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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 17.1</TITLE></HEAD><BODY><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>next:</B> <A HREF="s17_02.htm">17.2 Instruction Format</A><P><HR><P><H1>17.1 Operand-Size and Address-Size Attributes</H1>When executing an instruction, the 80386 can address memory using either 16or 32-bit addresses. Consequently, each instruction that uses memoryaddresses has associated with it an address-size attribute of either 16 or32 bits. 16-bit addresses imply both the use of a 16-bit displacement inthe instruction and the generation of a 16-bit address offset (segmentrelative address) as the result of the effective address calculation.32-bit addresses imply the use of a 32-bit displacement and the generationof a 32-bit address offset. Similarly, an instruction that accesses words(16 bits) or doublewords (32 bits) has an operand-size attribute of either16 or 32 bits.<P>The attributes are determined by a combination of defaults, instructionprefixes, and (for programs executing in protected mode) size-specificationbits in segment descriptors.<H2>17.1.1 Default Segment Attribute</H2>For programs executed in protected mode, the D-bit in executable-segmentdescriptors determines the default attribute for both address size andoperand size. These default attributes apply to the execution of allinstructions in the segment. A value of zero in the D-bit sets the defaultaddress size and operand size to 16 bits; a value of one, to 32 bits.<P>Programs that execute in real mode or virtual-8086 mode have 16-bitaddresses and operands by default.<H2>17.1.2 Operand-Size and Address-Size Instruction Prefixes</H2>The internal encoding of an instruction can include two byte-long prefixes:the address-size prefix, 67H, and the operand-size prefix, 66H. (A latersection, "Instruction Format," shows the position of the prefixes in aninstruction's encoding.) These prefixes override the default segmentattributes for the instruction that follows. Table 17-1 shows the effect ofeach possible combination of defaults and overrides.<H2>17.1.3 Address-Size Attribute for Stack</H2>Instructions that use the stack implicitly (for example: <A HREF="POP.htm">POP</A> EAX also havea stack address-size attribute of either 16 or 32 bits. Instructions with astack address-size attribute of 16 use the 16-bit SP stack pointer register;instructions with a stack address-size attribute of 32 bits use the 32-bitESP register to form the address of the top of the stack.<P>The stack address-size attribute is controlled by the B-bit of thedata-segment descriptor in the SS register. A value of zero in the B-bitselects a stack address-size attribute of 16; a value of one selects a stackaddress-size attribute of 32.<PRE>Table 17-1. Effective Size AttributesSegment Default D = ... 0 0 0 0 1 1 1 1Operand-Size Prefix 66H N N Y Y N N Y YAddress-Size Prefix 67H N Y N Y N Y N YEffective Operand Size 16 16 32 32 32 32 16 16Effective Address Size 16 32 16 32 32 16 32 16Y = Yes, this instruction prefix is presentN = No, this instruction prefix is not present</PRE><P><HR><P><B>up:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>prev:</B> <A HREF="c17.htm">Chapter 17 -- 80386 Instruction Set</A><BR><B>next:</B> <A HREF="s17_02.htm">17.2 Instruction Format</A></BODY>⌨️ 快捷键说明
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