Intel IA-32 - Manual (Page 551)

Vol. 3A 15-1
CHAPTER 15
8086 EMULATION
IA-32 processors (beginning with the Intel386 processor) provide two ways to execute new or
legacy programs that are assembled and/or compiled to run on an Intel 8086 processor:
•
Real-address mode.
•
Virtual-8086 mode.
Figure 2-3 shows the relationship of these operating modes to protected mode and system
management mode (SMM).
When the processor is powered up or reset, it is placed in the real-address mode. This operating
mode almost exactly duplicates the execution environment of the Intel 8086 processor, with
some extensions. Virtually any program assembled and/or compiled to run on an Intel 8086
processor will run on an IA-32 processor in this mode.
When running in protected mode, the processor can be switched to virtual-8086 mode to run
8086 programs. This mode also duplicates the execution environment of the Intel 8086
processor, with extensions. In virtual-8086 mode, an 8086 program runs as a separate protected-
mode task. Legacy 8086 programs are thus able to run under an operating system (such as
Microsoft Windows*) that takes advantage of protected mode and to use protected-mode facil-
ities, such as the protected-mode interrupt- and exception-handling facilities. Protected-mode
multitasking permits multiple virtual-8086 mode tasks (with each task running a separate 8086
program) to be run on the processor along with other non-virtual-8086 mode tasks.
This section describes both the basic real-address mode execution environment and the virtual-
8086-mode execution environment, available on the IA-32 processors beginning with the
Intel386 processor.
15.1 REAL-ADDRESS MODE
The IA-32 architecture’s real-address mode runs programs written for the Intel 8086, Intel 8088,
Intel 80186, and Intel 80188 processors, or for the real-address mode of the Intel 286, Intel386,
Intel486, Pentium, P6 family, Pentium 4, and Intel Xeon processors.
The execution environment of the processor in real-address mode is designed to duplicate the
execution environment of the Intel 8086 processor. To an 8086 program, a processor operating
in real-address mode behaves like a high-speed 8086 processor. The principal features of this
architecture are defined in Chapter 3, “Basic Execution Environment”, of the IA-32 Intel®
Architecture Software Developer’s Manual, Volume 1.