Epson Apex 386/33 Canadian Product User Manual - Page 148
Bus Width, Memory Subsystem
View all Epson Apex 386/33 Canadian Product manuals
Add to My Manuals
Save this manual to your list of manuals |
Page 148 highlights
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 accomodate up to 16MB of system memory. Chapter 6: Appendix 29