A timid enemy might retreat often and attack rarely. Give the outcomes weights rather than duplicating branches.
Use it in a project
Three weights describe half-open intervals; zero excludes an outcome. Their sum must be 1–255 and ROLL must be below that sum. CHOICE is 0–2 on success, 255 on failure. TOTAL reports the sum only when it fits a byte. The caller supplies a uniformly distributed bounded roll; this routine does not generate randomness.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: WEIGHT0, WEIGHT1, WEIGHT2, ROLL, CHOICE, TOTAL, VALID. 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 weighted_choice_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 weighted_choice_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 between choices |
| Supply before service | WEIGHT0–WEIGHT2 and ROLL |
| Read after service | CHOICE, TOTAL and VALID |
Lay the weights end to end as intervals. A roll names a position in that combined interval. The outcome containing it wins; an outcome with zero weight has no positions and cannot be selected.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Choose from Weighted Outcomes. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
weighted_choice_init:
move.b #0,d0
move.b d0,WEIGHT0
move.b d0,WEIGHT1
move.b d0,WEIGHT2
move.b d0,ROLL
move.b d0,CHOICE
move.b d0,TOTAL
move.b d0,VALID
move.b d0,T0
move.b d0,T1
rts
weighted_choice_step:
; CHOICE = 255
move.b #255,d0
move.b d0,CHOICE
; TOTAL = 0
move.b #0,d0
move.b d0,TOTAL
; VALID = 0
move.b #0,d0
move.b d0,VALID
; T0 = WEIGHT0 + WEIGHT1
move.b WEIGHT0,d0
move.b WEIGHT1,d1
add.b d1,d0
move.b d0,T0
; if T0 < WEIGHT0:
move.b T0,d0
move.b WEIGHT0,d1
cmp.b d1,d0
bcs weighted_choice_L1
jmp weighted_choice_L2
weighted_choice_L1:
; return
rts
jmp weighted_choice_L3
weighted_choice_L2:
weighted_choice_L3:
; T1 = T0 + WEIGHT2
move.b T0,d0
move.b WEIGHT2,d1
add.b d1,d0
move.b d0,T1
; if T1 < T0:
move.b T1,d0
move.b T0,d1
cmp.b d1,d0
bcs weighted_choice_L4
jmp weighted_choice_L5
weighted_choice_L4:
; return
rts
jmp weighted_choice_L6
weighted_choice_L5:
weighted_choice_L6:
; TOTAL = T1
move.b T1,d0
move.b d0,TOTAL
; if TOTAL == 0:
move.b TOTAL,d0
cmp.b #0,d0
beq weighted_choice_L7
jmp weighted_choice_L8
weighted_choice_L7:
; return
rts
jmp weighted_choice_L9
weighted_choice_L8:
weighted_choice_L9:
; if ROLL >= TOTAL:
move.b ROLL,d0
move.b TOTAL,d1
cmp.b d1,d0
bcc weighted_choice_L10
jmp weighted_choice_L11
weighted_choice_L10:
; return
rts
jmp weighted_choice_L12
weighted_choice_L11:
weighted_choice_L12:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; CHOICE = 0
move.b #0,d0
move.b d0,CHOICE
; if ROLL < WEIGHT0:
move.b ROLL,d0
move.b WEIGHT0,d1
cmp.b d1,d0
bcs weighted_choice_L13
jmp weighted_choice_L14
weighted_choice_L13:
; return
rts
jmp weighted_choice_L15
weighted_choice_L14:
weighted_choice_L15:
; CHOICE = 1
move.b #1,d0
move.b d0,CHOICE
; if ROLL < T0:
move.b ROLL,d0
move.b T0,d1
cmp.b d1,d0
bcs weighted_choice_L16
jmp weighted_choice_L17
weighted_choice_L16:
; return
rts
jmp weighted_choice_L18
weighted_choice_L17:
weighted_choice_L18:
; CHOICE = 2
move.b #2,d0
move.b d0,CHOICE
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:
WEIGHT0: dc.b 0
WEIGHT1: dc.b 0
WEIGHT2: dc.b 0
ROLL: dc.b 0
CHOICE: dc.b 0
TOTAL: dc.b 0
VALID: 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 "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
; WEIGHT0 = 1
move.b #1,d0
move.b d0,WEIGHT0
; WEIGHT1 = 1
move.b #1,d0
move.b d0,WEIGHT1
; WEIGHT2 = 4
move.b #4,d0
move.b d0,WEIGHT2
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
; ROLL = 2
move.b #2,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
; ROLL = 5
move.b #5,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
rts
else
; WEIGHT0 = 2
move.b #2,d0
move.b d0,WEIGHT0
; WEIGHT1 = 3
move.b #3,d0
move.b d0,WEIGHT1
; WEIGHT2 = 1
move.b #1,d0
move.b d0,WEIGHT2
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
; ROLL = 2
move.b #2,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
; ROLL = 5
move.b #5,d0
move.b d0,ROLL
ifne BROKEN
; ROLL = 0
move.b #0,d0
move.b d0,ROLL
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Use weights 1,1,4 instead of 2,3,1. 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:
WEIGHT0: dc.b 0
WEIGHT1: dc.b 0
WEIGHT2: dc.b 0
ROLL: dc.b 0
CHOICE: dc.b 0
TOTAL: dc.b 0
VALID: 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 "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 04 05 02 06 01 00. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong caller replaces every supplied ROLL with zero. Positive weight zero always wins and the intended variation disappears. Keep the supplied bounded roll; checking its range cannot establish that it was random.
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:
WEIGHT0: dc.b 0
WEIGHT1: dc.b 0
WEIGHT2: dc.b 0
ROLL: dc.b 0
CHOICE: dc.b 0
TOTAL: dc.b 0
VALID: 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 "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 302 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 for three choices. A linear scan grows with outcome count; cumulative tables and binary search repay their storage on larger distributions.
Reducing an arbitrary random byte modulo TOTAL can bias the choice. Use rejection sampling or an appropriate bounded generator if equal interval positions matter. Three fixed weights are readable; larger tables favour cumulative totals and search.
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
Seeded randomness: A reproducible generator supplies randomness; bounded sampling and weighted intervals decide how to use it.