Moving arrows from a chest to a pack should never remove more than the pack can hold.
Use it in a project
Zero count requires kind zero; a nonempty stack requires a nonzero kind. Both counts must be at most CAPACITY. AMOUNT requests up to that many items from source into destination. A nonempty destination accepts only the same kind. MOVED reports the actual minimum of requested, available and free space; zero is a valid refusal. On success clear an emptied source kind and assign the source kind to an empty destination. Invalid stack representation or capacity sets VALID=0 without changing either stack.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: MOVED, VALID, SOURCE_KIND, SOURCE_COUNT, DEST_KIND, DEST_COUNT, CAPACITY, AMOUNT. They occupy 8 contiguous bytes. Also reserve the private workspace: T0 (1 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call inventory_transfer_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_transfer_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 | source and destination kinds/counts |
| Supply before service | CAPACITY and AMOUNT |
| Read after service | updated stacks, MOVED and VALID |
Validate both representations before writing, calculate the complete accepted amount, then commit both counts and kinds. Conservation of the total item count is the central invariant.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Transfer Inventory Stacks Atomically. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
inventory_transfer_init:
lda #0
sta MOVED
sta VALID
sta SOURCE_KIND
sta SOURCE_COUNT
sta DEST_KIND
sta DEST_COUNT
sta CAPACITY
sta AMOUNT
sta T0
rts
inventory_transfer_step:
; MOVED = 0
lda #0
sta MOVED
; VALID = 0
lda #0
sta VALID
; if SOURCE_COUNT > CAPACITY:
lda SOURCE_COUNT
cmp CAPACITY
beq inventory_transfer_L5
bcs inventory_transfer_L4
inventory_transfer_L5:
jmp inventory_transfer_L2
inventory_transfer_L4:
jmp inventory_transfer_L1
inventory_transfer_L1:
; return
rts
jmp inventory_transfer_L3
inventory_transfer_L2:
inventory_transfer_L3:
; if DEST_COUNT > CAPACITY:
lda DEST_COUNT
cmp CAPACITY
beq inventory_transfer_L10
bcs inventory_transfer_L9
inventory_transfer_L10:
jmp inventory_transfer_L7
inventory_transfer_L9:
jmp inventory_transfer_L6
inventory_transfer_L6:
; return
rts
jmp inventory_transfer_L8
inventory_transfer_L7:
inventory_transfer_L8:
; if SOURCE_COUNT == 0:
lda SOURCE_COUNT
cmp #0
beq inventory_transfer_L14
inventory_transfer_L15:
jmp inventory_transfer_L12
inventory_transfer_L14:
jmp inventory_transfer_L11
inventory_transfer_L11:
; if SOURCE_KIND != 0:
lda SOURCE_KIND
cmp #0
bne inventory_transfer_L19
inventory_transfer_L20:
jmp inventory_transfer_L17
inventory_transfer_L19:
jmp inventory_transfer_L16
inventory_transfer_L16:
; return
rts
jmp inventory_transfer_L18
inventory_transfer_L17:
inventory_transfer_L18:
jmp inventory_transfer_L13
inventory_transfer_L12:
; if SOURCE_KIND == 0:
lda SOURCE_KIND
cmp #0
beq inventory_transfer_L24
inventory_transfer_L25:
jmp inventory_transfer_L22
inventory_transfer_L24:
jmp inventory_transfer_L21
inventory_transfer_L21:
; return
rts
jmp inventory_transfer_L23
inventory_transfer_L22:
inventory_transfer_L23:
inventory_transfer_L13:
; if DEST_COUNT == 0:
lda DEST_COUNT
cmp #0
beq inventory_transfer_L29
inventory_transfer_L30:
jmp inventory_transfer_L27
inventory_transfer_L29:
jmp inventory_transfer_L26
inventory_transfer_L26:
; if DEST_KIND != 0:
lda DEST_KIND
cmp #0
bne inventory_transfer_L34
inventory_transfer_L35:
jmp inventory_transfer_L32
inventory_transfer_L34:
jmp inventory_transfer_L31
inventory_transfer_L31:
; return
rts
jmp inventory_transfer_L33
inventory_transfer_L32:
inventory_transfer_L33:
jmp inventory_transfer_L28
inventory_transfer_L27:
; if DEST_KIND == 0:
lda DEST_KIND
cmp #0
beq inventory_transfer_L39
inventory_transfer_L40:
jmp inventory_transfer_L37
inventory_transfer_L39:
jmp inventory_transfer_L36
inventory_transfer_L36:
; return
rts
jmp inventory_transfer_L38
inventory_transfer_L37:
inventory_transfer_L38:
inventory_transfer_L28:
; VALID = 1
lda #1
sta VALID
; if SOURCE_COUNT == 0:
lda SOURCE_COUNT
cmp #0
beq inventory_transfer_L44
inventory_transfer_L45:
jmp inventory_transfer_L42
inventory_transfer_L44:
jmp inventory_transfer_L41
inventory_transfer_L41:
; return
rts
jmp inventory_transfer_L43
inventory_transfer_L42:
inventory_transfer_L43:
; if DEST_COUNT != 0:
lda DEST_COUNT
cmp #0
bne inventory_transfer_L49
inventory_transfer_L50:
jmp inventory_transfer_L47
inventory_transfer_L49:
jmp inventory_transfer_L46
inventory_transfer_L46:
; if DEST_KIND != SOURCE_KIND:
