Author: Bo Persson
Date: 07:11:07 07/07/03
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On July 06, 2003 at 19:44:31, Tom Kerrigan wrote: >On July 06, 2003 at 18:41:42, Bo Persson wrote: > >>>Reading two bytes over a one byte bus? >> >>Except that it was actually a wider bus, but the full width wasn't used for >>data, just for addresses. >> >>The 20 bit address bus was multiplexed with an 8 bit data bus on the same pins. >>Elegant - no. Super kludge - yes! > >I still contend that just because something _lacks_ performance that it seems >like it should potentially have does not make it a kludge. It is not even "potentially", because its big sister the 8086 did use a wider bus on the very same pins. The higher performance of the 8086 was not so much becase its wider bus for data, but because it could actually fetch instructions almost as fast as they could be executed. That's a good idea for a processor. > >I'm not sure if you've ever wired up a board with one of these processors No I have not, but I have used one with an 8088. The wrapping was already done when I started working on the software. When it didn't boot, you gave the 1 foot high video board a good shake, and tried again... The next machine we built had such an extreme clock frequency (8 MHz) that a wire wrap didn't seem possible, so we started with a prototyped PCB. >(or a >similar processor) but I have, and let me tell you, it would have been great to >have an 8 bit data bus instead of a 16 bit one. Not only would it have meant >fewer (costly at the time) memory chips but also fewer supporting TTL chips and >much less wire wrapping for yours truly. But most of the wiring was needed anyway, becase the address and the data shared the 20-bit bus between the CPU and the memory sub system. The support chips were cheap, and you soon added more memory anyway when IBMs original 16k was not enough. > I'm sure if you took pictures of two >wire-wrapped boards, with the different width data busses, there would be NO >question which was more elegant. Of course IBM used PCBs, but the extra chips >and the extra space for the chips and wiring would have made the boards that >much more expensive. The boards were already huge as-is, IIRC... Yes, I know. By using a smaller bus, IBM got the price of the original PC down to a mere $4995. :-) > >>I still remember my amazement when I once realized that a slow multiply >>instruction was faster than shift and add, because a MUL took long enough that >>the poor 8088 had time to fill its 4(!) byte instruction queue with the next >>sequence. My "optimized" shift and add sequence used instructions that executed >>faster than they could be fetched, and left the processor starved. A definite >>Super kludge. > >Processors today have to wait hundreds of cycles for a main memory access... >what a kludge. So they have 64 or 128 bit buses, and several layers of caches. >Maybe you want to go buy a much more elegant 486? :) I already have a 486/33 in my desk drawer. Not used it much lately though. > >-Tom Bo Persson bop2@telia.com
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