Z800 и Z280: Несостоявшаяся эволюция Z80

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. 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.

The Z800 was, in most ways, a minicomputer inspired "super Z80" that would run existing, and larger, programs at considerably higher speeds. However the address and data buses were multiplexed and the chip was, also in other respects, somewhat complicated to program and interface to. Calculation of exact execution times was also very much harder to do than for the Z80. Moreover, the plain Z80 were good enough for most applications at the time so the extra computing power was, in many cases, not worth the added complexity. Bad marketing seems to have hurt the product as well. Hitachi developed the HD64180, as a less ambitious Z80 derivative. It had great success, probably because it is almost as simple to program and interface to as the original Z80.

Apart from the successful Zilog Z180 (developed largely by Hitachi) and Zilog Z182 other attempts were made to extend the Z80 architecture, the 32 bit Z380 (introduced 1994) was a commercial disappointment except for some specific telecom applications. On the other hand, the fast 24 bit eZ80 (introduced 2001) has been both commercially successful and won engineering awards. Unlike the Z800, Z280, and Z380, the eZ80 does not introduce many new instructions or addressing modes, in comparison to the original Z80, but instead primarily extends the 16 bit registers of the Z80 to 24 bits wide. This enables it to reach 256 times as much memory, and adds a fully pipelined execution unit that executes Z80 opcodes 4× as fast as the original.