R800

The R800 was designed by ASCII Corporation of Japan and built by Mitsui & Co The goal was a modern and pipelined CPU binary compatible with the Z80, and therefore with MSX software, while also maintaining compatibility with older MSX Z80-based hardware.

During the development of the MSX Turbo R, ASCII Corporation considered various processors, both compatible and incompatible with the Z80, as candidates.

At that time, Kazuya Kishioka (岸岡和也), a company employee, was researching and developing an ASIC that was a high-speed version of the Z80 and largely customized for the MSX architecture.

A review of the fetch mechanism in a typical MSX environment helps in explaining the R800: Since most implementations of MSX use RAM disposed in a 256×256 bytes block, two cycles are required to set the address for the fetch.

However, on the Z80, the refresh cycles destroy the information on the higher bits, so a workaround was needed.