Oziphantom wrote:
A 1.8Mhz 6502 will beat the 3.5Mhz Z80 but now your bus and ROM and RAM chips only need to be good for 1.8Mhz operation and not 3.5Mhz, which makes them cheaper
No, that's not true. A 6502 requires a memory system that can respond in half a clock cycle. A Z80 only requires memory to respond in one-and-a-half clocks on instruction fetches, or two clocks on normal reads and writes. A 1.79 MHz 6502 needs a memory system that can respond in 280ns (minus latency from the address decoding, etc.) A 3.58 MHz Z80 needs 420ns to operate without any wait states, or 560ns to operate with a wait state on instruction fetches (as in the MSX)
On the other hand, the fact that the 6502 only accesses memory on alternating phases means that it can perfectly share a bus with another device. That's how the Apple II and the various Commodore machines work: the CPU can freely read or write "VRAM" (really just the portion of regular RAM that's shared with the video hardware) at any time. On the Z80-based ZX Spectrum, the CPU may be halted for up to 6 clocks whenever it accesses shared VRAM outside of VBlank or HBlank.