The Free On-line Dictionary of Computing (30 December 2018):

Zilog Z80
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 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 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 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.  It was also used in the Sega Genesis
   for hardware reverse compatibility with the Sega Master System
   through a special cartridge.

   Gaby Chaudry site (http://gaby.de/z80/).

   (2004-06-10)