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Bus Width
The 80386 microprocessor supports two types of accesses: Memory,
and Input/Output. Each type of access can be 32, 24, 16, or 8 bits wide.
Memory and I/O devices can have paths 32, 16, or 8 bits wide. Your
mainboard allows any type of access to a device of any width. If
necessary, the hardware can break up a 80386 cycle into the required
number of cycles(up to 3218 = 4) to allow access to a 16 or 8 bit device.
All the onboard memory devices except the EPROM which contains
the BIOS are organized into a 32-bit wide memory. These include the
DRAM and the high-speed cache memory.
Memory Subsystem
In the IBM PC-AT, conventional memory or base memory extends from
0 to 640KB. This is the user area, and is available for use by application
software.
Physical memory address space from 640KB to 1MB is reserved for
the system.
DOS can recognize and use the memory area from 0 to 1MB only.
Refer to the figure for the memory map on page 41.
One way of overcoming the 640KB barrier is by using expanded
memory. This requires the use of additional bank-switched physical
memory (memory organized in banks which can individually be
switched on or off) along with LIM Expanded Memory Specification
(EMS) compatible Expanded Memory Manager (EMM) software and
an application program that is capable of working with the EMM
software.
The EMM software first finds a 64KB page frame in the unused part of
system memory, divides the frame into four 16KB windows and swaps
in four 16KB pages from different areas of the additional physical
memory. The Additional page memory used along with an EMS
emulator is known as Expanded Memory.
Applications programs (Netware and the XENIX operating system) can
use physical memory beyond 1MB without the EMM manager. This
additional memory is referred to as Extended Memory.
Your mainboard can have up to 8MB DRAM onboard with different
types of DRAM in various configurations (refer to the Configuration
section). If necessary, the PEI-306 32-bit memory board can ac-
comodate up to 16MB of system memory.
Chapter 6: Appendix
29
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