Chiptune

This tune uses the Sound chip, the MSX's AY-3-8910: channel A plays the melody, B the bass and C the drums. Choose Build, then Run, with your sound on.

The chip has three square-wave channels, one shared noise generator and one shared envelope generator, which changes volume over time. A volume value of 16 makes a channel follow the envelope. Here C follows shape 0: fall from full volume to silence and stay there. Writing 0 to shape register 13 restarts that fall, striking the drum; even writing the same shape restarts it.

Each setup line pairs a register number with its value. The initialization loop selects the register through port 0xA0, then writes the value through 0xA1.

Pair in setup	Purpose
7, 0x9c	Mixer: tones on A and B, noise on C
8, 11	A's fixed volume: 11
9, 14	B's fixed volume: 14
10, 0x10	C follows the envelope (0x10 is 16)
11, 0x58	Low byte of the envelope period
12, 0x02	High byte: together 0x0258, or 600, for a fall of about 86 ms

A 0 bit in the mixer enables a source. In 0x9C (10011100), bits 0 and 1 enable A's and B's tones; bit 5 enables C's noise. Bit 7 controls an unconnected I/O port and has no effect here. The noise period sets the drum's brightness: 24 gives a low thud, 6 a snare-like sound and 1 a bright hi-hat-like sound.

Each step occupies five bytes. The first row, .word 214, 854 followed by .byte 24, means:

Offset from ix	Value	Meaning
0–1	214	A's tone period, low byte first
2–3	854	B's tone period, low byte first
4	24	Noise period; restart the drum envelope

A zero in either tone field leaves that channel unchanged. A zero drum byte means no new hit: any previous envelope fall continues. Neither commands immediate silence.

        .org 0x8000
        ld hl, setup        ; pairs of a register and its value
        ld b, 6
init:   ld a, (hl)
        out (0xa0), a       ; select the register
        inc hl
        ld a, (hl)
        out (0xa1), a       ; and write it
        inc hl
        djnz init

        ld ix, song
step:   ld l, (ix+0)        ; the lead's period
        ld h, (ix+1)
        ld a, h
        and l
        inc a               ; 0xFFFF ends the song
        jr z, done
        ld a, 0             ; registers 0 and 1: channel A's period
        call tone
        ld l, (ix+2)        ; the bass's period
        ld h, (ix+3)
        ld a, 2             ; registers 2 and 3: channel B's period
        call tone
        ld a, (ix+4)        ; the drum's noise period
        or a
        jr z, wait          ; 0: no drum on this step
        ld c, a
        ld a, 6             ; register 6: the noise period
        out (0xa0), a
        ld a, c
        out (0xa1), a
        ld a, 13            ; register 13: the envelope's shape,
        out (0xa0), a
        xor a               ; 0, falling once to silence: writing it strikes the drum
        out (0xa1), a
wait:   ld bc, 0x0f50       ; b = 15 hundredths of a second, c = the wait port
        in a, (c)
        ld de, 5
        add ix, de          ; the next row
        jr step

done:   ld bc, 0x3c50       ; let the last notes ring for 60 hundredths
        in a, (c)
        halt                ; the program's end silences the chip

; Writes the period in hl to the registers a and a + 1, unless it is 0.
tone:   ld c, a
        ld a, h
        or l
        ret z               ; 0 leaves the channel as it is
        ld a, c
        out (0xa0), a
        ld a, l
        out (0xa1), a       ; the low byte
        ld a, c
        inc a
        out (0xa0), a
        ld a, h
        out (0xa1), a       ; the high four bits
        ret

setup:  .byte 7, 0x9c       ; mixer: tones on A and B, noise on C
        .byte 8, 11         ; channel A's volume
        .byte 9, 14         ; channel B's volume
        .byte 10, 0x10      ; channel C follows the envelope
        .byte 11, 0x58      ; the envelope's period, 600: about 86 ms to fall
        .byte 12, 0x02

; One row a step, an eighth of a bar: the periods of the lead and the bass, then the drum's noise
; period. 0 leaves that channel as it is; a lead of 0xFFFF ends the song.
song:
        .word 214, 854
        .byte 24
        .word 170, 0
        .byte 0
        .word 143, 0
        .byte 1
        .word 170, 0
        .byte 0
        .word 214, 854
        .byte 6
        .word 170, 0
        .byte 0
        .word 143, 0
        .byte 1
        .word 107, 0
        .byte 0
        .word 254, 1017
        .byte 24
        .word 214, 0
        .byte 0
        .word 170, 0
        .byte 1
        .word 214, 0
        .byte 0
        .word 254, 1017
        .byte 6
        .word 214, 0
        .byte 0
        .word 170, 0
        .byte 1
        .word 127, 0
        .byte 0
        .word 321, 1286
        .byte 24
        .word 254, 0
        .byte 0
        .word 214, 0
        .byte 1
        .word 254, 0
        .byte 0
        .word 321, 1286
        .byte 6
        .word 254, 0
        .byte 0
        .word 214, 0
        .byte 1
        .word 160, 0
        .byte 0
        .word 285, 1141
        .byte 24
        .word 226, 0
        .byte 0
        .word 191, 0
        .byte 1
        .word 226, 0
        .byte 0
        .word 285, 1141
        .byte 6
        .word 226, 0
        .byte 0
        .word 191, 0
        .byte 1
        .word 143, 0
        .byte 0
        .word 214, 854
        .byte 24
        .word 0xffff1
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The main 32 steps make four bars of eight, over C, A minor, F and G. Each step waits 15 hundredths of a second through the wait port. A 33rd step returns to the opening C notes and strikes the drum. The following 0xFFFF word ends the table; after another 60 hundredths, halt silences the chip.

The tone period is 1,789,772.5 / (16 × frequency in Hz): 214 gives about 523 Hz, the C an octave above middle C, and the bass's 854 gives about 131 Hz. A larger period gives a lower note.

At step, and l must produce 0xFF for inc a to wrap to zero. This happens only when both bytes are 0xFF, detecting the sentinel before reading the rest of a row. In tone, h OR l is zero only when the whole period is zero, so ret z skips the write. Adding 5 to ix advances to the next step.

Try it

Make the drums longer by editing setup: change .byte 11, 0x58 to .byte 11, 0x00, and .byte 12, 0x02 to .byte 12, 0x04. Change the values, keeping the register numbers 11 and 12. Period 0x0400, or 1,024, gives a fall of about 146 ms.

Keep those edits, then give the lead the envelope too: change .byte 8, 11 to .byte 8, 16 in setup. Each drum hit now restarts the same 146 ms volume fall for both lead and drums. A lead note alone does not restart it. On steps without a new hit, any unfinished fall continues briefly, then both channels stay silent until the next hit. The bass keeps its fixed volume. The Sound chip ports list the other registers.