A large score or resource total should stop at its ceiling rather than wrap back to a small number.
Use it in a project
Add two unsigned little-endian words. Return their sum capped at 65,535 and CLAMPED=1 exactly when the mathematical sum exceeds that limit. Preserve both inputs.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: SUM_LO, SUM_HI, CLAMPED, LEFT_LO, LEFT_HI, RIGHT_LO, RIGHT_HI. They occupy 7 contiguous bytes. Also reserve the private workspace: T0, T1 (2 bytes). Array names in the explanation refer to the numbered consecutive bindings, such as C0, C1 and so on.
Call saturating_word_add_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 saturating_word_add_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 | none |
| Supply before service | LEFT_LO/HI and RIGHT_LO/HI |
| Read after service | SUM_LO/HI and CLAMPED |
The low addition yields a carry into the high byte. Either overflow in the high addition or overflow when adding that carry saturates both result bytes.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Add Words with Saturation. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
saturating_word_add_init:
move.b #0,d0
move.b d0,SUM_LO
move.b d0,SUM_HI
move.b d0,CLAMPED
move.b d0,LEFT_LO
move.b d0,LEFT_HI
move.b d0,RIGHT_LO
move.b d0,RIGHT_HI
move.b d0,T0
move.b d0,T1
rts
saturating_word_add_step:
; SUM_LO = LEFT_LO + RIGHT_LO
move.b LEFT_LO,d0
move.b RIGHT_LO,d1
add.b d1,d0
move.b d0,SUM_LO
; T0 = 0
move.b #0,d0
move.b d0,T0
; if SUM_LO < LEFT_LO:
move.b SUM_LO,d0
move.b LEFT_LO,d1
cmp.b d1,d0
bcs saturating_word_add_L1
jmp saturating_word_add_L2
saturating_word_add_L1:
; T0 = 1
move.b #1,d0
move.b d0,T0
jmp saturating_word_add_L3
saturating_word_add_L2:
saturating_word_add_L3:
; SUM_HI = LEFT_HI + RIGHT_HI
move.b LEFT_HI,d0
move.b RIGHT_HI,d1
add.b d1,d0
move.b d0,SUM_HI
; CLAMPED = 0
move.b #0,d0
move.b d0,CLAMPED
; if SUM_HI < LEFT_HI:
move.b SUM_HI,d0
move.b LEFT_HI,d1
cmp.b d1,d0
bcs saturating_word_add_L4
jmp saturating_word_add_L5
saturating_word_add_L4:
; CLAMPED = 1
move.b #1,d0
move.b d0,CLAMPED
jmp saturating_word_add_L6
saturating_word_add_L5:
saturating_word_add_L6:
; T1 = SUM_HI
move.b SUM_HI,d0
move.b d0,T1
; SUM_HI = SUM_HI + T0
move.b SUM_HI,d0
move.b T0,d1
add.b d1,d0
move.b d0,SUM_HI
; if SUM_HI < T1:
move.b SUM_HI,d0
move.b T1,d1
cmp.b d1,d0
bcs saturating_word_add_L7
jmp saturating_word_add_L8
saturating_word_add_L7:
; CLAMPED = 1
move.b #1,d0
move.b d0,CLAMPED
jmp saturating_word_add_L9
saturating_word_add_L8:
saturating_word_add_L9:
; if CLAMPED != 0:
move.b CLAMPED,d0
cmp.b #0,d0
bne saturating_word_add_L10
jmp saturating_word_add_L11
saturating_word_add_L10:
; SUM_LO = 255
move.b #255,d0
move.b d0,SUM_LO
; SUM_HI = 255
move.b #255,d0
move.b d0,SUM_HI
jmp saturating_word_add_L12
saturating_word_add_L11:
saturating_word_add_L12:
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. 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 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:
SUM_LO: dc.b 0
SUM_HI: dc.b 0
CLAMPED: dc.b 0
LEFT_LO: dc.b 0
LEFT_HI: dc.b 0
RIGHT_LO: dc.b 0
RIGHT_HI: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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
; LEFT_LO = 255
move.b #255,d0
move.b d0,LEFT_LO
; LEFT_HI = 0
move.b #0,d0
move.b d0,LEFT_HI
; RIGHT_LO = 1
move.b #1,d0
move.b d0,RIGHT_LO
; RIGHT_HI = 0
move.b #0,d0
move.b d0,RIGHT_HI
ifne BROKEN
; RIGHT_HI = 0
move.b #0,d0
move.b d0,RIGHT_HI
endif
bsr checked_step
bsr emit_state
rts
else
; LEFT_LO = 250
move.b #250,d0
move.b d0,LEFT_LO
; LEFT_HI = 254
move.b #254,d0
move.b d0,LEFT_HI
; RIGHT_LO = 10
move.b #10,d0
move.b d0,RIGHT_LO
; RIGHT_HI = 1
move.b #1,d0
move.b d0,RIGHT_HI
ifne BROKEN
; RIGHT_HI = 0
move.b #0,d0
move.b d0,RIGHT_HI
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Exercise a low-byte carry without saturating the full word. 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:
SUM_LO: dc.b 0
SUM_HI: dc.b 0
CLAMPED: dc.b 0
LEFT_LO: dc.b 0
LEFT_HI: dc.b 0
RIGHT_LO: dc.b 0
RIGHT_HI: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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 00 01 00 FF 00 01 00 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller loses the high byte of the amount being added. Small rewards can hide the mistake; a word-sized reward exposes it. Pass both bytes.
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:
SUM_LO: dc.b 0
SUM_HI: dc.b 0
CLAMPED: dc.b 0
LEFT_LO: dc.b 0
LEFT_HI: dc.b 0
RIGHT_LO: dc.b 0
RIGHT_HI: dc.b 0
T0: dc.b 0
T1: dc.b 0
even
ready: dc.w 0
preview: dcb.b 8,0
OUTPUT: dcb.b 1300,0
even
signature: dc.b "198XTOOLKIT0000",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 266 code bytes; public state is 7 bytes and private scratch is 2 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. Native 16-bit addition is simpler on the 68000; explicit byte carries keep the MOS and Z80 representations clear.
The fixed ceiling is 65,535. Signed saturation and a game-specific maximum need different comparisons.
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
Score counters: Saturating arithmetic is a policy choice distinct from decimal display.