Code Like It's 198x

Service a Logical Attack–Decay–Sustain–Release Envelope on C64

A sound can swell, settle and fade instead of switching between full volume and silence.

A sound can swell, settle and fade instead of switching between full volume and silence.

C64 shows the working diagnostic row 06 03 01 01 01 03 02 06.
A completed native caller displays 06 03 01 01 01 03 02 06. 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

LEVEL is 0–15; PHASE is 0 off, 1 attack, 2 decay, 3 sustain or 4 release. ATTACK, DECAY and RELEASE are per-service steps 1–15; SUSTAIN is 0–15. A rising GATE restarts from zero. A low gate sends any active phase to release. Return a clamped logical level and VALID=1; invalid parameters or state return VALID=0 without changing envelope state.

Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: LEVEL, PHASE, PREVIOUS, VALID, GATE, ATTACK, DECAY, SUSTAIN, RELEASE. 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 logical_envelope_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 logical_envelope_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 LEVEL, PHASE and PREVIOUS
Supply before service GATE, ATTACK, DECAY, SUSTAIN and RELEASE
Read after service LEVEL, PHASE and VALID

Gate edges choose a phase; each phase moves towards its own bound. Clamp before changing phase so a large step cannot wrap below silence or past the peak.

Reusable initialisation and service

6502.inc
; SPDX-License-Identifier: MIT
; Service a Logical Attack–Decay–Sustain–Release Envelope. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
logical_envelope_init:
    lda #0
    sta LEVEL
    sta PHASE
    sta PREVIOUS
    sta VALID
    sta GATE
    sta ATTACK
    sta DECAY
    sta SUSTAIN
    sta RELEASE
    sta T0
    rts
logical_envelope_step:
; VALID = 0
    lda #0
    sta VALID
; if LEVEL > 15:
    lda LEVEL
    cmp #15
    beq logical_envelope_L5
    bcs logical_envelope_L4
logical_envelope_L5:
    jmp logical_envelope_L2
logical_envelope_L4:
    jmp logical_envelope_L1
logical_envelope_L1:
; return
    rts
    jmp logical_envelope_L3
logical_envelope_L2:
logical_envelope_L3:
; if PHASE > 4:
    lda PHASE
    cmp #4
    beq logical_envelope_L10
    bcs logical_envelope_L9
logical_envelope_L10:
    jmp logical_envelope_L7
logical_envelope_L9:
    jmp logical_envelope_L6
logical_envelope_L6:
; return
    rts
    jmp logical_envelope_L8
logical_envelope_L7:
logical_envelope_L8:
; if ATTACK == 0:
    lda ATTACK
    cmp #0
    beq logical_envelope_L14
logical_envelope_L15:
    jmp logical_envelope_L12
logical_envelope_L14:
    jmp logical_envelope_L11
logical_envelope_L11:
; return
    rts
    jmp logical_envelope_L13
logical_envelope_L12:
logical_envelope_L13:
; if ATTACK > 15:
    lda ATTACK
    cmp #15
    beq logical_envelope_L20
    bcs logical_envelope_L19
logical_envelope_L20:
    jmp logical_envelope_L17
logical_envelope_L19:
    jmp logical_envelope_L16
logical_envelope_L16:
; return
    rts
    jmp logical_envelope_L18
logical_envelope_L17:
logical_envelope_L18:
; if DECAY == 0:
    lda DECAY
    cmp #0
    beq logical_envelope_L24
logical_envelope_L25:
    jmp logical_envelope_L22
logical_envelope_L24:
    jmp logical_envelope_L21
logical_envelope_L21:
; return
    rts
    jmp logical_envelope_L23
logical_envelope_L22:
logical_envelope_L23:
; if DECAY > 15:
    lda DECAY
    cmp #15
    beq logical_envelope_L30
    bcs logical_envelope_L29
logical_envelope_L30:
    jmp logical_envelope_L27
logical_envelope_L29:
    jmp logical_envelope_L26
logical_envelope_L26:
; return
    rts
    jmp logical_envelope_L28
logical_envelope_L27:
logical_envelope_L28:
; if RELEASE == 0:
    lda RELEASE
    cmp #0
    beq logical_envelope_L34
logical_envelope_L35:
    jmp logical_envelope_L32
logical_envelope_L34:
    jmp logical_envelope_L31
logical_envelope_L31:
; return
    rts
    jmp logical_envelope_L33
logical_envelope_L32:
logical_envelope_L33:
; if RELEASE > 15:
    lda RELEASE
    cmp #15
    beq logical_envelope_L40
    bcs logical_envelope_L39
logical_envelope_L40:
    jmp logical_envelope_L37
logical_envelope_L39:
    jmp logical_envelope_L36
logical_envelope_L36:
; return
    rts
    jmp logical_envelope_L38
logical_envelope_L37:
logical_envelope_L38:
; if SUSTAIN > 15:
    lda SUSTAIN
    cmp #15
    beq logical_envelope_L45
    bcs logical_envelope_L44
