6-6 Technical Information; Microprocessor; Protected Virtual Address Mode - Datatech Enterprises Apex 386SX/20 User Manual

6-6 Technical information

Microprocessor

The 80386SX is a high-performance microprocessor with a 16-bit
external data path, up to 16 megabytes of directly addressable physi-
cal memory and up to 64 terabytes of virtual memory space. The
operating speed of the 80386SX chip is 10MHz in Normal mode and
20MHz in Turbo mode.
The 80386SX operates in two modes: protected virtual address mode
and real address mode.

Protected Virtual Address Mode

In protected mode, software can perform a task switch to enter virtual
8086 mode tasks. Each task behaves with 8086 semantics, thus
allowing 8086 software (an application program or an entire operation
system) to execute. The virtual 8086 tasks can be isolated and
protected from one another and the host 386SX microprocessor opera-
tion system by use of paging.
Protected mode will use one of two different address spaces, depend-
ing on whether or not paging is enabled. Every selector has a logical
base address of up to 32 bits in length. This 32-bit logical base address
is added to the effective address to form a final 32-bit linear address.
If paging is disabled, this final linear address reflects physical memory
and is truncated so that only the lower 24 bits of this address are used
to address the 16 megabyte memory address space. if paging is
enabled this final linear address reflects a 32-bit address. This is
translated through the paging unit to form
address.
16-megabyte physical
a
Chapter 6: Appendix 17