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:
ld a,0
ld (SLOT),a
ld (AMOUNT),a
ld (REMOVE),a
ld (CAP),a
ld (SUCCESS),a
ld (VALID),a
ld (C0),a
ld (C1),a
ld (C2),a
ld (C3),a
ld (T0),a
ld (T1),a
ld (EXPR_SAVE),a
ret
inventory_stacks_step:
; SUCCESS = 0
ld a,0
ld (SUCCESS),a
; VALID = 0
ld a,0
ld (VALID),a
; if SLOT >= 4:
ld a,(SLOT)
cp 4
jp nc,inventory_stacks_L1
jp inventory_stacks_L2
inventory_stacks_L1:
; return
ret
jp inventory_stacks_L3
inventory_stacks_L2:
inventory_stacks_L3:
; if CAP == 0:
ld a,(CAP)
cp 0
jp z,inventory_stacks_L4
jp inventory_stacks_L5
inventory_stacks_L4:
; return
ret
jp inventory_stacks_L6
inventory_stacks_L5:
inventory_stacks_L6:
; T0 = C[SLOT]
ld a,(SLOT)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld a,(hl)
ld (T0),a
; if T0 > CAP:
ld a,(T0)
ld b,a
ld a,(CAP)
ld c,a
ld a,b
cp c
jp z,inventory_stacks_L8
jp nc,inventory_stacks_L7
jp inventory_stacks_L8
inventory_stacks_L7:
; return
ret
jp inventory_stacks_L9
inventory_stacks_L8:
inventory_stacks_L9:
; VALID = 1
ld a,1
ld (VALID),a
; if REMOVE != 0:
ld a,(REMOVE)
cp 0
jp nz,inventory_stacks_L10
jp inventory_stacks_L11
inventory_stacks_L10:
; if AMOUNT > T0:
ld a,(AMOUNT)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp z,inventory_stacks_L14
jp nc,inventory_stacks_L13
jp inventory_stacks_L14
inventory_stacks_L13:
; return
ret
jp inventory_stacks_L15
inventory_stacks_L14:
inventory_stacks_L15:
; T0 = T0 - AMOUNT
ld a,(T0)
ld b,a
ld a,(AMOUNT)
ld c,a
ld a,b
sub c
ld (T0),a
jp inventory_stacks_L12
inventory_stacks_L11:
; T1 = CAP - T0
ld a,(CAP)
ld b,a
ld a,(T0)
ld c,a
ld a,b
sub c
ld (T1),a
; if AMOUNT > T1:
ld a,(AMOUNT)
ld b,a
ld a,(T1)
ld c,a
ld a,b
cp c
jp z,inventory_stacks_L17
jp nc,inventory_stacks_L16
jp inventory_stacks_L17
inventory_stacks_L16:
; return
ret
jp inventory_stacks_L18
inventory_stacks_L17:
inventory_stacks_L18:
; T0 = T0 + AMOUNT
ld a,(T0)
ld b,a
ld a,(AMOUNT)
ld c,a
ld a,b
add a,c
ld (T0),a
inventory_stacks_L12:
; C[SLOT] = T0
ld a,(T0)
ld c,a
ld a,(SLOT)
ld e,a
ld d,0
ld hl,C0
add hl,de
ld (hl),c
; SUCCESS = 1
ld a,1
ld (SUCCESS),a
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. Scope or rename routine and field labels and allocate a separate block for each independent instance. 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:
SLOT: defb 0
AMOUNT: defb 0
REMOVE: defb 0
CAP: defb 0
SUCCESS: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
T0: defb 0
T1: 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
; CAP = 7
ld a,7
ld (CAP),a
; SLOT = 1
ld a,1
ld (SLOT),a
; AMOUNT = 6
ld a,6
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
; AMOUNT = 5
ld a,5
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
; REMOVE = 1
ld a,1
ld (REMOVE),a
; AMOUNT = 2
ld a,2
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
ret
else
; CAP = 9
ld a,9
ld (CAP),a
; SLOT = 1
ld a,1
ld (SLOT),a
; AMOUNT = 6
ld a,6
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
; AMOUNT = 5
ld a,5
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
; REMOVE = 1
ld a,1
ld (REMOVE),a
; AMOUNT = 2
ld a,2
ld (AMOUNT),a
if BROKEN
; REMOVE = 1
ld a,1
ld (REMOVE),a
endif
call checked_step
call emit_state
endif
retCustomise 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
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:
SLOT: defb 0
AMOUNT: defb 0
REMOVE: defb 0
CAP: defb 0
SUCCESS: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
T0: defb 0
T1: 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 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
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:
SLOT: defb 0
AMOUNT: defb 0
REMOVE: defb 0
CAP: defb 0
SUCCESS: defb 0
VALID: defb 0
C0: defb 0
C1: defb 0
C2: defb 0
C3: defb 0
T0: defb 0
T1: 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 244 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 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
Resource wallets: Both protect a bounded quantity; indexed slots add a choice of which stack owns the count.