Author: Robert Hyatt
Date: 08:00:17 07/07/03
Go up one level in this thread
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. > >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 did "wire one up". In my chess board. And as Bo mentioned, it _was_ a kludge. You had to take the 16 address lines (it had a pure 16 bit address space to start with limiting memory to 64K) and route them to the memory controller to do the addressing. You used the same 16 lines and used 8 of them for the data that came back. Kludge. Kludge. Kludge. Later memory makers added the "software bank select" so that you could with one instruction turn off one bank and turn on another bank, usually 32 K bytes at a time. But with two banks, wouldn't it be nice to read 8 bytes from each and to 2x the memory bandwidth. But not with the 8088 kludge. > >>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? :) Not the same thing. More like building a two-lane highway between two cities, but in one direction, both lanes can be used, in the other direction, only one lane can be used. That's a kludge, and a dangerous one to boot. And very similar to the 8088/80c88/etc kludge. The 8088 was simply a kludged-up 8086 cobbled together to use old 8-bit memory modules. Fortunately it didn't last very long. And not all PCs used it. > >-Tom
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