Virtual 8086 Mode Addressing Mechanism; Paging In Virtual Mode; Virtual 8086 Environment Memory Management - Intel Quark SoC X1000 Core Developer's Manual

Figure 48.

Virtual 8086 Environment Memory Management

6.5.2

Virtual 8086 Mode Addressing Mechanism

One of the major differences between Real and Protected Modes is how the segment
selectors are interpreted. When the Intel
Virtual 8086 Mode, the segment registers are used in an identical fashion to Real Mode.
The contents of the segment register are shifted left four bits and added to the offset to
form the segment base linear address.
The Intel
programs use Real Mode and which programs use Protected Mode addressing. Through
the use of paging, the one megabyte address space of the Virtual Mode task can be
mapped to anywhere in the 4-Gbyte linear address space of the Intel
X1000 Core. Like Real Mode, Virtual Mode effective addresses (i.e., segment offsets)
that exceed 64 Kbyte cause an exception 13. However, these restrictions should not
prove to be important, because most tasks running in Virtual 8086 Mode are legacy
application programs.
6.5.3

Paging in Virtual Mode

The paging hardware allows the concurrent running of multiple Virtual Mode tasks, and
provides protection and operating system isolation. Although it is not strictly necessary
to have the paging hardware enabled to run Virtual Mode tasks, it is needed in order to
run multiple Virtual Mode tasks or to relocate the address space of a Virtual Mode task
to physical address space greater than one Mbyte.
®
Intel Quark SoC X1000 Core
Developer's Manual
108
®
Quark SoC X1000 Core allows the operating system to specify which
®
Intel Quark Core—Protected Mode Architecture
®
Quark SoC X1000 Core is executing in
Order Number: 329679-001US
®
Quark SoC
October 2013