Source : Free On-Line Dictionary of Computing

Zilog Z80
     
         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 code}s 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 switch}es.  It features 3 types of
        interrupt mode.
     
        The Z80 also added two {index register}s (IX and IY) and
        relocatable {vectored interrupt}s (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 {Zilog
        Z80A} (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 code}s.
     
        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 (http://www.gaby.de/z80/)}.
     
        (2003-07-11)