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.