I didn't bring those up, Allison. You're the one who brought up the 10 MHz part and made the ill-considered statement that there exists any basis for comparison between a Z80 and a 6502 at the same clock rate, or even with the Z80 at twice the clock rate. A 10 MHz part didn't exist back then either, nor did the latest shrink of the CMOS 6502. What I've been going on about is a way of reconciling the major architectural differences that make a comparison on the basis of performance of the processor core/instruction set possible. Talking simply about clock rate is almost meaningless, since the way in which the processors behave with respect to the clock differes greatly. If you want to insist that you can devise a task, any task at all, mind you, that you can code in Z80 code to run at 2x the clock rate of a 6502, in, say, <80 lines of assembler, that's strictly a computing task, just to leave I/O out of it, or any I/O task that you think you can code in legitimate Z80 instructions, providing a precise spec for the I/O task, then I'd certainly come out and say you can't cook a legitimate one up that a 6502 can't accomplish in less time, in the case of the computing task, and that, since the I/O task spec limits the rate, the 6502 will be able to do it as well. I'd like to see a task that meets those spec's. I'm pretty sure that a 2 MHz 6502 can easily handle transfers to/from an 8" floppy drive via a WD-179x using MFM without any hardware synchronization using, say, the wait line. I'm also pretty comfortable that the Z80 would have to strain. I certainly don't know that it's impossible, but every controller I've seen that handles MFM on 8" floppy drives for use with a Z80 uses the Wait line to sync up with the DRQ signal. For a 2 MHz 6502, it's a leisurely task requiring no hardware for synchronizing the process at all. The crux of making a reasonable comparison between a 6502, avaialble in the '70's, and a Z80, also available in the '70's, is selecting a clock and hardware arrangement that makes it possible for the Z80 to compete with the 6502 on a fair basis at all. To run without waits, the Z80 requires relatively fast memory, which, in its day, would make the system too costly. Running the Z80 with the same memory that the 6502 requires would make the Z80 too slow to compete. Dick ----- Original Message ----- From: "ajp166" &lt;ajp166(a)bellatlantic.net&gt; To: &lt;classiccmp(a)classiccmp.org&gt; Sent: Friday, December 21, 2001 7:27 PM Subject: Re: 6502/Z80 speed comparison (was MITS 2SIO serial chip?) You brought 'em up, I didn't. My main 6502, back in '79, was a Synertek NMOS 6502C, specified to run at a 4 MHz clock. They were production parts, avaiable through every one of their distributors. They stopped making those when the CMOS part became available, since it was more profitable. A Z180 is not a Z80.