As mentioned earlier in this document, the 80386 has a third mode called the V86 mode. In this mode, the 80386 behaves exactly like an 8086 with a few minor exceptions. In V86 mode, the CPU may not issue any of the privileged instructions such as LGDT, LIDT, LLDT, SLDT, ARPL, LAR, LSL, LTR, VERR, VERW, INVD, WBINVD, WRMSR, RDMSR, etc. However, unlike the real mode, the MMU is active in the V86 mode. Therefore, the operating system still has control over a V86 task, although it runs just like in real mode. Since privileged instructions cannot be executed, the OS can prevent system crashes or malicious programs from destroying any data.
A task is said to be a V86 task when the VM bit in the EFLAGS register is set. Only a PL0 program can set this bit. The bit can also be set in the TSS before the task is executed. The advantage of a having a V86 task rather than a real mode task is obvious - it's due to security reasons. The other reason is that the time taken by the CPU to switch from protected mode to real mode is considerable and greatly slows down the program. By running the program in V86 mode, the program is actually executed faster.
A V86 task, just like a real mode program, uses segments and offsets rather than a selector:offset pair. It can access 1MB of memory. A V86 task always runs at PL3. They can do I/O only if the IOPL is 3 or if the TSS bitmap allows them to do so. If IOPL is less than 3, then the CPU generates a General Protection Fault whenever the V86 task attempts to execute the following instructions - CLI, STI, INT, IRET, LOCK, PUSHF and POPF.
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