
sub 1
ld (T1),a
jp attack_hit_window_L19
attack_hit_window_L21:
; T1 = SEEN & T0
ld a,(SEEN)
ld b,a
ld a,(T0)
ld c,a
ld a,b
and c
ld (T1),a
; if T1 == 0:
ld a,(T1)
cp 0
jp z,attack_hit_window_L22
jp attack_hit_window_L23
attack_hit_window_L22:
; SEEN = SEEN | T0
ld a,(SEEN)
ld b,a
ld a,(T0)
ld c,a
ld a,b
or c
ld (SEEN),a
; HIT = 1
ld a,1
ld (HIT),a
jp attack_hit_window_L24
attack_hit_window_L23:
attack_hit_window_L24:
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:
HIT: defb 0
VALID: defb 0
SEEN: defb 0
FRAME: defb 0
START_FRAME: defb 0
END_FRAME: defb 0
TARGET: defb 0
CONTACT: defb 0
NEW_ATTACK: 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
; START_FRAME = 1
ld a,1
ld (START_FRAME),a
; END_FRAME = 3
ld a,3
ld (END_FRAME),a
; FRAME = 1
ld a,1
ld (FRAME),a
; TARGET = 7
ld a,7
ld (TARGET),a
; CONTACT = 1
ld a,1
ld (CONTACT),a
; NEW_ATTACK = 1
ld a,1
ld (NEW_ATTACK),a
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
; NEW_ATTACK = 0
ld a,0
ld (NEW_ATTACK),a
; TARGET = 0
ld a,0
ld (TARGET),a
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
; FRAME = 3
ld a,3
ld (FRAME),a
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
ret
else
; START_FRAME = 2
ld a,2
ld (START_FRAME),a
; END_FRAME = 5
ld a,5
ld (END_FRAME),a
; FRAME = 2
ld a,2
ld (FRAME),a
; TARGET = 1
ld a,1
ld (TARGET),a
; CONTACT = 1
ld a,1
ld (CONTACT),a
; NEW_ATTACK = 1
ld a,1
ld (NEW_ATTACK),a
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
; NEW_ATTACK = 0
ld a,0
ld (NEW_ATTACK),a
; FRAME = 3
ld a,3
ld (FRAME),a
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
if BROKEN
; SEEN = 0
ld a,0
ld (SEEN),a
endif
call checked_step
call emit_state
endif
retCustomise it
Strike target seven, then a different target, and stop exactly at the exclusive end frame. 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:
HIT: defb 0
VALID: defb 0
SEEN: defb 0
FRAME: defb 0
START_FRAME: defb 0
END_FRAME: defb 0
TARGET: defb 0
CONTACT: defb 0
NEW_ATTACK: 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 00 01 81 03 01 03 00 01. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong client clears the hit history every service. The same target is hit repeatedly while the attack remains active. Reset SEEN once per new attack, not once per collision candidate.
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:
HIT: defb 0
VALID: defb 0
SEEN: defb 0
FRAME: defb 0
START_FRAME: defb 0
END_FRAME: defb 0
TARGET: defb 0
CONTACT: defb 0
NEW_ATTACK: 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 247 code bytes; public state is 9 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.
At most seven shifts and one bitset update. Larger target sets need more bits or handles; per-attack storage grows with the identity space.
Eight stable target IDs are available per attack. Slot reuse during one attack requires generation-aware identity or clearing the relevant bit under an explicit policy. The caller owns animation, overlap, faction rules and actual damage. Do not set NEW_ATTACK on every candidate in one frame.
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
Damage rules: After this attack grants a hit, apply the target’s invulnerability and health rules separately.