The Zilog Z800 was a 16-bit microprocessor designed by Zilog and meant to be released in 1985. It was instruction compatible with their existing Z80, and differed primarily in having on-chip cache and a memory management unit (MMU) to provide a 16 MB address range. It also added a huge number of new more orthogonal instructions and addressing modes.
Zilog essentially ignored the Z800 in favor of their 32-bit Z80000 and the Z800 never entered mass production. After more than five years had elapsed since it was originally introduced, the effort was redubbed the Z280 in 1986.[1] An actual product, the Z280 would ship in 1987 with almost the same design as the Z800, but this time implemented in CMOS.
The Z800 contrasts with Zilog's first 16-bit effort, the Zilog Z8000, in that the Z800 was intended to be Z80 compatible, while the Z8000 was only Z80-like and did not offer any direct compatibility. Zilog sought to rectify the lack of Z80 compatibility exhibited by the Z8000 when introducing the Z800, seeking to offer Z80 binary compatibility with an eightfold performance increase over the Z80, mirroring plans by National Semiconductor to incorporate emulation of the Intel 8080 in certain products in its own 32000 series of microprocessors.[2]