On Mon, Apr 22, 2024 at 01:06:42AM +0100, Peter Coghlan via cctalk wrote: [...] > This was implemented by a humble 6502 running at (mostly) 2MHz, with one 8 > bit arithmetic register, two 8 bit index registers, one 8 bit stack > pointer, a 16 bit program counter and a few flag bits.
> I would have expected that a computers featuring a Z80 with its larger > register set, 16 bit arithmetic operations, richer instruction set and > general bells and whistles would have been able to produce a much superior > implementation in terms of speed or features or both but I never came > across one. > Why is that? Did the Z80 take more cycles to implement it's more complex > instructions? Is this an early example of RISC vs CISC? Technically yes, but the implicit assumption in the question is wrong. The Z80 takes three or four memory cycles to perform a memory access versus the 6502 accessing memory on every cycle, but Z80 systems tend to be clocked 3-4 times faster so the memory bandwidth is pretty much the same. This shouldn't be too surprising: they were designed to use the same RAM chips. So the Z80 takes more cycles, but it was designed to use a faster clock and do simpler operations per clock as that saved die space. Clock speeds have *never* been a good way to compare CPUs. In the hands of an expert in their respective instruction sets, both architectures perform about as well as each other for a given memory bandwidth (which was and still remains the limiting factor on CPUs without caches). The 6502 could be said to "win" only in as much as the modern drop-in replacement is the 14Mhz 65C02S, whereas the Z80's is the Z804C00xx which tops out at 20MHz so is only equivalent to a ~5MHz 6502. For the same reason, a 14MHz 65C02S will leave a 68000 (maximum 16.67Mhz) in the dust, especially when working with byte-oriented data such as text where the wider bus doesn't help. The 68000 takes four cycles to perform a memory access, and inserts a couple of extra cycles of dead time for certain addressing modes which require extra work in the address-generation circuitry. Even back in the day, it was noted that the Sinclair's ZX Spectrum with its 3.5MHz Z80 could outperform their later QL with its 7.5MHz 68008.
