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R800
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R800

The R800 is a RISC processor running on 7,16MHz and is opcode compatible with the Z80. Although it's data bus is 8-bit to ensure compatibility with the MSX architecture it is a 16bit processor internally. Apart from the standard Z80 instruction set two instructions were added, MULUB and MULUW, to allow for the multiplication of both unsigned bytes and unsigned words. Furthermore, a number of unofficial Z80 instructions were formalized. Unlike the Z80, the R800 is a RISC processor. The major advantage of its architecture is that it allows most simple instructions to be completed in one cycle, which is equal to one T-State. Since the Z80 has four T-States in each cycle the increase in speed is significant. Theoretically, the R800 is at least 4 times as fast as the Z80 clock for clock. Since the R800 also runs at twice the clockspeed this means that it is theoretically 8 times as fast as the standard Z80. Due to slower DRAM, the additional waits for DRAM refreshing and the waits introduced to maintain compatibility with the slower components of the system it is in practice between 5 and 6 times as fast as a standard Z80A MSX System. As an additional feature the R800 allows shadowing of BIOS and BASIC ROMss in the high 64kB of the Main RAM. Since the RAM memory is significantly faster than the ROM memory used this allows for a further gain in speed.