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Re: x86 NT(Nested Task) flag vs VM flag?

Posted on 2019-03-28, 09:40

So, if the NT flag is essentially unsupported in V86 mode(only being cleared/set during task switches, but having no effect on IRET), is it affected by task switches to V86 tasks? So a CALL to a V86 task, will that set the NT flag of said task? Will it have effect on the V86 task(besides it being …

Re: x86 NT(Nested Task) flag vs VM flag?

Posted on 2019-03-26, 06:31

What I mean is that you can't, for example, use V86 mode to handle an interrupt(e.g. MS-DOS) and then return(IRET) from it back to the caller when it's finished. You'll have to go the long way, installing an INT instruction to call the monitor at the IRET return address(e.g. MS-DOS style exit …

x86 NT(Nested Task) flag in Virtual 8086 mode?

Posted on 2019-03-24, 15:12

What happens when an IRET is executed while both the NT(Nested Task) and VM(Virtual 8086 mode) flags in EFLAGS are set, while EFLAGS.IOPL is 3? Will the CPU perform a return to the parent task(undocumented behaviour)? Or will the CPU ignore the NT flag and perform a V86-mode IRET to the caller in …

Re: UniPCemu Windows 95/NT progress and issues

Posted on 2019-03-22, 11:00

Hmmm... Debugging segment loads from PL3 protected mode, I see a far jmp(to 3b:20c) using a 16-bit jmp, which is marked as an user-mode segment, while locafed in kernel memory? That can't be right? That's from 9f:cac2. Edit: Hmmmm.... It simply fetches an INT instruction(0xCD) from there? That can't …

Re: UniPCemu Windows 95/NT progress and issues

Posted on 2019-03-21, 21:17

Hmmmm... I remember having set a protected mode-style IRET(to a non-V86 destination), but don't seeing it triggered somehow. And Windows 95 is crashing on a CPL 3 'task'? So is it using RETF to get to that task, or is some privilege rule being broken somewhere? Edit: Nope. It's an IRET from the …

Re: Windows 95 bootlog Initializing KERNEL

Posted on 2019-03-19, 23:35

Well, afaik Windows itself(on the emulated harddisk) isn't the issue. There's some CPU emulation issue in UniPCemu that's causing it. It's booting fine when loading the disk image in the Bochs emulator. But afaik the CPU instructions have all been tested and verified(except some remaining Protected …

Re: Windows 95 bootlog Initializing KERNEL

Posted on 2019-03-19, 22:37

Hmmmm... Interestingly enough, at https://www.google.com/amp/s/www.techsupportforum.com/forums/f6/msgsrv32-exe-kernel32-dll-win95-boot-probs-66168.html%3famp=1 , the Initializing KERNEL is followed by: LoadStart = system.drv on the next row. Does that mean that the issue is somewhere within that …

Re: What CPU state is reversed when a fault occurs?

Posted on 2019-03-17, 13:55

All those registers aren't restored when it repeats. They're saved before each of those repeated instructions starts executing(before every single repeated step during REP CMPS*, so before it starts running and after each time it successfully(without faults) completes said action(that is, e.g. …

IRET to V86 mode from non-zero CPL?

Posted on 2019-03-17, 11:58

If I have to believe the 80386 and all related documentation, if you IRET with EFLAGS.VM on the stack set with a non-zero CPL, it results in a normal protected-mode IRET, with (undocumented) EFLAGS.VM being cleared(to make it use normal protected mode)? Is that correct behaviour? Also the 80386 …

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