logical_envelope_L45:
    jmp logical_envelope_L42
logical_envelope_L44:
    jmp logical_envelope_L41
logical_envelope_L41:
; return
    rts
    jmp logical_envelope_L43
logical_envelope_L42:
logical_envelope_L43:
; VALID = 1
    lda #1
    sta VALID
; if GATE != 0:
    lda GATE
    cmp #0
    bne logical_envelope_L49
logical_envelope_L50:
    jmp logical_envelope_L47
logical_envelope_L49:
    jmp logical_envelope_L46
logical_envelope_L46:
; if PREVIOUS == 0:
    lda PREVIOUS
    cmp #0
    beq logical_envelope_L54
logical_envelope_L55:
    jmp logical_envelope_L52
logical_envelope_L54:
    jmp logical_envelope_L51
logical_envelope_L51:
; LEVEL = 0
    lda #0
    sta LEVEL
; PHASE = 1
    lda #1
    sta PHASE
    jmp logical_envelope_L53
logical_envelope_L52:
logical_envelope_L53:
; PREVIOUS = 1
    lda #1
    sta PREVIOUS
    jmp logical_envelope_L48
logical_envelope_L47:
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
; if PHASE != 0:
    lda PHASE
    cmp #0
    bne logical_envelope_L59
logical_envelope_L60:
    jmp logical_envelope_L57
logical_envelope_L59:
    jmp logical_envelope_L56
logical_envelope_L56:
; PHASE = 4
    lda #4
    sta PHASE
    jmp logical_envelope_L58
logical_envelope_L57:
logical_envelope_L58:
logical_envelope_L48:
; if PHASE == 0:
    lda PHASE
    cmp #0
    beq logical_envelope_L64
logical_envelope_L65:
    jmp logical_envelope_L62
logical_envelope_L64:
    jmp logical_envelope_L61
logical_envelope_L61:
; LEVEL = 0
    lda #0
    sta LEVEL
; return
    rts
    jmp logical_envelope_L63
logical_envelope_L62:
logical_envelope_L63:
; if PHASE == 1:
    lda PHASE
    cmp #1
    beq logical_envelope_L69
logical_envelope_L70:
    jmp logical_envelope_L67
logical_envelope_L69:
    jmp logical_envelope_L66
logical_envelope_L66:
; T0 = 15 - LEVEL
    lda #15
    sec
    sbc LEVEL
    sta T0
; if ATTACK >= T0:
    lda ATTACK
    cmp T0
    bcs logical_envelope_L74
logical_envelope_L75:
    jmp logical_envelope_L72
logical_envelope_L74:
    jmp logical_envelope_L71
logical_envelope_L71:
; LEVEL = 15
    lda #15
    sta LEVEL
; PHASE = 2
    lda #2
    sta PHASE
    jmp logical_envelope_L73
logical_envelope_L72:
; LEVEL = LEVEL + ATTACK
    lda LEVEL
    clc
    adc ATTACK
    sta LEVEL
logical_envelope_L73:
; return
    rts
    jmp logical_envelope_L68
logical_envelope_L67:
logical_envelope_L68:
; if PHASE == 2:
    lda PHASE
    cmp #2
    beq logical_envelope_L79
logical_envelope_L80:
    jmp logical_envelope_L77
logical_envelope_L79:
    jmp logical_envelope_L76
logical_envelope_L76:
; if LEVEL <= SUSTAIN:
    lda LEVEL
    cmp SUSTAIN
    bcc logical_envelope_L84
    beq logical_envelope_L84
logical_envelope_L85:
    jmp logical_envelope_L82
logical_envelope_L84:
    jmp logical_envelope_L81
logical_envelope_L81:
; LEVEL = SUSTAIN
    lda SUSTAIN
    sta LEVEL
; PHASE = 3
    lda #3
    sta PHASE
; return
    rts
    jmp logical_envelope_L83
logical_envelope_L82:
logical_envelope_L83:
; T0 = LEVEL - SUSTAIN
    lda LEVEL
    sec
    sbc SUSTAIN
    sta T0
; if DECAY >= T0:
    lda DECAY
    cmp T0
    bcs logical_envelope_L89
logical_envelope_L90:
    jmp logical_envelope_L87
logical_envelope_L89:
    jmp logical_envelope_L86
logical_envelope_L86:
; LEVEL = SUSTAIN
    lda SUSTAIN
    sta LEVEL
; PHASE = 3
    lda #3
    sta PHASE
    jmp logical_envelope_L88
logical_envelope_L87:
; LEVEL = LEVEL - DECAY
    lda LEVEL
    sec
    sbc DECAY
    sta LEVEL
logical_envelope_L88:
; return
    rts
    jmp logical_envelope_L78
logical_envelope_L77:
logical_envelope_L78:
; if PHASE == 3:
    lda PHASE
    cmp #3
    beq logical_envelope_L94
logical_envelope_L95:
    jmp logical_envelope_L92
logical_envelope_L94:
    jmp logical_envelope_L91
logical_envelope_L91:
; LEVEL = SUSTAIN
    lda SUSTAIN
    sta LEVEL
; return
    rts
    jmp logical_envelope_L93
logical_envelope_L92:
logical_envelope_L93:
; if RELEASE >= LEVEL:
    lda RELEASE
    cmp LEVEL
    bcs logical_envelope_L99
logical_envelope_L100:
    jmp logical_envelope_L97
logical_envelope_L99:
    jmp logical_envelope_L96
logical_envelope_L96:
; LEVEL = 0
    lda #0
    sta LEVEL
; PHASE = 0
    lda #0
    sta PHASE
    jmp logical_envelope_L98
logical_envelope_L97:
; LEVEL = LEVEL - RELEASE
    lda LEVEL
    sec
    sbc RELEASE
    sta LEVEL
logical_envelope_L98:
    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

