AMD AMD5K86 - Virtual-8086 Mode Extensions (VME)

AMD~
AMD5J!16 Processor Technical Reference Manual
18524BjO-Mar1996
3.1.4 Virtual-8086 Mode Extensions (VME)
Interrupt Redirection
in Virtual-8086 Mode
WithoutVME
Extensions
3-12
The Virtual-8086 Mode Extensions (VME) bit in CR4 (bit 0)
enable performance enhancements for 8086 programs running
as protected tasks in Virtual-8086 mode. These extensions
include:
• Virtualizing mask able external interrupt control and notifi-
cation via the VIF and VIP bits in EFLAGS
• Selectively intercepting software interrupts (INTn instruc-
tions) via the Interrupt Redirection Bitmap (IRB) in the
Task State Segment (TSS)
8086
programs expect to have full access to the interrupt flag
(IF) in the EFLAGS register, which enables maskable external
interrupts via the INTR signal. When 8086 programs run in Vir-
tual-8086 mode on a 386 or 486 processor, they run as pro-
tected tasks and access to the IF flag must be controlled by the
operating system on a task-by-task basis to prevent corruption
of system resources.
Without the VME extensions available on the AMDS
K
86 pro-
cessor, the operating system controls Virtual-8086 mode access
to the IF flag by trapping instructions that can read or write
this flag. These instructions include ST!, CLI, PUSHF, POPF,
INTn, and IRET. This method prevents changes to the real IF
when the I/O privilege level (IOPL) in EFLAGS is less than 3,
the privilege level at which all Virtual-8086 tasks run. The
operating system maintains an image of the IF flag for each
Virtual-8086 program by emulating the instructions that read
or write IF. When an external mask able interrupt occurs, the
operating system checks the state of the IF image for the cur-
rent Virtual-8086 program to determine whether the program
is allowing interrupts. If the program has disabled interrupts,
the operating system saves the interrupt information until the
program attempts to re-enable interrupts.
The overhead for trapping and emulating the instructions that
enable and disable interrupts, and the maintenance of virtual
interrupt flags for each Virtual-8086 program, can degrade the
processor's performance. This performance can be regained by
running Virtual-8086 programs with IOPL set to 3, thus allow-
ing changes to the real IF flag from any privilege level, but
with a loss in protection.
Software Environment and Extensions