Using LLDT and configuring the GDT for it

Question

I'm working on a small OS that will use a separate Local Descriptor Table for each process. I understand that I will need to use the lldt instruction to load a LDT segment from my GDT. I already have my kernel running in protected mode with a valid GDT, but I cannot figure out what the GDT entry for my LDT should look like. I understand that its base address should point to my LDT, but I don't know what the privilege level and other attributes should be. Here is the NASM code that represents the LDT entry in my GDT:

localTable equ $-gdt            ; GDT entry #5 (selector 20h)
dw 0x1FF                        ; limit to 64 descriptors
dw 0x8000                       ; base address
db 0x0
db 0x89                         ; probably incorrect...
db 0x1f                         ; possibly incorrect...
db 0x0

If you are not familiar with the NASM syntax, this table entry has a base address of 0x8000 and a limit of 511 (512 bytes total, or 64 entries). I have read the section about the GDT and LDT in the i486 programmer's reference manual, but I cannot fully understand what my GDT entry should look like.

Anyway, I load the LDT like so:

mov ax, 0x20
lldt ax

This code causes the processor to generate a general protection fault (I handle it with an interrupt). I would like to know two things:

1) Did I correctly describe my LDT in the GDT? If not, what needs to be changed? 2) Could the LLDT instruction be failing because there are invalid selectors in my LDT itself? I read the LLDT instruction spec, and it seems to me that it doesn't even read the memory of the LDT, but I just want to be sure the LLDT isn't failing because I have a typo in my LDT's data.

Answer 1

Ok, I figured it out. The type that I was using ( 1001b ) was not what I needed. I found that type 2 ( 10b ) is used for LDT entries. For the record, this information is in chapter 6, page 4, of the i486 Microprocessor Programmer's Manual . My functional GDT entry looks as follows:

localTable equ $-gdt            ; GDT entry #5 (selector 20h)
dw 0x1FF                        ; limit to 64 descriptors
dw 0x8000                       ; base address
db 0x0
db 0x82                         ; 10000010b (segment present set, WTM)
db 0x1f
db 0x0