IntelSoftwareDevelopersManual

2 23 system architecture overview 2 24 3 protected

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Unformatted text preview: re with the Pentium® processor. 2.6.8. Loading and Storing the Streaming SIMD Extensions Control/Status Word The LDMXCSR (load Streaming SIMD Extensions control/status word from memory) and STMXCSR (store Streaming SIMD Extensions control/status word to memory) allow the Pentium® III processor’s 32-bit control/status word to be read and written to, respectively. The MXCSR control/status register is used to enable masked/unmasked exception handling, to set rounding modes, to set flush-to-zero mode, and to view exception status flags. For more information on the LDMXCSR and STMXCSR instructions, refer to the Intel Architecture Software Developer’s Manual, Vol 2, for a complete description of these instructions. 2-23 SYSTEM ARCHITECTURE OVERVIEW 2-24 3 Protected-Mode Memory Management PROTECTED-MODE MEMORY MANAGEMENT CHAPTER 3 PROTECTED-MODE MEMORY MANAGEMENT This chapter describes the Intel Architecture’s protected-mode memory management facilities, including the physical memory requirements, the segmentation mechanism, and the paging mechanism. Refer to Chapter 4, Protection for a description of the processor’s protection mechanism. Refer to Chapter 16, 8086 Emulation for a description of memory addressing protection in real-address and virtual-8086 modes. 3.1. MEMORY MANAGEMENT OVERVIEW The memory management facilities of the Intel Architecture are divided into two parts: segmentation and paging. Segmentation provides a mechanism of isolating individual code, data, and stack modules so that multiple programs (or tasks) can run on the same processor without interfering with one another. Paging provides a mechanism for implementing a conventional demand-paged, virtual-memory system where sections of a program’s execution environment are mapped into physical memory as needed. Paging can also be used to provide isolation between multiple tasks. When operating in protected mode, some form of segmentation must be used. There is no mode bit to disable segmentation. The use of paging, however, is optional. These two mechanisms (segmentation and paging) can be configured to support simple singleprogram (or single-task) systems, multitasking systems, or multiple-processor systems that used shared memory. As shown in Figure 3-1, segmentation provides a mechanism for dividing the processor’s addressable memory space (called the linear address space) into smaller protected address spaces called segments. Segments can be used to hold the code, data, and stack for a program or to hold system data structures (such as a TSS or LDT). If more than one program (or task) is running on a processor, each program can be assigned its own set of segments. The processor then enforces the boundaries between these segments and insures that one program does not interfere with the execution of another program by writing into the other program’s segments. The segmentation mechanism also allows typing of segments so that the operations that may be performed on a particular type of segment can be restricted. All of the segments within a system are contained in the processor’s linear address space. To locate a byte in a particular segment, a logical address (sometimes called a far pointer) must be provided. A logical address consists of a segment selector and an offset. The segment selector is a unique identifier for a segment. Among other things it provides an offset into a descriptor table (such as the global descriptor table, GDT) to a data structure called a segment descriptor. Each segment has a segment descriptor, which specifies the size of the segment, the access rights and privilege level for the segment, the segment type, and the location of the first byte of the segment in the linear address space (called the base address of the segment). The offset part of the logical address is added to the base address for the segment to locate a byte within the segment. The base address plus the offset thus forms a linear address in the processor’s linear 3-1 PROTECTED-MODE MEMORY MANAGEMENT address space. Logical Address (or Far Pointer) Segment Selector Offset Linear Address Space Global Descriptor Table (GDT) Segment Segment Descriptor Dir Linear Address Table Offset Physical Address Space Page Phy. Addr. Page Table Page Directory Lin. Addr. Entry Segment Base Address Page Entry Segmentation Paging Figure 3-1. Segmentation and Paging If paging is not used, the linear address space of the processor is mapped directly into the physical address space of processor. The physical address space is defined as the range of addresses that the processor can generate on its address bus. Because multitasking computing systems commonly define a linear address space much larger than it is economically feasible to contain all at once in physical memory, some method of “virtualizing” the linear address space is needed. This virtualization of the linear address space is handled through the processor’s paging mechanism. Paging supports a “virtual...
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