The Global Descriptor Table

The Global Descriptor Table (often called just GDT) is a data structure containing entries telling the CPU about memory segments. In this example, as in most of the Operating Systems, the GDT has the null descriptor, the code descriptor and the data descriptor.

• The null descriptor, describe to the CPU the meaning of a NULL value.
• The code descriptor, describe to the CPU how to handle the code saved in the memory.
• The data descriptor, describe to the CPU how to handle the data saved in the memory.

GDT example

We start this guide from this example, where we can see a GDT for the Protected Mode.

align 8
GDTStart:
    nullDescriptor:
        dd 0
        dd 0

    codeDescriptor:
        dw 0xffff
        dw 0
        db 0

        db 0b10011010

        db 0b11001111

        db 0

    dataDescriptor:
        dw 0xffff
        dw 0
        db 0

        db 0b10010010

        db 0b11001111

        db 0
GDTEnd:

GDTDescriptor:
    dw GDTEnd - GDTStart - 1
    dd GDTStart

Commented version

align 8                 ; Align the GDT to 8-bit blocks (REQUIRED!)
GDTStart:               ; The address of the start of the GDT
    nullDescriptor:     ; The null descriptor
        dd 0            ; Allocate 32 bits of 0
        dd 0            ; Allocate 32 bits of 0

    codeDescriptor:     ; The code descriptor
        dw 0xffff       ; Allocate 16 bits with value 0xFFFF (all bits to 1)
        dw 0            ; Allocate 16 bits of 0
        db 0            ; Allocate 8 bits of 0

        db 0b10011010   ; Allocate 8 bits with value 10011010

        db 0b11001111   ; Allocate 8 bits with value 11001111

        db 0            ; Allocate 8 bits of 0

    dataDescriptor:     ; The data descriptor
        dw 0xffff
        dw 0            ; Allocate 16 bits of 0
        db 0            ; Allocate 8 bits of 0

        db 0b10010010   ; Allocate 8 bits with value 10010010

        db 0b11001111   ; Allocate 8 bits with value 11001111

        db 0            ; Allocate 8 bits of 0
GDTEnd:                 ; The address of the end of the GDT

GDTDescriptor:
    dw GDTEnd - GDTStart - 1    ; The size of the GDT in 16 bits
    dd GDTStart                 ; The 32 bits address of the start of the GDT

Null Descriptor

nullDescriptor:
    dd 0
    dd 0

I haven't a lot to say about the Null Descriptor. It must be this: 2 sets of 32-bits filled with 0.

Code Descriptor

db 0b10011010

This is really important for the code descriptor. Here the meaning of each bit.

We have 2 type of bitsets, both of 4 bits:

• Info flags
• Type flags

Info flags

Present, Privilege, Type (1001)

• Present = 1 - Means that it's a used segment, don't set to 0. Never set it to 0. Really, your OS may explode.
• Privilege = 00 - Means max privileges
• Type = 1 - Means that it's the code or data descriptor

Type flags

Code, Conforming, Readable, Accessed (1010)

• Code = 1 - Means that it's the code descriptor
• Conforming = 0 - Means that's non-conforming
• Readable = 1 - Means that it's readable
• Accessed = 0 - Always set to 0

db 0b11001111

This is additional informations flags. Also here, we have 2 type of bitsets, both of 4 bits:

• Segment informations
• Limit

Segment informations

Granularity, Default operand size, 64-bit segment, Available (1100)

• Granularity = 1 - We have an highter code limit
• Default operand size = 1 - You need to set it to 1 to enable the Protected Mode
• 64-bit segment = 0 - You must set it to 0 now
• Available = 0 - This is a bit that OS can use for their stuffs, but nobody uses it, so is set to 0

Limit

Limit (1111)

• Limit = 1111 - Yes, all to 1 to haven't size problems later

Data Descriptor

db 0b10010010

Now we are talking about the data descriptor. It's similar for most of the things.

We have 2 type of bitsets, both of 4 bits:

• Info flags
• Type flags

Info flags

Present, Privilege, Type (1001)

• Present = 1 - Means that it's a used segment, don't set to 0. Never set it to 0. Really, your OS may explode.
• Privilege = 00 - Means max privileges
• Type = 1 - Means that it's the code or data descriptor

Type flags

Code, Expand-down, Writable, Accessed (0010)

• Code = 0 - Means that it's the data descriptor
• Expand-down = 0 - Means that's normal (the expand-down is for stacks that needs to expand during execution - really advanced stuffs, set to 0 and don't worry)
• Writable = 1 - Means that it's writable
• Accessed = 0 - Always set to 0

db 0b11001111

This is additional informations flags. Also here, we have 2 type of bitsets, both of 4 bits:

• Segment informations
• Limit

Segment informations

Granularity, Default operand size, 64-bit segment, Available (1100)

• Granularity = 1 - We have an highter code limit
• Default operand size = 1 - You need to set it to 1 to enable the Protected Mode
• 64-bit segment = 0 - You must set it to 0 now
• Available = 0 - This is a bit that OS can use for their stuffs, but nobody uses it, so is set to 0

Limit

Limit (1111)

• Limit = 1111 - Yes, all to 1 to haven't size problems later

GDT differences for Protecetd Mode and Long Mode

The GDT structure for the Protected Mode and for the Long Mode are almost the same.

Here an example of the GDT for the Long Mode.

align 8
GDTStart64:
    nullDescriptor64:
        dd 0
        dd 0

    codeDescriptor64:
        dw 0xffff
        dw 0
        db 0

        db 0b10011010
        
        db 0b10101111

        db 0

    dataDescriptor64:
        dw 0xffff
        dw 0
        db 0

        db 0b10010010
        
        db 0b10101111

        db 0
GDTEnd64:

GDTDescriptor64:
    dw GDTEnd64 - GDTStart64 - 1
    dd GDTStart64

As you can see, the GDT for the Long Mode is almost the same of the GDT for the Protected Mode.

The only differences are 2 little bits:

• In the Code Descriptor:
  • db 0b10101111 instead of db 0b11001111
• In the Data Descriptor:
  • db 0b10101111 instead of db 0b11001111

What's the difference? The bit 6 and the bit 5 are switched. This pins are the Default operand size (bit 6) and the 64-bit segment (bit 5). For the Long Mode the 64-bit segment must be enabled, and the Default operand size set to 0 to return to the normal Operand Size.