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15.5 Virtual I/O

Many 8086 programs that were designed to execute on single-task systems use I/O devices directly. However, when these same programs are executed in a multitasking environment, such use of devices can be disruptive. The 80386 provides sufficient flexibility to control I/O in a manner that both suits the needs of the new environment and is transparent to the 8086 program. Designers may take any of several possible approaches to controlling I/O:
  • Implement or emulate the 8086 operating system as an 80386 program and require the 8086 application to do I/O via software interrupts to the operating system, trapping all attempts to do I/O directly.
  • Let the 8086 program take complete control of all I/O.
  • Selectively trap and emulate references that a task makes to specific I/O ports.
  • Trap or redirect references to memory-mapped I/O addresses.
The method of controlling I/O depends upon whether I/O ports are I/O mapped or memory mapped.

15.5.1 I/O-Mapped I/O

I/O-mapped I/O in V86 mode differs from protected mode only in that the protection mechanism does not consult IOPL when executing the I/O instructions IN, INS, OUT, OUTS. Only the I/O permission bit map controls the right for V86 tasks to execute these I/O instructions.

The I/O permission map traps I/O instructions selectively depending on the I/O addresses to which they refer. The I/O permission bit map of each V86 task determines which I/O addresses are trapped for that task. Because each task may have a different I/O permission bit map, the addresses trapped for one task may be different from those trapped for others . Refer to Chapter 8 for more information about the I/O permission map.

15.5.2 Memory-Mapped I/O

In hardware designs that utilize memory-mapped I/O, the paging facilities of the 80386 can be used to trap or redirect I/O operations. Each task that executes memory-mapped I/O must have a page (or pages) for the memory-mapped address space. The V86 monitor may control memory-mapped I/O by any of these means:
  • Assign the memory-mapped page to appropriate physical addresses. Different tasks may have different physical addresses, thereby preventing the tasks from interfering with each other.
  • Cause a trap to the monitor by forcing a page fault on the memory-mapped page. Read-only pages trap writes. Not-present pages trap both reads and writes.
Intervention for every I/O might be excessive for some kinds of I/O devices. A page fault can still be used in this case to cause intervention on the first I/O operation. The monitor can then at least make sure that the task has exclusive access to the device. Then the monitor can change the page status to present and read/write, allowing subsequent I/O to proceed at full speed.

15.5.3 Special I/O Buffers

Buffers of intelligent controllers (for example, a bit-mapped graphics buffer) can also be virtualized via page mapping. The linear space for the buffer can be mapped to a different physical space for each virtual 8086 task. The V86 monitor can then assume responsibility for spooling the data or assigning the virtual buffer to the real buffer at appropriate times.

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