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Aucbvax.1945 fa.works utzoo!duke!decvax!ucbvax!DP@MIT-ML Fri Jun 26 05:28:30 1981 Address space vs. physical memory. DP@MIT-ML 06/26/81 01:26:23 Re: Address space vs. physical memory. To: WorkS at MIT-AI

The two are not equal. It is perfectly reasonable to have far more of one than you have the other (and there are examples of both in common use). There are problems that cannot be made to fit in the small address space of a 16 bit machine. But wait there is overlaying, certainly that can solve things. Unfortunately there comes a time when the largest branch of the overlay tree just wont fit in the address space. This happend at DEC with the Bliss compiler for the pdp-11. It to this day remains a cross compiler, because no matter how clever they were with overlays something or other just wouldn't fit.

The only thing that mapping and large physical memory give you with a small address space, is speed it saves you reading overlay segments (or users in a multi user system) in and out of memory from the disk.

BTW, even large systems are not immune. There are people complaining that their problems wont fit into the 16mb of the 370. (even with the crockish region scheme that ibm built (memory mapping, but done painfully I am told)). All these new machines with huge (4gb) address spaces are claimed to satisfy medium range requirements. It is apparently expected that in 8 - 10 years or so, 32 bits wont be enough.

Jeff

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