Picking up a stack should either fit completely or leave the inventory as it was.
Use it in a project
Four count slots each have caller-selected CAP 1–255. SLOT must be 0–3 and its current count at most CAP. REMOVE=0 adds, nonzero subtracts. AMOUNT=0 succeeds without a change. SUCCESS reports the complete transaction; insufficient capacity or stock is a valid but unsuccessful request. An invalid slot or zero cap rejects before the count is read. An out-of-range current count rejects before any count is written.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SLOT, AMOUNT, REMOVE, CAP, SUCCESS, VALID, C0, C1, C2, C3. They occupy 10 contiguous bytes. Also reserve the private workspace: T0, T1, EXPR_SAVE (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call inventory_stacks_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 inventory_stacks_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 | C0–C3 |
| Supply before service | SLOT, AMOUNT, REMOVE and CAP |
| Read after service | SUCCESS and VALID |
Check the room left before adding, or the stock available before subtracting. These comparisons avoid using wrapped byte arithmetic as an overflow test.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Add and Remove Inventory Stacks. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
inventory_stacks_init:
move.b #0,d0
move.b d0,SLOT
move.b d0,AMOUNT
move.b d0,REMOVE
move.b d0,CAP
move.b d0,SUCCESS
move.b d0,VALID
move.b d0,C0
move.b d0,C1
move.b d0,C2
move.b d0,C3
move.b d0,T0
move.b d0,T1
move.b d0,EXPR_SAVE
rts
inventory_stacks_step:
; SUCCESS = 0
move.b #0,d0
move.b d0,SUCCESS
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if SLOT >= 4:
move.b SLOT,d0
cmp.b #4,d0
bcc inventory_stacks_L1
jmp inventory_stacks_L2
inventory_stacks_L1:
; return
rts
jmp inventory_stacks_L3
inventory_stacks_L2:
inventory_stacks_L3:
; if CAP == 0:
move.b CAP,d0
cmp.b #0,d0
beq inventory_stacks_L4
jmp inventory_stacks_L5
inventory_stacks_L4:
; return
rts
jmp inventory_stacks_L6
inventory_stacks_L5:
inventory_stacks_L6:
; T0 = C[SLOT]
moveq #0,d1
move.b SLOT,d1
lea C0,a0
move.b 0(a0,d1.w),d0
move.b d0,T0
; if T0 > CAP:
move.b T0,d0
move.b CAP,d1
cmp.b d1,d0
bhi inventory_stacks_L7
jmp inventory_stacks_L8
inventory_stacks_L7:
; return
rts
jmp inventory_stacks_L9
inventory_stacks_L8:
inventory_stacks_L9:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; if REMOVE != 0:
move.b REMOVE,d0
cmp.b #0,d0
bne inventory_stacks_L10
jmp inventory_stacks_L11
inventory_stacks_L10:
; if AMOUNT > T0:
move.b AMOUNT,d0
move.b T0,d1
cmp.b d1,d0
bhi inventory_stacks_L13
jmp inventory_stacks_L14
inventory_stacks_L13:
; return
rts
jmp inventory_stacks_L15
inventory_stacks_L14:
inventory_stacks_L15:
; T0 = T0 - AMOUNT
move.b T0,d0
move.b AMOUNT,d1
sub.b d1,d0
move.b d0,T0
jmp inventory_stacks_L12
inventory_stacks_L11:
; T1 = CAP - T0
move.b CAP,d0
move.b T0,d1
sub.b d1,d0
move.b d0,T1
; if AMOUNT > T1:
move.b AMOUNT,d0
move.b T1,d1
cmp.b d1,d0
bhi inventory_stacks_L16
jmp inventory_stacks_L17
inventory_stacks_L16:
; return
rts
jmp inventory_stacks_L18
inventory_stacks_L17:
inventory_stacks_L18:
; T0 = T0 + AMOUNT
move.b T0,d0
move.b AMOUNT,d1
add.b d1,d0
move.b d0,T0
inventory_stacks_L12:
; C[SLOT] = T0
move.b T0,d0
move.b d0,d2
moveq #0,d1
move.b SLOT,d1
lea C0,a0
move.b d2,0(a0,d1.w)
; SUCCESS = 1
move.b #1,d0
move.b d0,SUCCESS
rts
component_end:Machine contract
D0–D3/A0–A1/CCR may change; D4–D7/A2–A6 are preserved. A direct BSR/JSR needs four 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 A500 OCS diagnostic cold-boots an OFS disk, owns the Copper and bitplane and disables interrupt/DMA sources. The helper does not take over hardware or allocate Chip RAM.
