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<!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"><HTML><HEAD><TITLE>80386 Programmer's Reference Manual -- Section 15.5</TITLE></HEAD><BODY><B>up:</B> <A HREF="c15.htm">Chapter 15 -- Virtual 8086 Mode</A><BR><B>prev:</B> <A HREF="s15_04.htm">15.4 Additional Sensitive Instructions</A><BR><B>next:</B> <A HREF="s15_06.htm">15.6 Differences From 8086</A><P><HR><P><H1>15.5 Virtual I/O</H1>Many 8086 programs that were designed to execute on single-task systems useI/O devices directly. However, when these same programs are executed in amultitasking environment, such use of devices can be disruptive. The 80386provides sufficient flexibility to control I/O in a manner that both suitsthe needs of the new environment and is transparent to the 8086 program.Designers may take any of several possible approaches to controlling I/O:<UL><LI> Implement or emulate the 8086 operating system as an 80386 program andrequire the 8086 application to do I/O via software interrupts to theoperating system, trapping all attempts to do I/O directly.<LI> Let the 8086 program take complete control of all I/O.<LI> Selectively trap and emulate references that a task makes to specificI/O ports.<LI> Trap or redirect references to memory-mapped I/O addresses.</UL>The method of controlling I/O depends upon whether I/O ports are I/O mappedor memory mapped.<H2>15.5.1 I/O-Mapped I/O</H2>I/O-mapped I/O in V86 mode differs from protected mode only in that theprotection mechanism does not consult IOPL when executing the I/Oinstructions <A HREF="IN.htm">IN</A>, <A HREF="INS.htm">INS</A>, <A HREF="OUT.htm">OUT</A>, <A HREF="OUTS.htm">OUTS</A>. Only the I/O permission bit map controlsthe right for V86 tasks to execute these I/O instructions.<P>The I/O permission map traps I/O instructions selectively depending on theI/O addresses to which they refer. The I/O permission bit map of each V86task determines which I/O addresses are trapped for that task. Because eachtask may have a different I/O permission bit map, the addresses trapped forone task may be different from those trapped for others . Refer to <A HREF="c08.htm">Chapter 8</A> for more information about the I/O permission map.<H2>15.5.2 Memory-Mapped I/O</H2>In hardware designs that utilize memory-mapped I/O, the paging facilitiesof the 80386 can be used to trap or redirect I/O operations. Each task thatexecutes memory-mapped I/O must have a page (or pages) for the memory-mappedaddress space. The V86 monitor may control memory-mapped I/O by any ofthese means:<UL><LI> Assign the memory-mapped page to appropriate physical addresses.Different tasks may have different physical addresses, therebypreventing the tasks from interfering with each other.<LI> Cause a trap to the monitor by forcing a page fault on thememory-mapped page. Read-only pages trap writes. Not-present pages trapboth reads and writes.</UL>Intervention for every I/O might be excessive for some kinds of I/Odevices. A page fault can still be used in this case to cause interventionon the first I/O operation. The monitor can then at least make sure that thetask has exclusive access to the device. Then the monitor can change thepage status to present and read/write, allowing subsequent I/O to proceed atfull speed.<H2>15.5.3 Special I/O Buffers</H2>Buffers of intelligent controllers (for example, a bit-mapped graphicsbuffer) can also be virtualized via page mapping. The linear space for thebuffer can be mapped to a different physical space for each virtual 8086task. The V86 monitor can then assume responsibility for spooling the dataor assigning the virtual buffer to the real buffer at appropriate times.<P><HR><P><B>up:</B> <A HREF="c15.htm">Chapter 15 -- Virtual 8086 Mode</A><BR><B>prev:</B> <A HREF="s15_04.htm">15.4 Additional Sensitive Instructions</A><BR><B>next:</B> <A HREF="s15_06.htm">15.6 Differences From 8086</A></BODY>⌨️ 快捷键说明
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