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 for Zilog Z80
From The Free On-line Dictionary of Computing (30 December 2018) :

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

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