The IO Address Range
The IO address range supported by the Pentium® 4 processor is from 000000000h through 000010002h (the overall range is 64KB+3 in size). This is backward-compatible with previous x86 processors. Consider the following:
A 2-byte IO access starting at IO address FFFFh. In this case, the 2-bytes of data straddles the 64KB address boundary. Since these two bytes reside in different qwords, the processor would perform this as two separate single-qword transactions.
A 4-byte IO access starting at IO address FFFFh, FFFEh, or FFFDh. As before, the target dword straddles the 64KB address boundary, and the processor would perform this as two separate single-qword transactions.
In both cases, when accessing above the 64KB boundary, the ...
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