lda DEST_KIND
cmp SOURCE_KIND
bne inventory_transfer_L54
inventory_transfer_L55:
jmp inventory_transfer_L52
inventory_transfer_L54:
jmp inventory_transfer_L51
inventory_transfer_L51:
; return
rts
jmp inventory_transfer_L53
inventory_transfer_L52:
inventory_transfer_L53:
jmp inventory_transfer_L48
inventory_transfer_L47:
inventory_transfer_L48:
; T0 = CAPACITY - DEST_COUNT
lda CAPACITY
sec
sbc DEST_COUNT
sta T0
; if AMOUNT < T0:
lda AMOUNT
cmp T0
bcc inventory_transfer_L59
inventory_transfer_L60:
jmp inventory_transfer_L57
inventory_transfer_L59:
jmp inventory_transfer_L56
inventory_transfer_L56:
; T0 = AMOUNT
lda AMOUNT
sta T0
jmp inventory_transfer_L58
inventory_transfer_L57:
inventory_transfer_L58:
; if SOURCE_COUNT < T0:
lda SOURCE_COUNT
cmp T0
bcc inventory_transfer_L64
inventory_transfer_L65:
jmp inventory_transfer_L62
inventory_transfer_L64:
jmp inventory_transfer_L61
inventory_transfer_L61:
; T0 = SOURCE_COUNT
lda SOURCE_COUNT
sta T0
jmp inventory_transfer_L63
inventory_transfer_L62:
inventory_transfer_L63:
; if T0 == 0:
lda T0
cmp #0
beq inventory_transfer_L69
inventory_transfer_L70:
jmp inventory_transfer_L67
inventory_transfer_L69:
jmp inventory_transfer_L66
inventory_transfer_L66:
; return
rts
jmp inventory_transfer_L68
inventory_transfer_L67:
inventory_transfer_L68:
; DEST_KIND = SOURCE_KIND
lda SOURCE_KIND
sta DEST_KIND
; DEST_COUNT = DEST_COUNT + T0
lda DEST_COUNT
clc
adc T0
sta DEST_COUNT
; SOURCE_COUNT = SOURCE_COUNT - T0
lda SOURCE_COUNT
sec
sbc T0
sta SOURCE_COUNT
; MOVED = T0
lda T0
sta MOVED
; if SOURCE_COUNT == 0:
lda SOURCE_COUNT
cmp #0
beq inventory_transfer_L74
inventory_transfer_L75:
jmp inventory_transfer_L72
inventory_transfer_L74:
jmp inventory_transfer_L71
inventory_transfer_L71:
; SOURCE_KIND = 0
lda #0
sta SOURCE_KIND
jmp inventory_transfer_L73
inventory_transfer_L72:
inventory_transfer_L73:
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. 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 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
MOVED=$80
VALID=$81
SOURCE_KIND=$82
SOURCE_COUNT=$83
DEST_KIND=$84
DEST_COUNT=$85
CAPACITY=$86
AMOUNT=$87
T0=$88
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
preview_rules:
!if CUSTOM {
; SOURCE_KIND = 2
lda #2
sta SOURCE_KIND
; SOURCE_COUNT = 4
lda #4
sta SOURCE_COUNT
; CAPACITY = 4
lda #4
sta CAPACITY
; AMOUNT = 255
lda #255
sta AMOUNT
!if BROKEN {
; AMOUNT = 0
lda #0
sta AMOUNT
}
jsr checked_step
jsr emit_state
!if BROKEN {
; AMOUNT = 0
lda #0
sta AMOUNT
}
jsr checked_step
jsr emit_state
rts
} else {
; SOURCE_KIND = 3
lda #3
sta SOURCE_KIND
; SOURCE_COUNT = 7
lda #7
sta SOURCE_COUNT
; DEST_KIND = 3
lda #3
sta DEST_KIND
; DEST_COUNT = 8
lda #8
sta DEST_COUNT
; CAPACITY = 10
lda #10
sta CAPACITY
; AMOUNT = 5
lda #5
sta AMOUNT
!if BROKEN {
; AMOUNT = 0
lda #0
sta AMOUNT
}
jsr checked_step
jsr emit_state
; DEST_KIND = 0
lda #0
sta DEST_KIND
; DEST_COUNT = 0
lda #0
sta DEST_COUNT
; AMOUNT = 5
lda #5
sta AMOUNT
!if BROKEN {
; AMOUNT = 0
lda #0
sta AMOUNT
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Move the whole source into an empty four-item destination, then try the now-empty source again. 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
MOVED=$80
VALID=$81
SOURCE_KIND=$82
SOURCE_COUNT=$83
DEST_KIND=$84
DEST_COUNT=$85
CAPACITY=$86
AMOUNT=$87
T0=$88
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 00 01 00 00 02 04 04 FF. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong client clears the transfer request before service. Both stacks remain unchanged and MOVED is zero. Keep the requested amount until it has been consumed.
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
MOVED=$80
VALID=$81
SOURCE_KIND=$82
SOURCE_COUNT=$83
DEST_KIND=$84
DEST_COUNT=$85
CAPACITY=$86
AMOUNT=$87
T0=$88
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:
Cost and alternatives
The measured helper extent is 313 code bytes; public state is 8 bytes and private scratch is 1 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 with no array search. Full inventory placement can scan candidate stacks first and invoke this transaction under the same mutation owner.
One source and one destination are borrowed exclusively during a call. This does not find stacks, merge an entire inventory, reserve network transactions or support different per-item caps. The capacity is shared by these two stacks; use a larger transaction for several destinations.
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
Inventory stacks: Reuse the existing stack rules when adding or removing items without a second container.