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

Zilog Z8000
Z8000

    A microprocessor from Zilog introduced not
   long after the Intel 8086, but with superior features.  It
   was basically a 16-bit processor, but could address up to 23
   bits in some versions by using segment registers (to supply
   the upper 7 bits).  There was also an unsegmented version, but
   both could be extended further with an additional MMU that
   used 64 segment registers.

   Internally, the Z8000 had sixteen 16-bit registers, but
   register size and use were exceedingly flexible.  The Z-8000
   registers could be used as sixteen 8-bit registers (only the
   first half were used like this), sixteen 16-bit registers,
   eight 32-bit registers, or four 64-bit registers, and included
   32-bit multiply and divide.  They were all general purpose
   registers - the stack pointer was typically register 15,
   with register 14 holding the stack segment (both accessed as
   one 32-bit register for painless address calculations).

   The Z8000 featured two modes, one for the operating system
   and one for user programs.  The user mode prevented the user
   from messing about with interrupt handling and other
   potentially dangerous stuff.

   Finally, like the Zilog Z80, the Z8000 featured automatic
   DRAM refresh circuitry.  Unfortunately it was somewhat slow,
   but the features generally made up for that.  Initial bugs
   also hindered its acceptance (partly because it did not use
   microcode).  There was a radiation resistant military
   version.

   There was a later 32-bit, pipelined version, the Zilog
   Z80000.

   (1997-12-16)