AY Tone

Overview

The 128K, +2 and +3 add what the 48K never had: a real sound chip, the AY-3-8912. Where the beeper needs the CPU to toggle a bit thousands of times a second, the AY has three tone channels that oscillate in hardware — you set a channel up once and it plays while your game runs. It’s a register-file chip: you pick a register through port $FFFD, write a value through $BFFD, and sixteen registers between them control the whole chip. A tone is four writes: the 12-bit pitch, the mixer to switch the channel on, and a volume.

Code

; =============================================================================
; AY TONE - ZX SPECTRUM 128K (AY-3-8912)
; Hold a continuous tone on channel A
; =============================================================================

; The two AY ports on the 128K
AY_SELECT equ $fffd        ; write a register number here to select it
; data is written to $BFFD (we build that port in B below)

R_A_LO  equ 0              ; channel A tone period, low 8 bits
R_A_HI  equ 1              ; channel A tone period, high 4 bits
R_MIXER equ 7              ; tone / noise enables — ACTIVE LOW
R_A_VOL equ 8              ; channel A volume (0-15; bit 4 = follow envelope)

start:
    ld   a,R_A_LO          ; channel A pitch — a 12-bit period
    ld   e,$fc             ; period 252 -> ~440 Hz (A-4) at the 1.77 MHz clock
    call ay_out
    ld   a,R_A_HI
    ld   e,$00             ; high nibble of the period
    call ay_out

    ld   a,R_MIXER
    ld   e,%00111110       ; tone A ON (bit 0 = 0); B, C and all noise off
    call ay_out

    ld   a,R_A_VOL
    ld   e,15              ; full, fixed volume (bit 4 clear = not envelope)
    call ay_out

    ret                    ; the tone plays on — the AY needs no more CPU

; A = register number, E = value to write
ay_out:
    ld   bc,AY_SELECT      ; BC = $FFFD
    out  (c),a             ; select the register
    ld   b,$bf             ; BC = $BFFD
    out  (c),e             ; write the value
    ret

Trade-offs

Aspect	Cost
CPU	None once set — the chip oscillates by itself
Memory	A few register writes
Limitation	128K only; a 48K game has to fall back to the beeper

When to use: Any sound on a 128K machine — it’s better than the beeper in every way that matters.

When to avoid: A game that must also run on a 48K, where the AY simply isn’t there.

Two ports, sixteen registers

The AY hides behind just two I/O ports. $FFFD is the address latch — write a register number (0–15) and that register becomes “current”. $BFFD is the data port — the next write lands in whichever register you latched. The ld bc,$fffd / out (c),a / ld b,$bf / out (c),e idiom is the whole access method: the two ports differ only in their high byte, so you swap B from $FF to $BF between the two outs. Every AY routine is built on this pair of writes.

Pitch is a 12-bit period

Each channel’s pitch is a 12-bit period split across two registers — the low byte and the low nibble of the next. As with most such chips, a bigger period is a lower note:

frequency (Hz) = 1773400 / (16 * period)

The 1773400 is the Spectrum 128K’s AY clock. Period 252 gives ≈440 Hz. Music routines keep a table of periods, one per semitone; the same chip in a CPC or an Atari ST uses a different clock, so only the table changes when you port a tune.

The mixer is active-low

Register 7 is the trap that silences every first AY tone. Its low bits enable tone on channels A, B, C (bits 0–2) and noise on the same channels (bits 3–5) — but a 0 turns a source ON, not off. So %00111110 means “tone A on, everything else off”: bit 0 clear for tone A, the rest set. Forget the inversion and write %00000001, and you switch off the very channel you meant to play.

Patterns: Beeper Music — the 48K alternative

Vault: AY-3-8912 | ZX Spectrum