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:
move.b #0,d0
move.b d0,HAVE0
move.b d0,HAVE1
move.b d0,RESULT
move.b d0,NEED0
move.b d0,NEED1
move.b d0,PRODUCE
move.b d0,CAP
move.b d0,CRAFTED
move.b d0,VALID
move.b d0,T0
rts
recipe_transaction_step:
; CRAFTED = 0
move.b #0,d0
move.b d0,CRAFTED
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if CAP == 0:
move.b CAP,d0
cmp.b #0,d0
beq recipe_transaction_L1
jmp recipe_transaction_L2
recipe_transaction_L1:
; return
rts
jmp recipe_transaction_L3
recipe_transaction_L2:
recipe_transaction_L3:
; if RESULT > CAP:
move.b RESULT,d0
move.b CAP,d1
cmp.b d1,d0
bhi recipe_transaction_L4
jmp recipe_transaction_L5
recipe_transaction_L4:
; return
rts
jmp recipe_transaction_L6
recipe_transaction_L5:
recipe_transaction_L6:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; if HAVE0 < NEED0:
move.b HAVE0,d0
move.b NEED0,d1
cmp.b d1,d0
bcs recipe_transaction_L7
jmp recipe_transaction_L8
recipe_transaction_L7:
; return
rts
jmp recipe_transaction_L9
recipe_transaction_L8:
recipe_transaction_L9:
; if HAVE1 < NEED1:
move.b HAVE1,d0
move.b NEED1,d1
cmp.b d1,d0
bcs recipe_transaction_L10
jmp recipe_transaction_L11
recipe_transaction_L10:
; return
rts
jmp recipe_transaction_L12
recipe_transaction_L11:
recipe_transaction_L12:
; T0 = CAP - RESULT
move.b CAP,d0
move.b RESULT,d1
sub.b d1,d0
move.b d0,T0
; if PRODUCE > T0:
move.b PRODUCE,d0
move.b T0,d1
cmp.b d1,d0
bhi recipe_transaction_L13
jmp recipe_transaction_L14
recipe_transaction_L13:
; return
rts
jmp recipe_transaction_L15
recipe_transaction_L14:
recipe_transaction_L15:
; HAVE0 = HAVE0 - NEED0
move.b HAVE0,d0
move.b NEED0,d1
sub.b d1,d0
move.b d0,HAVE0
; HAVE1 = HAVE1 - NEED1
move.b HAVE1,d0
move.b NEED1,d1
sub.b d1,d0
move.b d0,HAVE1
; RESULT = RESULT + PRODUCE
move.b RESULT,d0
move.b PRODUCE,d1
add.b d1,d0
move.b d0,RESULT
; CRAFTED = 1
move.b #1,d0
move.b d0,CRAFTED
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:
HAVE0: dc.b 0
HAVE1: dc.b 0
RESULT: dc.b 0
NEED0: dc.b 0
NEED1: dc.b 0
PRODUCE: dc.b 0
CAP: dc.b 0
CRAFTED: dc.b 0
VALID: dc.b 0
T0: 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
; HAVE0 = 3
move.b #3,d0
move.b d0,HAVE0
; HAVE1 = 2
move.b #2,d0
move.b d0,HAVE1
; NEED0 = 2
move.b #2,d0
move.b d0,NEED0
; NEED1 = 1
move.b #1,d0
move.b d0,NEED1
; PRODUCE = 2
move.b #2,d0
move.b d0,PRODUCE
; CAP = 4
move.b #4,d0
move.b d0,CAP
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
rts
else
; HAVE0 = 3
move.b #3,d0
move.b d0,HAVE0
; HAVE1 = 2
move.b #2,d0
move.b d0,HAVE1
; NEED0 = 2
move.b #2,d0
move.b d0,NEED0
; NEED1 = 1
move.b #1,d0
move.b d0,NEED1
; PRODUCE = 1
move.b #1,d0
move.b d0,PRODUCE
; CAP = 4
move.b #4,d0
move.b d0,CAP
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; HAVE0 = 0
move.b #0,d0
move.b d0,HAVE0
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise 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
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:
HAVE0: dc.b 0
HAVE1: dc.b 0
RESULT: dc.b 0
NEED0: dc.b 0
NEED1: dc.b 0
PRODUCE: dc.b 0
CAP: dc.b 0
CRAFTED: dc.b 0
VALID: dc.b 0
T0: 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 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
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:
HAVE0: dc.b 0
HAVE1: dc.b 0
RESULT: dc.b 0
NEED0: dc.b 0
NEED1: dc.b 0
PRODUCE: dc.b 0
CAP: dc.b 0
CRAFTED: dc.b 0
VALID: dc.b 0
T0: 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 278 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 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
Inventory stacks: Reuse the same capacity and stock rules, but validate the whole recipe before committing any one stack.