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TSS Question
Posted: Tue Dec 23, 2008 10:49 am
by System123
I have looked through old posts on the forum and have checked the Intel manuals but I can't find the answer to my question. I am setting up my TSS in my GDT but I don't know what the Ganularity and Accessiblity values must be. so far I have this in my GDT
Code: Select all
SetGDTGate(0,0,0,0,0); // nil descriptor
SetGDTGate(1,0,$FFFFFFFF,$9A,$CF); // Kernel space code
SetGDTGate(2,0,$FFFFFFFF,$92,$CF); // Kernel space data
SetGDTGate(3,0,$FFFFFFFF,$FA,$CF); // User space code
SetGDTGate(4,0,$FFFFFFFF,$F2,$CF); // User space data
SetGDTGate(5,PtrUInt(TSS),SizeOf(TSS)-1, Access??, Gran??);
Re: TSS Question
Posted: Tue Dec 23, 2008 12:20 pm
by quirck
Granularity bit stands for multiplying limit by 4K. Here this is not necessary => we have gran = $40 (32-bit, upper bits of limit are zero).
Access byte must include: P = 1 (present), DPL = 0, S = 0 (system), Type = 9 (32-bit available TSS). Thus, access = $89.
Re: TSS Question
Posted: Tue Dec 23, 2008 1:31 pm
by System123
Thanks.
Re: TSS Question
Posted: Wed Dec 24, 2008 2:08 pm
by System123
I now have my tss installed in the gdt. But when i try call ltr the cpu tripple faults. What value should be used with ltr? Like lgdt you use the gdt pointer which contains a limit and a base what does ltr use?
Re: TSS Question
Posted: Thu Dec 25, 2008 1:03 am
by Brendan
Hi,
System123 wrote:I now have my tss installed in the gdt. But when i try call ltr the cpu tripple faults. What value should be used with ltr? Like lgdt you use the gdt pointer which contains a limit and a base what does ltr use?
LTR is like LLDT - you use an offset in the GDT to refer to the corresponding GDT entry. For example, if the GDT has a NULL entry then the TSS entry (then other entries) you'd do "mov eax,0x0008; ltr ax".
Cheers,
Brendan
Re: TSS Question
Posted: Thu Dec 25, 2008 8:04 am
by Love4Boobies
Note however that the use of hardware-based task switching is ill advised - it's slower than software and support for TSS's is dropped for 64-bit modes altogether.
Re: TSS Question
Posted: Thu Dec 25, 2008 10:22 am
by System123
Don't you still need 1 TSS for Software based switching? Because that is what I am trying to implement however all the docs I have read say you need a TSS.
Re: TSS Question
Posted: Thu Dec 25, 2008 5:04 pm
by pcmattman
Correct, if you plan to be switching to and from user-mode code.
Re: TSS Question
Posted: Thu Dec 25, 2008 5:17 pm
by Combuster
Basically, the TSS is used when entering ring 0 from ring 3. If its broken (or absent) when the processor needs it, it will reset instead. During this jump the CPU will grab two entries: SS0 and ESP0, so that's the only part you need to do.
Re: TSS Question
Posted: Fri Dec 26, 2008 3:15 pm
by System123
I initialized my TSS with SS0 = 0x10 and Esp0 = the start of the kernel stack. But when i ltr 0x0028 (the gdt segment with it in) i get a general protection fault? Any common reasons?
I think it is due to my access byte being 0x89? As this makes the dpl = 2?
Re: TSS Question
Posted: Fri Dec 26, 2008 5:07 pm
by Combuster
Could you tell us what bochs has to say/complain about it?
Re: TSS Question
Posted: Sat Dec 27, 2008 3:00 am
by System123
Don't worry I found the problem. I had my granularity and access byte mixed up. It works fine now.