The Unabridged Pentium 4 IA32 Processor Genealogy

The Virtual Machine Monitor (VMM)

When the processor's internal hardware associated with VM86 mode detects that the currently executing DOS task is attempting a potentially disruptive action, it suspends the VM86 task and jumps to the GP (General Protection) exception handler. As with any exception, before jumping to the exception handler, the processor first stores the current EFlags register contents (along with CS and EIP) on the stack. It then clears the EFlags[VM] bit, disabling VM86 mode. Upon entry to the GP exception handler, the programmer examines the VM bit in the EFlags image stored on the stack to determine if the exception was generated by a DOS task (i.e., EFlags[VM] = 1). If it was, the GP exception handler jumps to the watchdog ...