SPECIAL PENTIUM PRO FEATURES
The Pentium Pro is essentially the same microprocessor as the 80386, 80486, and Pentium, except that some additional features and changes to the control register set have occurred. This section highlights the differences between the 80386 control register structure and the Pentium Pro control register.
Control Register 4
Figure 18–17 shows control register 4 of the Pentium Pro microprocessor. Notice that CR4 has two new control bits that are added to the control register array.
This section of the text explains only the two new Pentium Pro components in the control register 4. (Refer to Figure 18–8 for a description and illustration of the Pentium control registers.) Following is a description of the Pentium CR4 bits and the new Pentium Pro control bits in control register CRM4:
VME Virtual mode extension enables support for the virtual interrupt flag in protected mode. If VME = 0, virtual interrupt support is disabled.
PVI Protected mode virtual interrupt enables support for the virtual interrupt flag in protected mode.
TSD Time stamp disable controls the RDTSC instruction.
DE Debugging extension enables I/O breakpoint debugging extensions when set.
PSE Page size extension enables 4M-byte memory pages when set in the Pentium, or 2M-byte pages when set in the Pentium Pro whenever PSE is also set.
PAE Page address extension enables address lines A35–A32 whenever a special new addressing mode, controlled by PGE, is enabled for the Pentium Pro.
MCE Machine check enable enables the machine checking interrupt.
PGE Page extension controls the new, larger 64G addressing mode whenever it is set along with PAE and PSE.
