Add and Remove Inventory Stacks on C64 | Pattern Library

Picking up a stack should either fit completely or leave the inventory as it was.

C64 shows the working diagnostic row 01 02 01 09 01 01 00 04. — A completed native caller displays 01 02 01 09 01 01 00 04. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

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

6502.inc

; 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:
    lda #0
    sta SLOT
    sta AMOUNT
    sta REMOVE
    sta CAP
    sta SUCCESS
    sta VALID
    sta C0
    sta C1
    sta C2
    sta C3
    sta T0
    sta T1
    sta EXPR_SAVE
    rts
inventory_stacks_step:
; SUCCESS = 0
    lda #0
    sta SUCCESS
; VALID = 0
    lda #0
    sta VALID
; if SLOT >= 4:
    lda SLOT
    cmp #4
    bcs inventory_stacks_L4
inventory_stacks_L5:
    jmp inventory_stacks_L2
inventory_stacks_L4:
    jmp inventory_stacks_L1
inventory_stacks_L1:
; return
    rts
    jmp inventory_stacks_L3
inventory_stacks_L2:
inventory_stacks_L3:
; if CAP == 0:
    lda CAP
    cmp #0
    beq inventory_stacks_L9
inventory_stacks_L10:
    jmp inventory_stacks_L7
inventory_stacks_L9:
    jmp inventory_stacks_L6
inventory_stacks_L6:
; return
    rts
    jmp inventory_stacks_L8
inventory_stacks_L7:
inventory_stacks_L8:
; T0 = C[SLOT]
    ldx SLOT
    lda C0,x
    sta T0
; if T0 > CAP:
    lda T0
    cmp CAP
    beq inventory_stacks_L15
    bcs inventory_stacks_L14
inventory_stacks_L15:
    jmp inventory_stacks_L12
inventory_stacks_L14:
    jmp inventory_stacks_L11
inventory_stacks_L11:
; return
    rts
    jmp inventory_stacks_L13
inventory_stacks_L12:
inventory_stacks_L13:
; VALID = 1
    lda #1
    sta VALID
; if REMOVE != 0:
    lda REMOVE
    cmp #0
    bne inventory_stacks_L19
inventory_stacks_L20:
    jmp inventory_stacks_L17
inventory_stacks_L19:
    jmp inventory_stacks_L16
inventory_stacks_L16:
; if AMOUNT > T0:
    lda AMOUNT
    cmp T0
    beq inventory_stacks_L25
    bcs inventory_stacks_L24
inventory_stacks_L25:
    jmp inventory_stacks_L22
inventory_stacks_L24:
    jmp inventory_stacks_L21
inventory_stacks_L21:
; return
    rts
    jmp inventory_stacks_L23
inventory_stacks_L22:
inventory_stacks_L23:
; T0 = T0 - AMOUNT
    lda T0
    sec
    sbc AMOUNT
    sta T0
    jmp inventory_stacks_L18
inventory_stacks_L17:
; T1 = CAP - T0
    lda CAP
    sec
    sbc T0
    sta T1
; if AMOUNT > T1:
    lda AMOUNT
    cmp T1
    beq inventory_stacks_L30
    bcs inventory_stacks_L29
inventory_stacks_L30:
    jmp inventory_stacks_L27
inventory_stacks_L29:
    jmp inventory_stacks_L26
inventory_stacks_L26:
; return
    rts
    jmp inventory_stacks_L28
inventory_stacks_L27:
inventory_stacks_L28:
; T0 = T0 + AMOUNT
    lda T0
    clc
    adc AMOUNT
    sta T0
inventory_stacks_L18:
; C[SLOT] = T0
    lda T0
    sta EXPR_SAVE
    ldx SLOT
    lda EXPR_SAVE
    sta C0,x
; SUCCESS = 1
    lda #1
    sta SUCCESS
    rts
component_end:

Machine contract

A/X/status may change; Y and decimal mode are preserved. Use CLD for binary arithmetic. A direct JSR 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 diagnostic PRG owns screen, font and colour RAM and disables interrupts; it does not return to BASIC. The reusable helper does none of those things.

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

demo.asm

SLOT=$80
AMOUNT=$81
REMOVE=$82
CAP=$83
SUCCESS=$84
VALID=$85
C0=$86
C1=$87
C2=$88
C3=$89
T0=$8a
T1=$8b
EXPR_SAVE=$8c
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=0
BROKEN=0
COYOTE_STEPS=4
BUFFER_STEPS=4
TEXT_CAP=8
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=7
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

A client retaining state between calls

exercise.inc, excerpt

preview_rules:
!if CUSTOM {
; CAP = 7
    lda #7
    sta CAP
; SLOT = 1
    lda #1
    sta SLOT
; AMOUNT = 6
    lda #6
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
; AMOUNT = 5
    lda #5
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
; REMOVE = 1
    lda #1
    sta REMOVE
; AMOUNT = 2
    lda #2
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; CAP = 9
    lda #9
    sta CAP
; SLOT = 1
    lda #1
    sta SLOT
; AMOUNT = 6
    lda #6
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
; AMOUNT = 5
    lda #5
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
; REMOVE = 1
    lda #1
    sta REMOVE
; AMOUNT = 2
    lda #2
    sta AMOUNT
!if BROKEN {
; REMOVE = 1
    lda #1
    sta REMOVE
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise 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

custom.asm

SLOT=$80
AMOUNT=$81
REMOVE=$82
CAP=$83
SUCCESS=$84
VALID=$85
C0=$86
C1=$87
C2=$88
C3=$89
T0=$8a
T1=$8b
EXPR_SAVE=$8c
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=1
BROKEN=0
COYOTE_STEPS=2
BUFFER_STEPS=2
TEXT_CAP=4
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=49
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

The 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

wrong.asm

SLOT=$80
AMOUNT=$81
REMOVE=$82
CAP=$83
SUCCESS=$84
VALID=$85
C0=$86
C1=$87
C2=$88
C3=$89
T0=$8a
T1=$8b
EXPR_SAVE=$8c
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$c000
preview=OUTPUT+1400
ready=OUTPUT+1408
CUSTOM=0
BROKEN=1
COYOTE_STEPS=4
BUFFER_STEPS=4
TEXT_CAP=8
PROPERTY_COUNT=16
STREAM_BYTES=13
CRC_POLY=7
!source "caller.inc"
!source "exercise.inc"
component_start:
!source "component.inc"
!source "data.inc"
!source "cases.inc"
hex_font:
    !byte 0,0,0,0,0,0,0,0,0,56,40,40,40,56,0,0,0,16,48,16,16,56,0,0,0,56,8,56,32,56,0,0,0,56,8,56,8,56,0,0,0,40,40,56,8,8,0,0,0,56,32,56,8,56,0,0,0,56,32,56,40,56,0,0,0,56,8,8,8,8,0,0,0,56,40,56,40,56,0,0,0,56,40,56,8,56,0,0,0,16,40,56,40,40,0,0,0,48,40,48,40,48,0,0,0,56,32,32,32,56,0,0,0,48,40,40,40,48,0,0,0,56,32,48,32,56,0,0,0,56,32,48,32,32,0,0
code_end:

C64 shows the deliberate error row 01 02 01 09 00 01 00 00. — The wrong client completes with 01 02 01 09 00 01 00 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 184 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 PAL stock-ROM PRG, 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.

The Craft · Maths for Games · Source and native evidence.