This preview shows page 1. Sign up to view the full content.
Unformatted text preview: t result in the destination general-purpose register. 32-bit operands generate a 32-bit result, zero-extended to a 64-bit result in the destination general-purpose register. 8-bit and 16-bit operands generate an 8-bit or 16-bit result. The upper 56 bits or 48 bits (respectively) of the destination general-purpose register are not be modified by the operation. If the result of an 8-bit or 16-bit operation is intended for 64-bit address calculation, explicitly sign-extend the register to the full 64-bits. Because the upper 32 bits of 64-bit general-purpose registers are undefined in 32-bit modes, the upper 32 bits of any general-purpose register are not preserved when switching from 64-bit mode to a 32-bit mode (to protected mode or compatibility mode). Software must not depend on these bits to maintain a value after a 64-bit to 32-bit mode switch. 3.4.2 Segment Registers The segment registers (CS, DS, SS, ES, FS, and GS) hold 16-bit segment selectors. A segment selector is a special pointer that identifies a segment in memory. To access a particular segment in memory, the segment selector for that segment must be present in the appropriate segment register. Vol. 1 3-17 BASIC EXECUTION ENVIRONMENT When writing application code, programmers generally create segment selectors with assembler directives and symbols. The assembler and other tools then create the actual segment selector values associated with these directives and symbols. If writing system code, programmers may need to create segment selectors directly. See Chapter 3, "Protected-Mode Memory Management," in the Intel 64 and IA-32 Architectures Software Developer's Manual, Volume 3A. How segment registers are used depends on the type of memory management model that the operating system or executive is using. When using the flat (unsegmented) memory model, segment registers are loaded with segment selectors that point to overlapping segments, each of which begins at address 0 of the linear address space (see Figure 3-6). These overlapping segments then comprise the linear address space for the program. Typically, two overlapping segments are defined: one for code and another for data and stacks. The CS segment register points to the code segment and all the other segment registers point to the data and stack segment. When using the segmented memory model, each segment register is ordinarily loaded with a different segment selector so that each segment register points to a different segment within the linear address space (see Figure 3-7). At any time, a program can thus access up to six segments in the linear address space. To access a segment not pointed to by one of the segment registers, a program must first load the segment selector for the segment to be accessed into a segment register. Linear Address Space for Program Segment Registers CS DS SS ES FS GS The segment selector in each segment register points to an overlapping segment in the linear address space. Overlapping Segmen...
View Full Document
This note was uploaded on 10/01/2013 for the course CPE 103 taught by Professor Watlins during the Winter '11 term at Mississippi State.
- Winter '11