On 2012-07-03 19:17, David Laight wrote:
On Tue, Jul 03, 2012 at 01:36:59PM +0300, ..I'd rather be coding ASM! wrote:..and here i was thinking that 20Mhz Z80 was blistering speed compared to 2-6Mhz. :>IIRC the Z80 required a 4x clock, so a 20MHz Z80 is about as fast as a 5MHz 8080 or 8085.
Assuming that the 8080 did one instruction per clock cycle... Which it don't. If I remember right, it needs about the same number of cycles as the Z80.
It's not that the Z80 "needs" as 4x clock. It's just that the fastest instructions takes four clock cycles, meaning the actual number of instructions executed is (at most) 1/4 of the clock frequency.
The 6502, which was the popular contemporary CPU to compare against, did do about one instruction per clock cycle, which is why there was somewhat of an equivalence between a 4 MHz Z80 and a 1 MHz 6502.
Johnny