Triple fault when loading GDT
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Author:  callumperks1 [ Tue Dec 07, 2021 11:50 am ]
Post subject:  Triple fault when loading GDT

I am trying to load a GDT but keep getting a triple fault. I've been trying to do this for far too long so any help would be hugely apppreciated

These are my GDT structs

struct GlobalDescriptorTableEntry
        unsigned short limit_low;  /* The lower 16 bits of the limit */
        unsigned short base_low;   /* The lower 16 bits of the base */
        unsigned char base_mid;    /* The next 8 bits of the base */
        unsigned char access;      /* Contains access flags */
        unsigned char granularity; /* Specify granularity, and 4 bits of limit */
        unsigned char base_high;   /* The last 8 bits of the base; */
    } __attribute__((packed));     /* It needs to be packed like this, 64 bits */

    struct GlobalDescriptorTablePointer
        unsigned short limit; /* Size of gdt table in bytes*/
        unsigned int base;    /* Address to the first gdt entry */
    } __attribute__((packed));

This is the assembly function to load the gdt
/* Build GDT pointer and load it */
.global gdt_load_and_set
   mov 4(%esp),%eax
   lgdt (%eax)

   mov $0x10, %ax
   mov %ax, %ds
   mov %ax, %es
   mov %ax, %fs
   mov %ax, %gs
   mov %ax, %ss
   jmp  $0x08,$flush


And this is the code that setupds the GDT
        GlobalDescriptorTablePointer gtdPointer;
        gtdPointer.limit = (sizeof(GlobalDescriptorTableEntry) * GDT_NUM_ENTRIES) - 1 ;
        gtdPointer.base = (unsigned int)&gdt_entries;

        CreateEntry(0, 0, 0, 0, 0);                // Null segment
        CreateEntry(1, 0, 0xFFFFFFFF, 0x9A, 0xCF); // Code segment
        CreateEntry(2, 0, 0xFFFFFFFF, 0x92, 0xCF); // Data segment
        // CreateEntry(3, 0, 0xFFFFFFFF, 0xFA, 0xCF); // User mode code segment
        // CreateEntry(4, 0, 0xFFFFFFFF, 0xF2, 0xCF); // User mode data segment
        gdt_load_and_set((unsigned int)&gtdPointer);

    void GlobalDescriptorTable::CreateEntry(int num, unsigned int base, unsigned int limit, unsigned char access, unsigned char gran)
        gdt_entries[num].base_low = (base & 0xFFFF);
        gdt_entries[num].base_mid = (base >> 16) & 0xFF;
        gdt_entries[num].base_high = (base >> 24) & 0xFF;

        gdt_entries[num].limit_low = (limit & 0xFFFF);
        gdt_entries[num].granularity = (limit >> 16) & 0x0F;

        gdt_entries[num].granularity |= gran & 0xF0;
        gdt_entries[num].access = access;

Author:  nullplan [ Wed Dec 08, 2021 11:20 am ]
Post subject:  Re: Triple fault when loading GDT

And what is the instruction that is failing? How is the GDT declared (that is, what is gdt_entries)? Is it possibly an alignment issue? The way you declare the GDT structure, the GDT entries have an alignment of 2, but they ought to have one of 8. Is the GDT pointer correct? Have you tried exhausting the debugging capabilities of whatever emulator you are using?

In short, what steps have you taken to investigate or rectify the issue?

There is nothing obviously wrong from what I can see here, though. So the issue may be elsewhere.

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