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

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VM86 Extensions

The VME feature was first implemented in the Pentium® processor. It was migrated backwards into the later versions of the 486 and is present in all IA32 processor subsequent to the Pentium®.

Introduction

The chapter entitled “Virtual 8086 Mode” on page 329 provided a detailed description of VM86 Mode as implemented on the 386 processor. VM86 Mode operation on the early versions of the 486 was identical to operation on the 386. The Pentium® processor introduced some improvements to VM86 Mode. Whether or not these improvements are activated is controlled by CR4[VME] (VM86 Mode Extensions; see Figure 21-1 on page 491):

  • When CR4[VME] = 0, an IA32 processor's VM86 Mode is 100% compatible with the 386 version of VM86 Mode.

  • If the OS ...

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