Author: Tom Kerrigan
Date: 16:44:31 07/06/03
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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. I'm not sure if you've ever wired up a board with one of these processors (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. 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... >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. Maybe you want to go buy a much more elegant 486? :) -Tom
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