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 and BUILD198X=/path/to/build198x. The Makefile produces normal, custom, deliberately wrong and repaired programs. Tools must already be installed; the sample downloads nothing.
Bindings and include order
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
AMOUNT: dc.b 0
REMOVE: dc.b 0
CAP: dc.b 0
SUCCESS: dc.b 0
VALID: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
T0: dc.b 0
T1: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0A client retaining state between calls
preview_rules:
ifne CUSTOM
; CAP = 7
move.b #7,d0
move.b d0,CAP
; SLOT = 1
move.b #1,d0
move.b d0,SLOT
; AMOUNT = 6
move.b #6,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
; AMOUNT = 5
move.b #5,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
; AMOUNT = 2
move.b #2,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
rts
else
; CAP = 9
move.b #9,d0
move.b d0,CAP
; SLOT = 1
move.b #1,d0
move.b d0,SLOT
; AMOUNT = 6
move.b #6,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
; AMOUNT = 5
move.b #5,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
; AMOUNT = 2
move.b #2,d0
move.b d0,AMOUNT
ifne BROKEN
; REMOVE = 1
move.b #1,d0
move.b d0,REMOVE
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Use a stack cap of seven rather than nine. 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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
AMOUNT: dc.b 0
REMOVE: dc.b 0
CAP: dc.b 0
SUCCESS: dc.b 0
VALID: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
T0: dc.b 0
T1: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0The custom diagnostic row is 01 02 01 07 01 01 00 04. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller labels a pickup as REMOVE. The helper correctly refuses missing stock or subtracts from an existing stack. Keep the meaning of the request separate from whether a button was pressed.
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
section code,code_c
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"
even
include "data.inc"
include "cases.inc"
even
include "display.inc"
include "../../framework/turn-deadline/font.inc"
code_end:
STATE:
SLOT: dc.b 0
AMOUNT: dc.b 0
REMOVE: dc.b 0
CAP: dc.b 0
SUCCESS: dc.b 0
VALID: dc.b 0
C0: dc.b 0
C1: dc.b 0
C2: dc.b 0
C3: dc.b 0
T0: dc.b 0
T1: dc.b 0
EXPR_SAVE: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XGAMEKIT0000",0
even
copperlist:
dc.w $08e,$2c81,$090,$2cc1,$092,$0038,$094,$00d0
dc.w $100,$1200,$102,0,$104,0,$108,0,$10a,0
dc.w $180,$012,$182,$fff
copbpl:
dc.w $0e0,0,$0e2,0
dc.w $ffff,$fffe
plane: dcb.b 10240,0
Cost and alternatives
The measured helper extent is 334 code bytes; public state is 10 bytes and private scratch is 3 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 a known slot. Finding a matching item or distributing a stack needs a bounded search and a separate transaction policy.
Slots represent item kinds, not objects with metadata. This neither finds a free slot nor automatically spreads a pickup across stacks. Those policies can be layered over a read-only capacity check or a multi-slot transaction.
Execution evidence
Four variants run on A500 OCS PAL, Kickstart1.3 OFS cold boot, using Asm198x 0.0.72, Emu198x 0.44.2 and Build198x 0.2.9. 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
Resource wallets: Both protect a bounded quantity; indexed slots add a choice of which stack owns the count.