Do not take the player’s ingredients and only then discover there is nowhere to put the result.
Use it in a project
The recipe consumes NEED0 and NEED1 from separate ingredient counts and adds PRODUCE to RESULT. CAP bounds the result stack. Validate all stock and destination capacity before any count changes. CRAFTED=1 means the complete change committed. CAP=0 or RESULT above CAP is invalid. Zero-cost and zero-output recipes are allowed and still atomic.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: HAVE0, HAVE1, RESULT, NEED0, NEED1, PRODUCE, CAP, CRAFTED, VALID. They occupy 9 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 recipe_transaction_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 recipe_transaction_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 | HAVE0, HAVE1 and RESULT |
| Supply before service | NEED0, NEED1, PRODUCE and CAP |
| Read after service | CRAFTED and VALID |
Validation and commit are separate phases within one call. Only after every ingredient and destination check passes does the helper write any of the three counts.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Commit a Crafting Recipe Atomically. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
recipe_transaction_init:
ld a,0
ld (HAVE0),a
ld (HAVE1),a
ld (RESULT),a
ld (NEED0),a
ld (NEED1),a
ld (PRODUCE),a
ld (CAP),a
ld (CRAFTED),a
ld (VALID),a
ld (T0),a
ret
recipe_transaction_step:
; CRAFTED = 0
ld a,0
ld (CRAFTED),a
; VALID = 0
ld a,0
ld (VALID),a
; if CAP == 0:
ld a,(CAP)
cp 0
jp z,recipe_transaction_L1
jp recipe_transaction_L2
recipe_transaction_L1:
; return
ret
jp recipe_transaction_L3
recipe_transaction_L2:
recipe_transaction_L3:
; if RESULT > CAP:
ld a,(RESULT)
ld b,a
ld a,(CAP)
ld c,a
ld a,b
cp c
jp z,recipe_transaction_L5
jp nc,recipe_transaction_L4
jp recipe_transaction_L5
recipe_transaction_L4:
; return
ret
jp recipe_transaction_L6
recipe_transaction_L5:
recipe_transaction_L6:
; VALID = 1
ld a,1
ld (VALID),a
; if HAVE0 < NEED0:
ld a,(HAVE0)
ld b,a
ld a,(NEED0)
ld c,a
ld a,b
cp c
jp c,recipe_transaction_L7
jp recipe_transaction_L8
recipe_transaction_L7:
; return
ret
jp recipe_transaction_L9
recipe_transaction_L8:
recipe_transaction_L9:
; if HAVE1 < NEED1:
ld a,(HAVE1)
ld b,a
ld a,(NEED1)
ld c,a
ld a,b
cp c
jp c,recipe_transaction_L10
jp recipe_transaction_L11
recipe_transaction_L10:
; return
ret
jp recipe_transaction_L12
recipe_transaction_L11:
recipe_transaction_L12:
; T0 = CAP - RESULT
ld a,(CAP)
ld b,a
ld a,(RESULT)
ld c,a
ld a,b
sub c
ld (T0),a
; if PRODUCE > T0:
ld a,(PRODUCE)
ld b,a
ld a,(T0)
ld c,a
ld a,b
cp c
jp z,recipe_transaction_L14
jp nc,recipe_transaction_L13
jp recipe_transaction_L14
recipe_transaction_L13:
; return
ret
jp recipe_transaction_L15
recipe_transaction_L14:
recipe_transaction_L15:
; HAVE0 = HAVE0 - NEED0
ld a,(HAVE0)
ld b,a
ld a,(NEED0)
ld c,a
ld a,b
sub c
ld (HAVE0),a
; HAVE1 = HAVE1 - NEED1
ld a,(HAVE1)
ld b,a
ld a,(NEED1)
ld c,a
ld a,b
sub c
ld (HAVE1),a
; RESULT = RESULT + PRODUCE
ld a,(RESULT)
ld b,a
ld a,(PRODUCE)
ld c,a
ld a,b
add a,c
ld (RESULT),a
; CRAFTED = 1
ld a,1
ld (CRAFTED),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:
HAVE0: defb 0
HAVE1: defb 0
RESULT: defb 0
NEED0: defb 0
NEED1: defb 0
PRODUCE: defb 0
CAP: defb 0
CRAFTED: defb 0
VALID: defb 0
T0: 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
; HAVE0 = 3
ld a,3
ld (HAVE0),a
; HAVE1 = 2
ld a,2
ld (HAVE1),a
; NEED0 = 2
ld a,2
ld (NEED0),a
; NEED1 = 1
ld a,1
ld (NEED1),a
; PRODUCE = 2
ld a,2
ld (PRODUCE),a
; CAP = 4
ld a,4
ld (CAP),a
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
ret
else
; HAVE0 = 3
ld a,3
ld (HAVE0),a
; HAVE1 = 2
ld a,2
ld (HAVE1),a
; NEED0 = 2
ld a,2
ld (NEED0),a
; NEED1 = 1
ld a,1
ld (NEED1),a
; PRODUCE = 1
ld a,1
ld (PRODUCE),a
; CAP = 4
ld a,4
ld (CAP),a
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
if BROKEN
; HAVE0 = 0
ld a,0
ld (HAVE0),a
endif
call checked_step
call emit_state
endif
retCustomise it
Craft a two-unit output instead of one, retaining the same capacity checks. 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:
HAVE0: defb 0
HAVE1: defb 0
RESULT: defb 0
NEED0: defb 0
NEED1: defb 0
PRODUCE: defb 0
CAP: defb 0
CRAFTED: defb 0
VALID: defb 0
T0: 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 01 02 02 01 02 04 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller clears HAVE0 before asking whether crafting can succeed. The recipe refuses, but the caller has already destroyed the stock. Leave all inventory changes to the checked commit; pre-mutating an input breaks the all-or-nothing promise.
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:
HAVE0: defb 0
HAVE1: defb 0
RESULT: defb 0
NEED0: defb 0
NEED1: defb 0
PRODUCE: defb 0
CAP: defb 0
CRAFTED: defb 0
VALID: defb 0
T0: 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 207 code bytes; public state is 9 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 for two ingredients and one result. A general recipe table needs iteration and an aliasing policy before committing changes.
Atomic here means all-or-nothing in one main-loop call. It does not provide interrupt exclusion, durable saving or a general dependency graph. Ingredients and result are distinct counters; aliasing them changes the recipe contract.
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
Inventory stacks: Reuse the same capacity and stock rules, but validate the whole recipe before committing any one stack.