demo.asm
LEVEL=$80
PHASE=$81
PREVIOUS=$82
VALID=$83
GATE=$84
ATTACK=$85
DECAY=$86
SUSTAIN=$87
RELEASE=$88
T0=$89
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

exercise.inc, excerpt
preview_rules:
!if CUSTOM {
; GATE = 1
    lda #1
    sta GATE
; ATTACK = 15
    lda #15
    sta ATTACK
; DECAY = 4
    lda #4
    sta DECAY
; SUSTAIN = 4
    lda #4
    sta SUSTAIN
; RELEASE = 4
    lda #4
    sta RELEASE
!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state
; GATE = 0
    lda #0
    sta GATE
!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state
    rts
} else {
; GATE = 1
    lda #1
    sta GATE
; ATTACK = 3
    lda #3
    sta ATTACK
; DECAY = 2
    lda #2
    sta DECAY
; SUSTAIN = 6
    lda #6
    sta SUSTAIN
; RELEASE = 4
    lda #4
    sta RELEASE
!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state

!if BROKEN {
; PREVIOUS = 0
    lda #0
    sta PREVIOUS
}
    jsr checked_step
    jsr emit_state
}
    rts

Customise it

Use a fast attack and release the note before the default held sequence finishes. 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
LEVEL=$80
PHASE=$81
PREVIOUS=$82
VALID=$83
GATE=$84
ATTACK=$85
DECAY=$86
SUSTAIN=$87
RELEASE=$88
T0=$89
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 00 00 00 01 00 0F 04 04. The full client trace in the source explains how it gets there.

Mistake and repair

The wrong caller clears PREVIOUS each service. A held note continually retriggers its attack and never develops the intended shape. Retain the gate edge state independently of note servicing.

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
LEVEL=$80
PHASE=$81
PREVIOUS=$82
VALID=$83
GATE=$84
ATTACK=$85
DECAY=$86
SUSTAIN=$87
RELEASE=$88
T0=$89
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:
C64 shows the deliberate error row 03 01 01 01 01 03 02 06.
The wrong client completes with 03 01 01 01 01 03 02 06; its changed inputs and state explain the result.

Cost and alternatives

The measured helper extent is 455 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 per service. A hardware envelope can save CPU when its behaviour fits the sound. Software envelopes offer a common control shape but need scheduled updates and machine-specific output mapping.

This is a software control envelope with linear steps per service, not an emulation of a chip’s envelope rates or curves. A backend maps LEVEL to an owned voice or mix gain. Do not overwrite shared hardware volume or a hardware-envelope mode without a deliberate audio ownership policy.

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

Priority arbitration: Voice ownership and effect priority decide which sound receives an envelope; the envelope alone does not allocate audio hardware.

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