A damaged save should leave the current game alone rather than half-replacing it with invalid state.
Use it in a project
C0..C7 are exactly eight already-read bytes: tag198, version1, payload length4, room0..15, horizontal position0..31, lives0..9, flags0..15, then XOR of the first seven bytes. MODE=0 validates without committing; MODE=1 validates then copies all four payload bytes to L0..L3. VALID means the mode is recognised; ACCEPTED means the entire record passes. A rejected record leaves all live fields unchanged. Physical file length must be verified as exactly eight before supplying this buffer.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: ACCEPTED, VALID, MODE, L0, L1, L2, L3, C0, C1, C2, C3, C4, C5, C6, C7. They occupy 15 contiguous bytes. Also reserve the private workspace: T0, T1, T2 (3 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call save_record_validation_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 save_record_validation_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 | L0..L3 live state |
| Supply before service | MODE and immutable C0..C7 |
| Read after service | ACCEPTED, VALID and optionally L0..L3 |
Check the complete envelope, semantic ranges and integrity before the first live-state write. A separate validation mode allows a load screen to inspect compatibility without changing the running game.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Validate a Save Before Replacing Live State. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
save_record_validation_init:
lda #0
sta ACCEPTED
sta VALID
sta MODE
sta L0
sta L1
sta L2
sta L3
sta C0
sta C1
sta C2
sta C3
sta C4
sta C5
sta C6
sta C7
sta T0
sta T1
sta T2
rts
save_record_validation_step:
; ACCEPTED = 0
lda #0
sta ACCEPTED
; VALID = 0
lda #0
sta VALID
; if MODE > 1:
lda MODE
cmp #1
beq save_record_validation_L5
bcs save_record_validation_L4
save_record_validation_L5:
jmp save_record_validation_L2
save_record_validation_L4:
jmp save_record_validation_L1
save_record_validation_L1:
; return
rts
jmp save_record_validation_L3
save_record_validation_L2:
save_record_validation_L3:
; VALID = 1
lda #1
sta VALID
; if C0 != 198:
lda C0
cmp #198
bne save_record_validation_L9
save_record_validation_L10:
jmp save_record_validation_L7
save_record_validation_L9:
jmp save_record_validation_L6
save_record_validation_L6:
; return
rts
jmp save_record_validation_L8
save_record_validation_L7:
save_record_validation_L8:
; if C1 != 1:
lda C1
cmp #1
bne save_record_validation_L14
save_record_validation_L15:
jmp save_record_validation_L12
save_record_validation_L14:
jmp save_record_validation_L11
save_record_validation_L11:
; return
rts
jmp save_record_validation_L13
save_record_validation_L12:
save_record_validation_L13:
; if C2 != 4:
lda C2
cmp #4
bne save_record_validation_L19
save_record_validation_L20:
jmp save_record_validation_L17
save_record_validation_L19:
jmp save_record_validation_L16
save_record_validation_L16:
; return
rts
jmp save_record_validation_L18
save_record_validation_L17:
save_record_validation_L18:
; if C3 > 15:
lda C3
cmp #15
beq save_record_validation_L25
bcs save_record_validation_L24
save_record_validation_L25:
jmp save_record_validation_L22
save_record_validation_L24:
jmp save_record_validation_L21
save_record_validation_L21:
; return
rts
jmp save_record_validation_L23
save_record_validation_L22:
save_record_validation_L23:
; if C4 > 31:
lda C4
cmp #31
beq save_record_validation_L30
bcs save_record_validation_L29
save_record_validation_L30:
jmp save_record_validation_L27
save_record_validation_L29:
jmp save_record_validation_L26
save_record_validation_L26:
; return
rts
jmp save_record_validation_L28
save_record_validation_L27:
save_record_validation_L28:
; if C5 > 9:
lda C5
cmp #9
beq save_record_validation_L35
bcs save_record_validation_L34
save_record_validation_L35:
jmp save_record_validation_L32
save_record_validation_L34:
jmp save_record_validation_L31
save_record_validation_L31:
; return
rts
jmp save_record_validation_L33
save_record_validation_L32:
save_record_validation_L33:
; if C6 > 15:
lda C6
cmp #15
beq save_record_validation_L40
bcs save_record_validation_L39
save_record_validation_L40:
jmp save_record_validation_L37
