Intel 80386 manual Architectural Compatibility

ARCHITECTURAL COMPATIBILITY

Because the address space of an 8086 is one megabyte, the logical addresses generated by a Virtual 86 Mode task fall into the first megabyte of the 80386 linear address space. Multiple Virtual 86 Mode tasks could interfere with each other, since they would all share the low megabyte of the linear address space. An operating system can use 80386 paging to relocate the linear address spaces of Virtual 86 Mode tasks to different areas of the physical address space. Using paging in this way not only prevents interference among Virtual 86 Mode tasks, but enables a virtual memory operating system to swap the pages of Virtual 86 Mode tasks just as if they were 80386 tasks.

A Virtual 86 Mode task may execute a program that was written for execution on a single-task personal computer. Such a program can contain instructions that are potentially disruptive if executed in a multitasking environment. For example, allowing a Virtual 86 Mode task to execute .the Clear Interrupt Flag instruction, thereby disabling interrupts, could bring the entire system to a halt. To prevent such disrup- tions, the 80386 raises an exception when a Virtual 86 Mode task attempts to execute an I/O or interrupt-related instruction.

Preventing the execution of such instructions protects the rest of the system from a Virtual 86 Mode task, but does not satisfy the Virtual 86 Mode task's need to execute the instructions. The solution is to simulate the sensitive instruc- tions in an operating system procedure called a virtual machine monitor. When an exception handler is invoked, it can inspect the VM86 flag in the Flags image on the stack to see if the source of the exception is a Virtual 86 Mode task; if so, the exception handler can call the virtual machine monitor which can simulate the instruction and return to the Virtual 86 Mode task. Note that a virtual machine monitor simulates only a few 8086 instructions and that both the simulated instructions and those the 80386 executes directly benefit from the much higher performance of the

80386 compared to the 8086.

Working together, the 80386 and a virtual machine monitor implement the full 8086 instruc- tion set, and paging can provide each Virtual 86 M ode task with its own protected address space. However, most 8086 programs need additional resources provided by an operating system and peripheral hardware. An example ofthe former type of resource is a file system; an example of the latter is a bit-mapped display controller manipulated directly by an application program. These resources may not exist in the same form in the 80386-based system as they did in the system for which the 8086 program was designed. To simplify the job of providing these resources in a different environment, the 80386 can trap operating system and peripheral references made by Virtual 86 Mode tasks.

For example, most 8086 operating systems use the Interrupt instruction to implement operating system calls. The 80386 raises an exception when a Virtual 86 Mode task attempts to execute an Interrupt instruction. The virtual machine moni- tor can then translate the 8086 operating system call into a call on the 80386 operating system as shown in Figure 4-I.-If a Virtual 86 Mode task's 10PL is set to less than 3, the 80386 will likewise trap any I/O instruction the 8086 program executes. The 80386 paging facility can be used to redirect references to memory-mapped periph- erals to other addresses, if necessary. Such references can also be trapped by marking the corresponding pages read-only (to trap writes), or not-present (to trap both reads and writes).

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