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I know in these days this is very stupid question, but for study purposes. I read that when PC starts, CPU is set to read adress just under the 4GB. There should BIOS be mapped to by memory controller.

My question is, in old days, had disabling BIOS shadowing actually freed some RAM for you? I mean, even when BIOS was not shadowed to RAM directly, still adresses for BIOS MMIO access were wasted. And when you cant adress it, its like there is no extra space gained.

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I don't think so.

These areas are overlapping, switching to RAM instead of ROM just gives faster access to this code.

This is done in order to improve computer performance

DOS worked in Real CPU Mode, today's OSes are using Protected mode, so memory management is different

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