📄 s13_01.htm
字号:
<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 13.1</TITLE></HEAD><BODY><B>up:</B> <A HREF="c13.htm">Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR><B>prev:</B> <A HREF="c13.htm">Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR> <B>next:</B> <A HREF="s13_02.htm">13.2 Two ways to Execute 80286 Tasks</A><P><HR><P><H1>13.1 80286 Code Executes as a Subset of the 80386</H1>In general, programs designed for execution in protected mode on an 80286execute without modification on the 80386, because the features of the 80286are a subset of those of the 80386.<P>All the descriptors used by the 80286 are supported by the 80386 as long asthe Intel-reserved word (last word) of the 80286 descriptor is zero.<P>The descriptors for data segments, executable segments, local descriptortables, and task gates are common to both the 80286 and the 80386. Other80286 -- segment, call gate, interrupt gate, and trap-- supported by the 80386. The 80386 also has new versions ofdescriptors for TSS segment, call gate, interrupt gate, and trap gate thatsupport the 32-bit nature of the 80386. Both sets of descriptors can beused simultaneously in the same system.<P>For those descriptors that are common to both the 80286 and the 80386, thepresence of zeros in the final word causes the 80386 to interpret thesedescriptors exactly as 80286 does; for example:<DL><DT>Base Address <DD>The high-order eight bits of the 32-bit base address arezero, limiting base addresses to 24 bits.<DT>Limit <DD>The high-order four bits of the limit field are zero,restricting the value of the limit field to 64K.<DT>Granularity bit <DD>The granularity bit is zero, which implies that the valueof the 16-bit limit is interpreted in units of one byte.<DT>B-bit <DD>In a data-segment descriptor, the B-bit is zero, implyingthat the segment is no larger than 64 Kbytes.<DT>D-bit <DD>In an executable-segment descriptor, the D-bit is zero,implying that 16-bit addressing and operands are thedefault.</DL>For formats of these descriptors and documentation of their use refer tothe iAPX 286 Programmer's Reference Manual.<P><HR><P><B>up:</B> <A HREF="c13.htm">Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR><B>prev:</B><A HREF="c13.htm">Chapter 13 -- Executing 80286 Protected-Mode Code</A><BR> <B>next:</B> <A HREF="s13_02.htm">13.2 Two ways to Execute 80286 Tasks</A></BODY>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -