Zilog 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 ZilogZ80000.

(1997-12-16)