Zilog Z80


Zilog Z80

(processor)An 8-bit microprocessor. It was released inJuly 1976 with a 2.5 MHz clock rate. The Z80 was a muchimproved Intel 8080 (as was the Intel 8085). It also used8-bit data and 16-bit addressing, and could execute all of the8080 op codes as well as 80 new ones, instructions thatincluded 1, 4, 8 and 16-bit operations and even block move andblock I/O instructions. The register set was doubled, withtwo banks of registers (including A and F) that could beswitched between. This allowed fast operating system orinterrupt context switches. It features 3 types ofinterrupt mode.

The Z80 also added two index registers (IX and IY) andrelocatable vectored interrupts (via the 8-bit IV register).Like many processors (including the 8085), the Z80 featuredmany undocumented op codes. Chip area near the edge was usedfor added instructions, but fabrication made the failure ofthese high. Instructions that often failed were just notdocumented, increasing chip yield. Later fabrication madethese more reliable.

The thing that really made the Z80 popular was the memoryinterface - the CPU generated it's own RAM refreshsignals, which meant easier design and lower system cost.That and its 8080 compatibility and CP/M, the firststandard microprocessor operating system, made it thefirst choice of many systems.

In addition to the original Z80 (2.5 MHz) there are the Zilog Z80A (4 MHz), Zilog Z80B (6MHz) and Zilog Z80H (8 MHz)versions. The popular Hitachi HD64180 processor family addsperipherals and an MMU to the Z80.

The Zilog Z280 was an enhanced version with an MMU andmany new op codes.

The Z80 was used in the first Nintendo Game Boy. ASharp Z80 work-alike was used in the GameBoy Color,running at 4 MHz for GameBoy software or at 8 MHz for Game BoyColor software. The Z80 was used in the Sega Master Systemand the Game Gear. It was also used in the Sega Genesisfor hardware reverse compatibility with the Sega Master Systemthrough a special cartridge.

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