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The Unabridged Pentium 4 IA32 Processor Genealogy by Bob Colwell, Tom Shanley

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An Overview of the 386DX FSB

Figure 5-2 on page 44 illustrates the address- and data-related signals on the 386DX processor's FSB. Although the 386 processor implemented a full 32-bit internal address bus, the two least-significant address lines, A[1:0], were not implemented as output pins on the FSB.

Figure 5-2. 386DX FSB

Address Bus Selects Dword

Whenever the processor initiated a transaction on the FSB, logic external to the processor behaved as if the least-significant two address lines are always zero. As a result, the processor could only output addresses divisible by four. As an example, it could address location 00000100h, but not 00000101h, ...

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