hw/i386.reference/s16_01.htm

   1 <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN">
   2 <HTML>
   3 <HEAD>
   4 <TITLE>80386 Programmer's Reference Manual -- Section 16.1</TITLE>
   5 </HEAD>
   6 <BODY>
   7 <B>up:</B> <A HREF="c16.htm">
   8 Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR>
   9 <B>prev:</B>
  10 <A HREF="c16.htm">Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR>
  11 <B>next:</B> <A HREF="s16_02.htm">16.2  Mixing 32-Bit and 16-Bit Operations</A>
  12 <P>
  13 <HR>
  14 <P>
  15 <H1>16.1  How the 80386 Implements 16-Bit and 32-Bit Features</H1>
  16 The features of the architecture that permit the 80386 to work equally well
  17 with 32-bit and 16-bit address and operand sizes include:
  18 <UL>
  19 <LI> The D-bit (default bit) of code-segment descriptors, which determines
  20 the default choice of operand-size and address-size for the
  21 instructions of a code segment. (In real-address mode and V86 mode,
  22 which do not use descriptors, the default is 16 bits.) A code segment
  23 whose D-bit is set is known as a USE32 segment; a code segment whose
  24 D-bit is zero is a USE16 segment. The D-bit eliminates the need to
  25 encode the operand size and address size in instructions when all
  26 instructions use operands and effective addresses of the same size.
  27 <LI> Instruction prefixes that explicitly override the default choice of
  28 operand size and address size (available in protected mode as well as
  29 in real-address mode and V86 mode).
  30 <LI> Separate 32-bit and 16-bit gates for intersegment control transfers
  31 (including call gates, interrupt gates, and trap gates). The operand
  32 size for the control transfer is determined by the type of gate, not by
  33 the D-bit or prefix of the transfer instruction.
  34 <LI> Registers that can be used both for 32-bit and 16-bit operands and
  35 effective-address calculations.
  36 <LI> The B-bit (big bit) of data-segment descriptors, which determines the
  37 size of stack pointer (32-bit ESP or 16-bit SP) used by the CPU for
  38 implicit stack references.
  39 </UL>
  40 <P>
  41 <HR>
  42 <P>
  43 <B>up:</B> <A HREF="c16.htm">
  44 Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR>
  45 <B>prev:</B><A HREF="c16.htm">
  46 Chapter 16 -- Mixing 16-Bit and 32 Bit Code</A><BR>
  47 <B>next:</B> <A HREF="s16_02.htm">16.2  Mixing 32-Bit and 16-Bit Operations</A>
  48 </BODY>