Code Like It's 198x

Choose from Weighted Outcomes on NES

A timid enemy might retreat often and attack rarely.

A timid enemy might retreat often and attack rarely. Give the outcomes weights rather than duplicating branches.

NES shows the working diagnostic row 02 03 01 05 02 06 01 00.
A completed native caller displays 02 03 01 05 02 06 01 00. The public field order below identifies these bytes; the capture is a state diagnostic, not a recording of a finished game.

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

6502.inc
; 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:
    lda #0
    sta WEIGHT0
    sta WEIGHT1
    sta WEIGHT2
    sta ROLL
    sta CHOICE
    sta TOTAL
    sta VALID
    sta T0
    sta T1
    rts
weighted_choice_step:
; CHOICE = 255
    lda #255
    sta CHOICE
; TOTAL = 0
    lda #0
    sta TOTAL
; VALID = 0
    lda #0
    sta VALID
; T0 = WEIGHT0 + WEIGHT1
    lda WEIGHT0
    clc
    adc WEIGHT1
    sta T0
; if T0 < WEIGHT0:
    lda T0
    cmp WEIGHT0
    bcc weighted_choice_L4
weighted_choice_L5:
    jmp weighted_choice_L2
weighted_choice_L4:
    jmp weighted_choice_L1
weighted_choice_L1:
; return
    rts
    jmp weighted_choice_L3
weighted_choice_L2:
weighted_choice_L3:
; T1 = T0 + WEIGHT2
    lda T0
    clc
    adc WEIGHT2
    sta T1
; if T1 < T0:
    lda T1
    cmp T0
    bcc weighted_choice_L9
weighted_choice_L10:
    jmp weighted_choice_L7
weighted_choice_L9:
    jmp weighted_choice_L6
weighted_choice_L6:
; return
    rts
    jmp weighted_choice_L8
weighted_choice_L7:
weighted_choice_L8:
; TOTAL = T1
    lda T1
    sta TOTAL
; if TOTAL == 0:
    lda TOTAL
    cmp #0
    beq weighted_choice_L14
weighted_choice_L15:
    jmp weighted_choice_L12
weighted_choice_L14:
    jmp weighted_choice_L11
weighted_choice_L11:
; return
    rts
    jmp weighted_choice_L13
weighted_choice_L12:
weighted_choice_L13:
; if ROLL >= TOTAL:
    lda ROLL
    cmp TOTAL
    bcs weighted_choice_L19
weighted_choice_L20:
    jmp weighted_choice_L17
weighted_choice_L19:
    jmp weighted_choice_L16
weighted_choice_L16:
; return
    rts
    jmp weighted_choice_L18
weighted_choice_L17:
weighted_choice_L18:
; VALID = 1
    lda #1
    sta VALID
; CHOICE = 0
    lda #0
    sta CHOICE
; if ROLL < WEIGHT0:
    lda ROLL
    cmp WEIGHT0
    bcc weighted_choice_L24
weighted_choice_L25:
    jmp weighted_choice_L22
weighted_choice_L24:
    jmp weighted_choice_L21
weighted_choice_L21:
; return
    rts
    jmp weighted_choice_L23
weighted_choice_L22:
weighted_choice_L23:
; CHOICE = 1
    lda #1
    sta CHOICE
; if ROLL < T0:
    lda ROLL
    cmp T0
    bcc weighted_choice_L29
weighted_choice_L30:
    jmp weighted_choice_L27
weighted_choice_L29:
    jmp weighted_choice_L26
weighted_choice_L26:
; return
    rts
    jmp weighted_choice_L28
weighted_choice_L27:
weighted_choice_L28:
; CHOICE = 2
    lda #2
    sta CHOICE
    rts
component_end:

Machine contract

A/X/status may change; Y is preserved. The 2A03 has no decimal arithmetic mode. 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. Scope or rename routine and field labels and allocate a separate block for each independent instance. Give each block one mutation owner.

The NROM caller owns PPU setup and background display, with NMI and DMC disabled. Table and packet sizes are bounded to fit the cartridge caller and internal RAM; the helper owns no PPU registers.

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

demo.asm
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
WEIGHT0=$80
WEIGHT1=$81
WEIGHT2=$82
ROLL=$83
CHOICE=$84
TOTAL=$85
VALID=$86
T0=$87
T1=$88
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

A client retaining state between calls

exercise.inc, excerpt
preview_rules:
.if CUSTOM
; WEIGHT0 = 1
    lda #1
    sta WEIGHT0
; WEIGHT1 = 1
    lda #1
    sta WEIGHT1
; WEIGHT2 = 4
    lda #4
    sta WEIGHT2
; ROLL = 0
    lda #0
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
; ROLL = 2
    lda #2
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
; ROLL = 5
    lda #5
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
    rts
.else
; WEIGHT0 = 2
    lda #2
    sta WEIGHT0
; WEIGHT1 = 3
    lda #3
    sta WEIGHT1
; WEIGHT2 = 1
    lda #1
    sta WEIGHT2
; ROLL = 0
    lda #0
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
; ROLL = 2
    lda #2
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
; ROLL = 5
    lda #5
    sta ROLL
.if BROKEN
; ROLL = 0
    lda #0
    sta ROLL
.endif
    jsr checked_step
    jsr emit_state
.endif
    rts

Customise 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

custom.asm
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
WEIGHT0=$80
WEIGHT1=$81
WEIGHT2=$82
ROLL=$83
CHOICE=$84
TOTAL=$85
VALID=$86
T0=$87
T1=$88
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0

The 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

wrong.asm
.segment "HEADER"
.byte "NES",$1a,2,1,0,0,0,0,0,0,0,0,0,0
WEIGHT0=$80
WEIGHT1=$81
WEIGHT2=$82
ROLL=$83
CHOICE=$84
TOTAL=$85
VALID=$86
T0=$87
T1=$88
STATE=$80
input_ptr=$70
cases_left=$72
ptr=$74
saved_y=$76
OUTPUT=$0200
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
.include "caller.inc"
.include "exercise.inc"
component_start:
.include "component.inc"
.include "data.inc"
.include "cases.inc"
code_end:
.segment "VECTORS"
.word nmi,reset,irq
.segment "CHARS"
.include "../../input/controller-reading/edges-font.inc"
.res 8192-272,0
NES shows the deliberate error row 02 03 01 00 00 06 01 00.
The wrong client completes with 02 03 01 00 00 06 01 00; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 164 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 NTSC NROM, background only, DMC/NMI disabled, 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

Seeded randomness: A reproducible generator supplies randomness; bounded sampling and weighted intervals decide how to use it.

The Craft · Maths for Games · Source and native evidence.