VIRTUAL 8086 MODE OF 80386DX

In its protected mode of operation, 80386DX provides a virtual 8086 operating environment to execute the 8086 programs. However, once the 80386 enters the protected mode from the real mode, it cannot return back to the real mode without a reset operation. The address forming mechanism in virtual 8086 mode is exactly identical with that of 8086 real mode. In virtual mode, 8086 can address 1Mbytes of physical memory that may be anywhere in the 4Gbytes address space of the protected mode of 80386. Like 80386 real mode, the addresses in virtual8086 mode lie within 1Mbytes of memory. In virtual mode, the paging mechanism and protection capabilities are available at the service of the programmers (note that the 80386 supports multiprogramming, hence more than one programmer may use the CPU at a time).Paging unit may not be necessarily enabled in virtual mode, but may be needed to run the 8086 programs which require more than 1Mbyte of memory for memory management functions.In virtual mode, the paging unit allows only 256 pages, each of 4Kbytes size. Each of the pages may be located anywhere within the maximum 4Gbytesphysical memory. The virtual mode allows the multiprogramming of 8086 applications. Figure 10.10 shows how the memory is managed in multitasking virtual 8086 environment.The virtual 8086 mode executes all the programs at privilege level-3. Any of the other programs may deny access to the virtual mode programs or data. However, the real mode programs are executed at the highest privilege level,Even in the virtual mode, all the interrupts and exceptions are handled by the protected, mode interrupt handler. To return to the protected mode from the virtual mode, any interrupt or exception may be used. As a part of interrupt service routine, the VM-bit may be reset to zero to pull back the 80386 into protected mode.Thus, the virtual 8086 mode of operation of 80386 offers an advantage of executing 8086 programs while in protected mode.

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