A reused pool slot should not let an old reference suddenly name a different enemy.
Use it in a project
Four slots use ACTIVE bits 0–3 and one generation byte each. MODE=0 allocates the lowest free slot and increments its generation modulo 256. MODE=1 frees a live SLOT with matching GENERATION; MODE=2 queries that handle. RESULT=1 returns OUT_SLOT/OUT_GENERATION; failure returns 255/0. ACTIVE above 15, unknown mode or out-of-range handle index returns VALID=0; ordinary full/stale requests are valid failures.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: RESULT, OUT_SLOT, OUT_GENERATION, VALID, MODE, SLOT, GENERATION, ACTIVE, G0, G1, G2, G3. They occupy 12 contiguous bytes. Also reserve the private workspace: T0, T1, T2, EXPR_SAVE (4 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call generation_handles_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 generation_handles_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 | ACTIVE and G0–G3 |
| Supply before service | MODE, SLOT and GENERATION |
| Read after service | RESULT, OUT_SLOT, OUT_GENERATION and VALID |
An index says where storage is; a generation says which lifetime occupies it. Releasing clears the live bit, while allocating advances the lifetime before returning a new reference.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Reject Stale Object Handles. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
generation_handles_init:
ld a,0
ld (RESULT),a
ld (OUT_SLOT),a
ld (OUT_GENERATION),a
ld (VALID),a
ld (MODE),a
ld (SLOT),a
ld (GENERATION),a
ld (ACTIVE),a
ld (G0),a
ld (G1),a
ld (G2),a
ld (G3),a
ld (T0),a
ld (T1),a
ld (T2),a
ld (EXPR_SAVE),a
ret
generation_handles_step:
; RESULT = 0
ld a,0
ld (RESULT),a
; OUT_SLOT = 255
ld a,255
ld (OUT_SLOT),a
; OUT_GENERATION = 0
ld a,0
ld (OUT_GENERATION),a
; VALID = 0
ld a,0
ld (VALID),a
; if ACTIVE > 15:
ld a,(ACTIVE)
cp 15
jp z,generation_handles_L2
jp nc,generation_handles_L1
jp generation_handles_L2
generation_handles_L1:
; return
ret
jp generation_handles_L3
generation_handles_L2:
generation_handles_L3:
; if MODE >= 3:
ld a,(MODE)
cp 3
jp nc,generation_handles_L4
jp generation_handles_L5
generation_handles_L4:
; return
ret
jp generation_handles_L6
generation_handles_L5:
generation_handles_L6:
; if MODE == 0:
ld a,(MODE)
cp 0
jp z,generation_handles_L7
jp generation_handles_L8
generation_handles_L7:
; VALID = 1
ld a,1
ld (VALID),a
; T0 = 0
ld a,0
ld (T0),a
; T1 = 1
ld a,1
ld (T1),a
; while T0 < 4:
generation_handles_L10:
ld a,(T0)
cp 4
jp c,generation_handles_L11
jp generation_handles_L12
generation_handles_L11:
; T2 = ACTIVE & T1
ld a,(ACTIVE)
ld b,a
ld a,(T1)
ld c,a
ld a,b
and c
ld (T2),a
; if T2 == 0:
ld a,(T2)
cp 0
jp z,generation_handles_L13
jp generation_handles_L14
generation_handles_L13:
; T2 = G[T0]
ld a,(T0)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld a,(hl)
ld (T2),a
; T2 = T2 + 1
ld a,(T2)
add a,1
ld (T2),a
; G[T0] = T2
ld a,(T2)
ld c,a
ld a,(T0)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld (hl),c
; ACTIVE = ACTIVE | T1
ld a,(ACTIVE)
ld b,a
ld a,(T1)
ld c,a
ld a,b
or c
ld (ACTIVE),a
; OUT_SLOT = T0
ld a,(T0)
ld (OUT_SLOT),a
; OUT_GENERATION = T2
ld a,(T2)
ld (OUT_GENERATION),a
; RESULT = 1
ld a,1
ld (RESULT),a
; return
ret
jp generation_handles_L15
generation_handles_L14:
generation_handles_L15:
; T0 = T0 + 1
ld a,(T0)
add a,1
ld (T0),a
; T1 = T1 << 1
ld a,(T1)
sla a
ld (T1),a
jp generation_handles_L10
generation_handles_L12:
; return
ret
jp generation_handles_L9
generation_handles_L8:
generation_handles_L9:
; if SLOT >= 4:
ld a,(SLOT)
cp 4
jp nc,generation_handles_L16
jp generation_handles_L17
generation_handles_L16:
; return
ret
jp generation_handles_L18
generation_handles_L17:
generation_handles_L18:
; VALID = 1
ld a,1
ld (VALID),a
; T0 = 1
ld a,1
ld (T0),a
; T1 = SLOT
ld a,(SLOT)
ld (T1),a
; while T1 != 0:
