Zilog Z8000

Zilog Z8000
Z8000

   <processor> 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)