The Zilog Z800 was a 16-bit microprocessor designed by Zilog to be released in 1985. It was instruction compatible with their existing Z80, and differed primarily in having on chip cache and MMU for a 16 MB address range, and also a huge number of new more orthogonal instructions and addressing modes. However Zilog essentially ignored the Z800 in favour 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. An actual product, the Z280 would ship in 1987 with almost the same design as the Z800, but this time implemented in CMOS.
Read more about Zilog Z800: Short Description, Reason For The Failure, More Successful Z80 Derivatives (from Zilog)