Zilog Z80

processor An 8-bit microprocessor. It was released in

July 1976 with a 2.5 MHz clock rate. The Z80 was a much

improved Intel 8080 (as was the Intel 8085). It also used

8-bit data and 16-bit addressing, and could execute all of the

8080 op codes as well as 80 new ones, instructions that

included 1, 4, 8 and 16-bit operations and even block move and

block I/O instructions. The register set was doubled, with

two banks of registers (including A and F) that could be

switched between. This allowed fast operating system or

interrupt context switches. It features 3 types of

interrupt mode.

The Z80 also added two index registers (IX and IY) and

relocatable vectored interrupts (via the 8-bit IV register).

Like many processors (including the 8085), the Z80 featured

many undocumented op codes. Chip area near the edge was used

for added instructions, but fabrication made the failure of

these high. Instructions that often failed were just not

documented, increasing chip yield. Later fabrication made

these more reliable.

The thing that really made the Z80 popular was the memory

interface - the CPU generated it's own RAM refresh

signals, which meant easier design and lower system cost.

That and its 8080 compatibility and CP/M, the first

standard microprocessor operating system, made it the

first choice of many systems.

In addition to the original Z80 (2.5 MHz) there are the ZilogZ80A (4 MHz), Zilog Z80B (6MHz) and Zilog Z80H (8 MHz)

versions. The popular Hitachi HD64180 processor family adds

peripherals and an MMU to the Z80.

The Zilog Z280 was an enhanced version with an MMU and

many new op codes.

The Z80 was used in the first Nintendo Game Boy. A

Sharp Z80 work-alike was used in the GameBoy Color,

running at 4 MHz for GameBoy software or at 8 MHz for Game Boy

Color software. The Z80 was used in the Sega Master System

and the Game Gear.

Gaby Chaudry Z80 site .

(2003-07-11)