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memorybootbios

0xffff0 and the BIOS

When a pc first boots up, it starts executing at physical address 0xffff0. This address contains a jmp instruction to the BIOS.

Now for my question, I always assume the physical addresses are mapped to RAM. If RAM initially contains garbage values, what exactly puts the jmp instruction in 0xffff0? Is the jmp instruction always the same or is it different for different BIOS's? Does 0xffff0 map from RAM to BIOS then (meaning it's "hard mapped")?

Solution

  • The top 64kB or so are mapped to BIOS ROM, not RAM.