AMD K5 User Manual

3-12 Software Environment and Extensions
AMD-K5 Processor Technical Reference Manual 18524C/0—Nov1996
3.1.4 Virtual-8086 Mode Extensions (VME)
The Virtual-8086 Mode Extensions (VME) bit in CR4 (bit 0)
enables performance enhancements for 8086 programs run-
ning as protected tasks in Virtual-8086 mode. These extensions
include:
■ Virtualizing maskable 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)
Interrupt Redirection
in Virtual-8086 Mode
Without VME
Extensions
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 AMD-K5 proces-
sor, 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 STI, 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 operat-
ing 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 maskable interrupt occurs, the operating
system checks the state of the IF image for the current Virtual-
8086 program to determine whether the program is allowing
interrupts. If the program has disabled interrupts, the operat-
ing 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.

AMD K5 Specifications

General

Manufacturer	AMD
Model	K5
Architecture	x86
Microarchitecture	K5
Introduction Year	1996
Clock Speed	75 - 133 MHz
Core Count	1
Socket	Socket 7
Core stepping	SSA/5, 5k86
Voltage	3.3V
Transistors	4.3 million
L1 Cache	8 KB (data) + 16 KB (instruction)
FSB	50 MHz to 66 MHz
Process Technology	350 nm