save_record_validation_L39:
jmp save_record_validation_L36
save_record_validation_L36:
; return
rts
jmp save_record_validation_L38
save_record_validation_L37:
save_record_validation_L38:
; T0 = 0
lda #0
sta T0
; T1 = 0
lda #0
sta T1
; while T0 < 7:
save_record_validation_L41:
lda T0
cmp #7
bcc save_record_validation_L44
save_record_validation_L45:
jmp save_record_validation_L43
save_record_validation_L44:
jmp save_record_validation_L42
save_record_validation_L42:
; T2 = C[T0]
ldx T0
lda C0,x
sta T2
; T1 = T1 ^ T2
lda T1
eor T2
sta T1
; T0 = T0 + 1
lda T0
clc
adc #1
sta T0
jmp save_record_validation_L41
save_record_validation_L43:
; if T1 != C7:
lda T1
cmp C7
bne save_record_validation_L49
save_record_validation_L50:
jmp save_record_validation_L47
save_record_validation_L49:
jmp save_record_validation_L46
save_record_validation_L46:
; return
rts
jmp save_record_validation_L48
save_record_validation_L47:
save_record_validation_L48:
; ACCEPTED = 1
lda #1
sta ACCEPTED
; if MODE == 1:
lda MODE
cmp #1
beq save_record_validation_L54
save_record_validation_L55:
jmp save_record_validation_L52
save_record_validation_L54:
jmp save_record_validation_L51
save_record_validation_L51:
; L0 = C3
lda C3
sta L0
; L1 = C4
lda C4
sta L1
; L2 = C5
lda C5
sta L2
; L3 = C6
lda C6
sta L3
jmp save_record_validation_L53
save_record_validation_L52:
save_record_validation_L53:
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
ACCEPTED=$80
VALID=$81
MODE=$82
L0=$83
L1=$84
L2=$85
L3=$86
C0=$87
C1=$88
C2=$89
C3=$8a
C4=$8b
C5=$8c
C6=$8d
C7=$8e
T0=$8f
T1=$90
T2=$91
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 {
; MODE = 0
lda #0
sta MODE
; L0 = 9
lda #9
sta L0
; C0 = 198
lda #198
sta C0
; C1 = 1
lda #1
sta C1
; C2 = 4
lda #4
sta C2
; C3 = 15
lda #15
sta C3
; C4 = 31
lda #31
sta C4
; C5 = 9
lda #9
sta C5
; C6 = 15
lda #15
sta C6
; C7 = 213
lda #213
sta C7
!if BROKEN {
; C7 = 0
lda #0
sta C7
}
jsr checked_step
jsr emit_state
; MODE = 1
lda #1
sta MODE
!if BROKEN {
; C7 = 0
lda #0
sta C7
}
jsr checked_step
jsr emit_state
rts
} else {
; MODE = 1
lda #1
sta MODE
; L0 = 2
lda #2
sta L0
; L1 = 3
lda #3
sta L1
; L2 = 4
lda #4
sta L2
; L3 = 5
lda #5
sta L3
; C0 = 198
lda #198
sta C0
; C1 = 1
lda #1
sta C1
; C2 = 4
lda #4
sta C2
; C3 = 7
lda #7
sta C3
; C4 = 20
lda #20
sta C4
; C5 = 3
lda #3
sta C5
; C6 = 2
lda #2
sta C6
; C7 = 209
lda #209
sta C7
!if BROKEN {
; C7 = 0
lda #0
sta C7
}
jsr checked_step
jsr emit_state
; C5 = 255
lda #255
sta C5
!if BROKEN {
; C7 = 0
lda #0
sta C7
}
jsr checked_step
jsr emit_state
}
rtsCustomise it
Validate a different legal record without commitment, then commit the same record on a second call. 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
ACCEPTED=$80
VALID=$81
MODE=$82
L0=$83
L1=$84
L2=$85
L3=$86
C0=$87
C1=$88
C2=$89
C3=$8a
C4=$8b
C5=$8c
C6=$8d
C7=$8e
T0=$8f
T1=$90
T2=$91
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 01 01 0F 1F 09 0F C6. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong client drops the stored checksum before validation. The otherwise valid record is rejected and live state stays intact. Supply all eight bytes, including the stored integrity byte.
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
ACCEPTED=$80
VALID=$81
MODE=$82
L0=$83
L1=$84
L2=$85
L3=$86
C0=$87
C1=$88
C2=$89
C3=$8a
C4=$8b
C5=$8c
C6=$8d
C7=$8e
T0=$8f
T1=$90
T2=$91
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 283 code bytes; public state is 15 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.
Seven XOR stages and four writes only after acceptance. A bigger record needs a bounded length and wider index; use the existing CRC helper when stronger error detection justifies its additional work.
XOR is intentionally small and weak: two cancelling changes can pass. The existing streaming CRC-8 gives stronger accidental-corruption detection, but neither authenticates a hostile file. Atomic disk replacement, I/O failure, migration and concurrent game-state writers are outside this in-memory transaction. Add semantic checks whenever the payload gains new rules.
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
Streaming CRC-8: Replace the weak XOR with a precisely specified CRC when its extra detection is worth the code and runtime cost.