Datatech Enterprises Apex 386SX/20 User Manual
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Contents
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6-6 Technical informationMicroprocessorThe 80386SX is a high-performance microprocessor with a 16-bitexternal data path, up to 16 megabytes of directly addressable physi-cal memory and up to 64 terabytes of virtual memory space. Theoperating speed of the 80386SX chip is 10MHz in Normal mode and20MHz in Turbo mode.The 80386SX operates in two modes: protected virtual address modeand real address mode.Protected Virtual Address ModeIn protected mode, software can perform a task switch to enter virtual8086 mode tasks. Each task behaves with 8086 semantics, thusallowing 8086 software (an application program or an entire operationsystem) to execute. The virtual 8086 tasks can be isolated andprotected 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 logicalbase address of up to 32 bits in length. This 32-bit logical base addressis added to the effective address to form a final 32-bit linear address.If paging is disabled, this final linear address reflects physical memoryand is truncated so that only the lower 24 bits of this address are usedto address the 16 megabyte memory address space. if paging isenabled this final linear address reflects a 32-bit address. This istranslated through the paging unit to form a 16-megabyte physicaladdress.Chapter 6: Appendix 17 |
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