Epson Apex 386/40 Canadian Product User Manual - Page 103

Memory Subsystem

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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 sofwares. Physical memory address space from 640KB to 1MB is reserved for the system. DOS can recognize and use the memory area from 0 to 1 MB only. 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 diferent 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. The mainboard can have up to 8MB DRAM onboard with different types of DRAM in various configurations. 10 Chapter 6: Appendix

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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
sofwares.
Physical memory address space from 640KB to 1MB is reserved for
the system.
DOS can recognize and use the memory area from 0 to 1 MB only.
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 diferent 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.
The mainboard can have up to 8MB DRAM onboard with different
types of DRAM in various configurations.
10
Chapter 6: Appendix