stepping machanism of Apple Disk ][ drive (was Re: Heatkit 51/4 floppies)

If you look at the instructions as opposed to getting too hung up on the register assets, and simply look at what it takes to "do something" e.g. load a value, do some arithmetic, store a value, etc. the two instruction sets have considerable overlap. However, the longest instructions for the 6502 take six (?) clock ticks, while the shortest ones for the Z-80 take four. (feel free to correct me, as I haven't even though about this, let alone looked at precise instruction details for over ten years.) I don't know how long the longest instruction for the Z-80 takes nor do I even know what its longest instruction is. I'd imagine it's one in which you load a 16-bit register from memory, indexed, or some such. That would have to take a fair number of clock ticks. In my comparisons, I've always fudged in favor of the Z-80, because it didn't make sense to me that a processor so cheap and simple would be as much faster than a Z-80 as the 6502 appeared. The hardware problems aside, the Z-80 instruction set is full of pretty complex instructions which take a number of clock ticks. The 6502 is full of fairly complex instructions too, but not as many. Fortunately, most programmers don't use ALL of either processor's instructions unless they are pushed. If you look at a simple load-the-accumulator then store-the-result, which occurs often in almost any application, the Z-80 takes about 6 ticks in the opcode fetch/decode (the instruction is decoded during the refresh cycle), three for each byte of the source address, then three to fetch the value. It then takes as long to store the result to memory. That's 6+3+3+3=15, twice is 30 ticks, or 7.5 microseconds at 4MHz. The 6502 takes one to fetch the load opcode, one to fetch each byte of the source address, one to execute the load, one to fetch the store opcode, one for each byte of the destination address, and one to accomplish the store. That's 5 to load, 5 to store, which is 10, right? It would appear that the 6502 would get through this pretty quickly at the same clock rate, i.e. 2.5 microseconds, but the problem is that external peripheral devices wouldn't run this fast. What's more, the 4 MHz parts, though priced a little higher than a 4 MHz Z-80, in 1980, the Z-80 had peripherals which would work very well together with the CPU at 4 MHz. Nevertheless, that's a 3:1 performance ratio which, based on the fact that there were many more register-to-register (quick) operations available to the Z-80 programmer, it's hard to imagine these would close the gap that much. They would have a significant impact, though. I think you can easily build a circuit with a 2 MHz 6502 and one with a 4 MHz Z-80 and make a comparison yourself. If you honestly try to take advantage of the instruction set to accomplish a task on both processors, you'll play HELL getting the Z-80 to keep up. It can be done, though, if the task is chosen properly. A pair of simulators would be really handy, wouldn't it? Dick -----Original Message----- From: Pete Turnbull <pete(a)dunnington.u-net.com> To: Discussion re-collecting of classic computers <classiccmp(a)u.washington.edu> Date: Sunday, April 11, 1999 10:55 AM Subject: Re: stepping machanism of Apple Disk ][ drive (was Re: Heatkit 51/4 floppies)