Operating Modes And Compatibility; Memory Management - Intel Embedded Intel486 Hardware Reference Manual

2.2.1

Operating Modes and Compatibility

The Intel486 processor can run in modes that give it object-code compatibility with software writ-
ten for the 8086, 80286, and Intel386 processor families. The operating mode is set in software
as one of the following:
• Real Mode: When the processor is powered up or reset, it is initialized in Real Mode. This
mode has the same base architecture as the 8086 processor but allows access to the 32-bit
register set of the Intel486 processor. The address mechanism, maximum memory size
(1 Mbyte), and interrupt handling are identical to the Real Mode of the 80286 processor.
Nearly all Intel486 processor instructions are available, but the default operand size is 16
bits; in order to use the 32-bit registers and addressing modes, override instruction prefixes
must be used. The primary purpose of Real Mode is to set up the processor for Protected
Mode operation.
• Protected Mode (also called Protected Virtual Address Mode): The complete capabilities of
the Intel486 processor become available when programs are run in Protected Mode. In
addition to segmentation protection, paging can be used in Protected Mode. The linear
address space is four gigabytes and virtual memory programs of up to 64 terabytes can be
run. All existing 8086, 80286, and Intel386 processor software can be run under the
Intel486 processor's hardware-assisted protection mechanism. The addressing mechanism
is more sophisticated in Protected Mode than in Real Mode.
• Virtual 8086 Mode, a sub-mode of Protected Mode, allows 8086 programs to be run with
the segmentation and paging protection mechanisms of Protected Mode. This mode offers
more flexibility than the Real Mode for running 8086 programs. Using this mode, the
Intel486 processor can execute 8086 operating systems and applications simultaneously
with an Intel486 operating system and both 80286 and Intel486 processor applications.
The hardware offers additional modes, which are described in greater detail in the Embedded
Intel486™ Processor Family Developer's Manual.
2.2.2

Memory Management

The memory management unit supports both segmentation and paging. Segmentation provides
several independent, protected address spaces. This security feature limits the damage a program
error can cause. For example, a program's stack space should be prevented from growing into its
code space. The segmentation unit maps the separate address spaces seen by programmers into
one unsegmented, linear address space.
Paging provides access to data structures larger than the available memory space by keeping them
partly in memory and partly on disk. Paging breaks the linear address space into units of 4 Kbytes
called pages. When a program makes its first reference to a page, the program can be stopped, the
new page copied from disk, and the program restarted. Programs tend to use only a few pages at
a time, so a processor with paging can simulate a large address space in RAM using a small
amount of RAM plus storage on a disk.
INTRODUCTION
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