generation_handles_L19:
ld a,(T1)
cp 0
jp nz,generation_handles_L20
jp generation_handles_L21
generation_handles_L20:
; T0 = T0 << 1
ld a,(T0)
sla a
ld (T0),a
; T1 = T1 - 1
ld a,(T1)
sub 1
ld (T1),a
jp generation_handles_L19
generation_handles_L21:
; T1 = ACTIVE & T0
ld a,(ACTIVE)
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,generation_handles_L22
jp generation_handles_L23
generation_handles_L22:
; return
ret
jp generation_handles_L24
generation_handles_L23:
generation_handles_L24:
; T1 = G[SLOT]
ld a,(SLOT)
ld e,a
ld d,0
ld hl,G0
add hl,de
ld a,(hl)
ld (T1),a
; if T1 != GENERATION:
ld a,(T1)
ld b,a
ld a,(GENERATION)
ld c,a
ld a,b
cp c
jp nz,generation_handles_L25
jp generation_handles_L26
generation_handles_L25:
; return
ret
jp generation_handles_L27
generation_handles_L26:
generation_handles_L27:
; OUT_SLOT = SLOT
ld a,(SLOT)
ld (OUT_SLOT),a
; OUT_GENERATION = T1
ld a,(T1)
ld (OUT_GENERATION),a
; RESULT = 1
ld a,1
ld (RESULT),a
; if MODE == 1:
ld a,(MODE)
cp 1
jp z,generation_handles_L28
jp generation_handles_L29
generation_handles_L28:
; T0 = T0 ^ 255
ld a,(T0)
xor 255
ld (T0),a
; ACTIVE = ACTIVE & T0
ld a,(ACTIVE)
ld b,a
ld a,(T0)
ld c,a
ld a,b
and c
ld (ACTIVE),a
jp generation_handles_L30
generation_handles_L29:
generation_handles_L30:
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:
RESULT: defb 0
OUT_SLOT: defb 0
OUT_GENERATION: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
T0: defb 0
T1: defb 0
T2: 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
; G0 = 255
ld a,255
ld (G0),a
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
; MODE = 2
ld a,2
ld (MODE),a
; SLOT = 0
ld a,0
ld (SLOT),a
; GENERATION = 0
ld a,0
ld (GENERATION),a
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
ret
else
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
; MODE = 1
ld a,1
ld (MODE),a
; SLOT = 0
ld a,0
ld (SLOT),a
; GENERATION = 1
ld a,1
ld (GENERATION),a
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
; MODE = 0
ld a,0
ld (MODE),a
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
; MODE = 2
ld a,2
ld (MODE),a
; SLOT = 0
ld a,0
ld (SLOT),a
; GENERATION = 2
ld a,2
ld (GENERATION),a
if BROKEN
; GENERATION = 0
ld a,0
ld (GENERATION),a
endif
call checked_step
call emit_state
endif
retCustomise it
Exercise a generation-byte wrap from 255 to zero. 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:
RESULT: defb 0
OUT_SLOT: defb 0
OUT_GENERATION: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
T0: defb 0
T1: defb 0
T2: 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 00 00 01 02 00 00 01. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller throws away the generation part of each reference. Free/query requests fail or eventually identify the wrong lifetime. Store and pass both parts of the returned handle.
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:
RESULT: defb 0
OUT_SLOT: defb 0
OUT_GENERATION: defb 0
VALID: defb 0
MODE: defb 0
SLOT: defb 0
GENERATION: defb 0
ACTIVE: defb 0
G0: defb 0
G1: defb 0
G2: defb 0
G3: defb 0
T0: defb 0
T1: defb 0
T2: 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 435 code bytes; public state is 12 bytes and private scratch is 4 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.
Allocation scans at most four slots; validated query/free uses at most three mask shifts. Stable indices avoid moving object records, at the cost of live flags and generation storage.
A byte generation eventually wraps: a sufficiently old handle can alias after 256 allocations of that slot. Wider generations reduce that risk but do not create eternal uniqueness. This validates identity, not object type, ownership or thread safety.
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
Object pools: Generation handles extend slot reuse with identity checks; they do not replace the pool’s actual object storage.