OSDev.org

Pype.Clicker wrote: okay, let's break that myth immediately: you don't need a descriptor per segment when you're in v86 mode. The segment registers within v86 mode are simply interpreted as they were in real mode.

In fact, almost everything happens as in real mode except:
- you may perform paging (from the kernel) in your address space (thus ensure you do the *right* paging for memory-mapped I/O
- any attempt to access priviledged instructions (hlt, lgdt, mov cr0 ...) will raise a GPF exception handled by the protected kernel
- any attempt to touch the interrupt flag (IRQ request, cli, sti, iret ..., int xx) will raise a GPF, unless you programmed the processor to defer interrupt management (avl. from pentium PRO : Virtual Interrupts extension in CR4)

Pype.Clicker wrote: you just need descriptors for the kernel supervisor (protected DPL0 code). But you'll need quite complex interrupt handlers for GPF ... and you'll also need complex code to setup and enter virtual tasks ...

Pype.Clicker wrote: well, as far as i remember (but i'm only basing on TRAN start32 tutorials & code), you cannot just jump/call a V86 task. Instead, you have to fake an interrupt return to that task, thus :
- setting up a TSS with all the required values in the registers and the VM bit set in flags (hum. not quite sure of that one, but it can't hurt)
- set the value of a trash-TSS selector with LTR. This one will hold the garbage state of the pmode starter task.
- set the "BACK" link of the trash-TSS to the value of the v86-TSS...
- create a fake 'virtual mode interrupted' frame on the stack (including values of ds,es,fs,gs,cs as 16 bits real-mode segments) and ip (as a 16 bits offset) and flags (including the VM bit set .. now i'm 100% sure you need that one ...)
- IRETD