R800 (CPU)
The R800 is the central processing unit used in the MSX Turbo-R home computer. The R800 was designed by ASCII Corporation of Japan and built by Mitsui & Co., Ltd.. The goals were to have the fastest CPU possible, while maintaining compatibility with old MSX Zilog Z80-based hardware and software.
Compatibility
In order to preserve software compatibility with old MSX software, the R800 uses a superset of the Z80 instruction set. In addition to all the Z80 opcodes, two multiplication instructions were added, MULUB (8-bit), and MULUW (16-bit). Also, many of the undocumented Z80 instructions were made official, including all the opcodes dealing with IX and IY as 8-bit registers (IXH, IXL, IYH, IYL).
As the R800 is not based directly on the Z80, but stems from the Z800 family, it lacks some of the other undocumented Z80 features. For instance, the undocumented flags represented in bits 3 and 5 of the F register don't assume the same values as in Z80 (causing it to fail ZEXALL tests) and the undocumented opcode often called SLL is replaced by another undocumented opcode called TST.
