A short tune needs a clock and a sequence of notes, not a fresh sound effect every frame.
Use it in a project
COUNT is 0–8 pairs N/U. N=0 is rest, 1–95 is a logical note, and 255 ends immediately. Other note codes or a zero duration on a note/rest are invalid. INDEX counts consumed pairs and must not exceed COUNT. If REMAINING is nonzero, decrement it first; when it reaches zero consume the next pair in the same call. A newly emitted note/rest holds for U service intervals, with REMAINING set to U. EVENT=1 emits NOTE; EVENT=2 is a one-shot stop on count exhaustion or N=255. FINISHED is sticky. Validate only the consumed pair before advancing INDEX. Invalid state returns VALID=0 and no event; an elapsed timer may already reach zero.
Include component.inc once (MOS callers include the shared 6502 implementation through it). Reserve the public fields in this exact order: NOTE, EVENT, FINISHED, VALID, INDEX, REMAINING, COUNT, N0, N1, N2, N3, N4, N5, N6, N7, U0, U1, U2, U3, U4, U5, U6, U7. They occupy 23 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 music_command_stream_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 music_command_stream_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 | INDEX, REMAINING, FINISHED and NOTE; immutable N/U pairs |
| Supply before service | COUNT and one service tick |
| Read after service | EVENT, NOTE, FINISHED and VALID |
Only consume a pair when its predecessor’s interval has expired. A bounded count prevents reading past the stream; explicit end and exhausted count each emit a stop exactly once.
Reusable initialisation and service
; SPDX-License-Identifier: MIT
; Sequence Counted Notes and Rests. See README.md for the byte interface and ownership.
; init clears the public fields and private workspace; set inputs afterwards.
music_command_stream_init:
move.b #0,d0
move.b d0,NOTE
move.b d0,EVENT
move.b d0,FINISHED
move.b d0,VALID
move.b d0,INDEX
move.b d0,REMAINING
move.b d0,COUNT
move.b d0,N0
move.b d0,N1
move.b d0,N2
move.b d0,N3
move.b d0,N4
move.b d0,N5
move.b d0,N6
move.b d0,N7
move.b d0,U0
move.b d0,U1
move.b d0,U2
move.b d0,U3
move.b d0,U4
move.b d0,U5
move.b d0,U6
move.b d0,U7
move.b d0,T0
move.b d0,T1
rts
music_command_stream_step:
; EVENT = 0
move.b #0,d0
move.b d0,EVENT
; VALID = 0
move.b #0,d0
move.b d0,VALID
; if COUNT > 8:
move.b COUNT,d0
cmp.b #8,d0
bhi music_command_stream_L1
jmp music_command_stream_L2
music_command_stream_L1:
; return
rts
jmp music_command_stream_L3
music_command_stream_L2:
music_command_stream_L3:
; if INDEX > COUNT:
move.b INDEX,d0
move.b COUNT,d1
cmp.b d1,d0
bhi music_command_stream_L4
jmp music_command_stream_L5
music_command_stream_L4:
; return
rts
jmp music_command_stream_L6
music_command_stream_L5:
music_command_stream_L6:
; VALID = 1
move.b #1,d0
move.b d0,VALID
; if FINISHED != 0:
move.b FINISHED,d0
cmp.b #0,d0
bne music_command_stream_L7
jmp music_command_stream_L8
music_command_stream_L7:
; return
rts
jmp music_command_stream_L9
music_command_stream_L8:
music_command_stream_L9:
; if REMAINING != 0:
move.b REMAINING,d0
cmp.b #0,d0
bne music_command_stream_L10
jmp music_command_stream_L11
music_command_stream_L10:
; REMAINING = REMAINING - 1
move.b REMAINING,d0
sub.b #1,d0
move.b d0,REMAINING
; if REMAINING != 0:
move.b REMAINING,d0
cmp.b #0,d0
bne music_command_stream_L13
jmp music_command_stream_L14
music_command_stream_L13:
; return
rts
jmp music_command_stream_L15
music_command_stream_L14:
music_command_stream_L15:
jmp music_command_stream_L12
music_command_stream_L11:
music_command_stream_L12:
; if INDEX == COUNT:
move.b INDEX,d0
move.b COUNT,d1
cmp.b d1,d0
beq music_command_stream_L16
jmp music_command_stream_L17
music_command_stream_L16:
; FINISHED = 1
move.b #1,d0
move.b d0,FINISHED
; NOTE = 0
move.b #0,d0
move.b d0,NOTE
; EVENT = 2
move.b #2,d0
move.b d0,EVENT
; return
rts
jmp music_command_stream_L18
music_command_stream_L17:
music_command_stream_L18:
; T0 = N[INDEX]
moveq #0,d1
move.b INDEX,d1
lea N0,a0
move.b 0(a0,d1.w),d0
move.b d0,T0
; T1 = U[INDEX]
moveq #0,d1
move.b INDEX,d1
lea U0,a0
move.b 0(a0,d1.w),d0
move.b d0,T1
; if T0 == 255:
move.b T0,d0
cmp.b #255,d0
beq music_command_stream_L19
jmp music_command_stream_L20
music_command_stream_L19:
; INDEX = INDEX + 1
move.b INDEX,d0
add.b #1,d0
