Zilog Z80

The Z80 Sound CPU and Sound RAM on the Model 1 Sega Mega Drive

The Zilog Z80 is a general-purpose 8-bit CPU. It is clocked at 3.58 MHz in the Mega Drive and is often used as the sound CPU, but is officially designated as the Co-Processor to the M68k. It has direct memory access to 8 kilobytes of dedicated sound RAM, as well as the YM2612 FM synthesis chip, and the SN76489 PSG chip. On top of this, it can access a 32KByte 'bank' of any area within M68k memory. This CPU is often used to run the code for the sound driver on many games, while some games only utilise it for DAC playback. The Z80's memory is laid out like the following:

Memory Map

Start	End	Description
0000h	1FFFh	Z80 RAM
2000h	3FFFh	Reserved
4000h		YM2612 A0
4001h		YM2612 D0
4002h		YM2612 A1
4003h		YM2612 D1
4004h	5FFFh	Reserved
6000h		Bank register
6001h	7F10h	Reserved
7F11h		SN76489 PSG
7F12h	7FFFh	Reserved
8000h	FFFFh	M68k memory bank

The current bank to be at the $8000 and up region can be controlled by the banking register.

Programming the Z80

Registers

Seeing as the Z80 was developed to be byte compatible with Intel's 8080 chip, the standard 8080 registers are also present on the Z80:

• AF: 8-bit accumulator (A) and flag bits (F), and an Add/Subtract flag (usually called N)
• BC: 16-bit data/address register or two 8-bit registers (A hybrid of an ax and dx register on the M68k)
• DE: 16-bit data/address register or two 8-bit registers (A hybrid of an ax and dx register on the M68k)
• HL: 16-bit accumulator/address register or two 8-bit registers
• SP: stack pointer, 16 bits (This register is also present on the M68k)
• PC: program counter, 16 bits (This register is also present on the M68k)

The Z80 has these added registers:

• IX: 16-bit index or base register for 8-bit immediate offsets
• IY: 16-bit index or base register for 8-bit immediate offsets
• I: interrupt vector base register, 8 bits
• R: DRAM refresh counter,
• AF': alternate (or shadow) accumulator and flags
• BC', DE' and HL': alternate (or shadow) registers