Why is address space allocation granularity 64K?

RE: Why is address space allocation granularity 64K?

Jordan Russell — Wed, 08 Oct 2003 23:58:00 GMT

Very interesting. But why, though, do the x86 versions of Windows have to use 64K granularity? Is it just for consistency?

RE: Why is address space allocation granularity 64K?

Peter Lund — Sun, 12 Oct 2003 18:42:00 GMT

Couldn't that have been solved by using a global offset table and a global pointer like they did in Digital Unix (OSF/1) ? Instead of constructing 32-bit values piece by piece with lda/ldah (that adds the 16-bit immediate field in the 32-bit instruction to a register) you could "just" load it from a table in memory.

RE: Why is address space allocation granularity 64K?

Raymond Chen — Mon, 13 Oct 2003 19:44:00 GMT

Yes, it could've been done that way, and in fact the MIPS did work that way. MIPS support actually predated Alpha support, and the 64K granularity existed even on the MIPS but for different reasons. (Or so I've been told by people who were alive back then.) The granularity exists even on the x86 because NT is a single code base for all architectures. You don't want to introduce gratuitous differences between architectures because it makes porting code between them harder. (Somebody will write code on x86 that assumes 4K granularity and run it on an Alpha and it will crash - or vice versa - and then they go nuts trying to figure out what went wrong, until they stumble across a small sentence buried somewhere in MSDN and then curse Microsoft for making gratuitous differences between their platforms.)

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