Virtual 8086 Mode: Difference between revisions

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Line 3: ==Entering V86== The CPU is executing in virtual 86 mode when the VM bit (bit 17) is set in the ''EFLAGS'' register. If you want to enter virtual 86 mode you must set this bit to 1. A way of modifying the ~~EFLAG~~EFLAGS register is to use the ~~pushf~~''PUSHF'' and ~~popf~~''POPF'' instructions., but ~~These~~these instructions ~~respectively~~are ~~push~~for user space and ~~pop~~cannot modify supervisor flags. The only way to set the ~~eflags~~VM ~~register~~flag onis to use the ~~stack~~iret instruction. SoThis ~~you~~instruction ~~could~~is ~~push~~normally ~~the~~used ~~register,~~to ~~modify~~return itfrom onan interrupt. When ''IRET'' is called with the VM=1 in the stack ~~and~~''EFLAGS'' ~~pop~~(the ~~it.~~context ~~But~~being returned to is V86), the ~~popf~~interrupt ~~instruction~~stack ~~doesn't~~frame ~~modify~~will ~~bits~~contain 16segments ''ES'', ''DS'', ''FS'', and 17''GS'', inso ~~the~~that ~~eflags~~they ~~register~~can be set before entry. The only way to set the VM flag is to use the iret instruction. This instruction is normally used to return from an interrupt. When executing an iret, the CPU pops eip, cs, eflags, esp, ss from the stack and continues executing at the new eip. A task gate can also be used to enter V86. This allows setting the segment registers. It is probably not necessary to do it this way unless the OS is using hardware multitasking. ~~<pre>~~ ~~; you should declare this function as :~~ ~~; extern void enter_v86(uint32_t ss, uint32_t esp, uint32_t cs, uint32_t eip);~~ ~~enter_v86:~~ ~~mov ebp, esp ; save stack pointer~~ [[File:Fig15-3.gif]] ~~push dword [ebp+4] ; ss~~ ~~push dword [ebp+8] ; esp~~ ~~pushfd ; eflags~~ ~~or dword [esp], (1 << 17) ; set VM flags~~ ~~push dword [ebp+12] ; cs~~ ~~push dword [ebp+16] ; eip~~ ~~iret~~ ~~</pre>~~ ==V86 Problem== Line 32 ⟶ 20: EFLAGS.VM is NEVER pushed onto the stack if the V86 task uses PUSHFD. You should check if CR0.PE=1 and then assume it's V86 if that bit is set. <syntaxhighlight lang="asm"> ~~<pre>~~ detect_v86: smsw ax and eax,1 ;CR0.PE bit ret </syntaxhighlight> ~~</pre>~~ VM mode detection is mainly useful when writing DOS extenders or other programs that could be started either in plain real mode or in virtual mode from a protected mode system. An 'ordinary' bootloader shouldn't worry about this since the BIOS will not set up VM86 to read the bootsector ;)