move.b d0,INDEX
; FINISHED = 1
move.b #1,d0
move.b d0,FINISHED
; NOTE = 0
move.b #0,d0
move.b d0,NOTE
; EVENT = 2
move.b #2,d0
move.b d0,EVENT
; return
rts
jmp music_command_stream_L21
music_command_stream_L20:
music_command_stream_L21:
; if T0 > 95:
move.b T0,d0
cmp.b #95,d0
bhi music_command_stream_L22
jmp music_command_stream_L23
music_command_stream_L22:
; VALID = 0
move.b #0,d0
move.b d0,VALID
; return
rts
jmp music_command_stream_L24
music_command_stream_L23:
music_command_stream_L24:
; if T1 == 0:
move.b T1,d0
cmp.b #0,d0
beq music_command_stream_L25
jmp music_command_stream_L26
music_command_stream_L25:
; VALID = 0
move.b #0,d0
move.b d0,VALID
; return
rts
jmp music_command_stream_L27
music_command_stream_L26:
music_command_stream_L27:
; NOTE = T0
move.b T0,d0
move.b d0,NOTE
; REMAINING = T1
move.b T1,d0
move.b d0,REMAINING
; INDEX = INDEX + 1
move.b INDEX,d0
add.b #1,d0
move.b d0,INDEX
; EVENT = 1
move.b #1,d0
move.b d0,EVENT
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:
NOTE: dc.b 0
EVENT: dc.b 0
FINISHED: dc.b 0
VALID: dc.b 0
INDEX: dc.b 0
REMAINING: dc.b 0
COUNT: dc.b 0
N0: dc.b 0
N1: dc.b 0
N2: dc.b 0
N3: dc.b 0
N4: dc.b 0
N5: dc.b 0
N6: dc.b 0
N7: dc.b 0
U0: dc.b 0
U1: dc.b 0
U2: dc.b 0
U3: dc.b 0
U4: dc.b 0
U5: dc.b 0
U6: dc.b 0
U7: 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 "198XNIGHTTWO000",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
; COUNT = 2
move.b #2,d0
move.b d0,COUNT
; N0 = 72
move.b #72,d0
move.b d0,N0
; U0 = 1
move.b #1,d0
move.b d0,U0
; N1 = 255
move.b #255,d0
move.b d0,N1
; U1 = 0
move.b #0,d0
move.b d0,U1
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
rts
else
; COUNT = 3
move.b #3,d0
move.b d0,COUNT
; N0 = 40
move.b #40,d0
move.b d0,N0
; U0 = 2
move.b #2,d0
move.b d0,U0
; N1 = 0
move.b #0,d0
move.b d0,N1
; U1 = 1
move.b #1,d0
move.b d0,U1
; N2 = 44
move.b #44,d0
move.b d0,N2
; U2 = 1
move.b #1,d0
move.b d0,U2
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
ifne BROKEN
; REMAINING = 0
move.b #0,d0
move.b d0,REMAINING
endif
bsr checked_step
bsr emit_state
endif
rtsCustomise it
Play one higher note then an explicit end command, whose duration is ignored. 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:
NOTE: dc.b 0
EVENT: dc.b 0
FINISHED: dc.b 0
VALID: dc.b 0
INDEX: dc.b 0
REMAINING: dc.b 0
COUNT: dc.b 0
N0: dc.b 0
N1: dc.b 0
N2: dc.b 0
N3: dc.b 0
N4: dc.b 0
N5: dc.b 0
N6: dc.b 0
N7: dc.b 0
U0: dc.b 0
U1: dc.b 0
U2: dc.b 0
U3: dc.b 0
U4: dc.b 0
U5: dc.b 0
U6: dc.b 0
U7: 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 "198XNIGHTTWO000",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 00 01 01 02 00 02 48. The full client trace in the source explains how it gets there.
Mistake and repair
The wrong client discards each note’s hold time. It advances to a rest or next note one service too early. Retain REMAINING and use one consistent service clock.
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:
NOTE: dc.b 0
EVENT: dc.b 0
FINISHED: dc.b 0
VALID: dc.b 0
INDEX: dc.b 0
REMAINING: dc.b 0
COUNT: dc.b 0
N0: dc.b 0
N1: dc.b 0
N2: dc.b 0
N3: dc.b 0
N4: dc.b 0
N5: dc.b 0
N6: dc.b 0
N7: dc.b 0
U0: dc.b 0
U1: dc.b 0
U2: dc.b 0
U3: dc.b 0
U4: dc.b 0
U5: dc.b 0
U6: dc.b 0
U7: 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 "198XNIGHTTWO000",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 548 code bytes; public state is 23 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.
One pair at most per service, with no scan over instant commands. Rich formats can add instrument/loop commands, but need a command budget to stop malformed streams looping forever.
This is a logical sequencer, not an audible chip driver, tracker, tempo converter or general music format. The caller maps note numbers to chip periods and handles rest/stop by silencing its owned voice. SID, APU and Paula have different tuning, envelope and channel rules; Spectrum beeper playback also consumes CPU time. No emitted PCM or audible-timing claim is made by the state diagnostic.
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 43 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
Logical envelope: Drive the existing logical envelope or the documented machine sound trigger from note events; avoid retriggering on every held tick.