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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 16.1</TITLE></HEAD><BODY><B>up:</B> <A HREF="c16.htm">Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR><B>prev:</B> <A HREF="c16.htm">Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR><B>next:</B> <A HREF="s16_02.htm">16.2 Mixing 32-Bit and 16-Bit Operations</A><P><HR><P><H1>16.1 How the 80386 Implements 16-Bit and 32-Bit Features</H1>The features of the architecture that permit the 80386 to work equally wellwith 32-bit and 16-bit address and operand sizes include:<UL><LI> The D-bit (default bit) of code-segment descriptors, which determinesthe default choice of operand-size and address-size for theinstructions of a code segment. (In real-address mode and V86 mode,which do not use descriptors, the default is 16 bits.) A code segmentwhose D-bit is set is known as a USE32 segment; a code segment whoseD-bit is zero is a USE16 segment. The D-bit eliminates the need toencode the operand size and address size in instructions when allinstructions use operands and effective addresses of the same size.<LI> Instruction prefixes that explicitly override the default choice ofoperand size and address size (available in protected mode as well asin real-address mode and V86 mode).<LI> Separate 32-bit and 16-bit gates for intersegment control transfers(including call gates, interrupt gates, and trap gates). The operandsize for the control transfer is determined by the type of gate, not bythe D-bit or prefix of the transfer instruction.<LI> Registers that can be used both for 32-bit and 16-bit operands andeffective-address calculations.<LI> The B-bit (big bit) of data-segment descriptors, which determines thesize of stack pointer (32-bit ESP or 16-bit SP) used by the CPU forimplicit stack references.</UL><P><HR><P><B>up:</B> <A HREF="c16.htm">Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR><B>prev:</B><A HREF="c16.htm">Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR><B>next:</B> <A HREF="s16_02.htm">16.2 Mixing 32-Bit and 16-Bit Operations</A></BODY>?? 快捷鍵說明
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