GDT Tutorial: Difference between revisions

{|class="wikitable" style="display: inline-table;"

|+ 32-bit

|-

| 0x0000 || Null Descriptor || <tt>Base = 0<br>Limit = 0x00000000<br>Access Byte = 0x00<br>Flags = 0x0</tt>

| 0x0020 || User Mode Data Segment || <tt>Base = 0<br>Limit = 0xFFFFF<br>Access Byte = 0xF2<br>Flags = 0xC</tt>

|-

|}

 

{|class="wikitable" style="display: inline-table;"

|+ 64-bit

|-

| 0x0000 || Null Descriptor || <tt>Base = 0<br>Limit = 0x00000000<br>Access Byte = 0x00<br>Flags = 0x0</tt>

| 0x0020 || User Mode Data Segment || <tt>Base = 0<br>Limit = 0xFFFFF<br>Access Byte = 0xF2<br>Flags = 0xC</tt>

|-

|}

 

{|class="wikitable" style="display: inline-table;"

|+ Small Kernel

|-

| 0x0000 || Null Descriptor || <tt>Base = 0<br>Limit = 0x00000000<br>Access Byte = 0x00<br>Flags = 0x0</tt>

| 0x0010 || Kernel Mode Data Segment || <tt>Base = 0x00800000<br>Limit = 0x003FFFFF<br>Access Byte = 0x92<br>Flags = 0xC</tt>

|-

|}

 

{|class="wikitable"

|+ GDT

|-

| Preceding Entries || Null Descriptor<br>Kernel Segments<br>etc.

The above structure of the '''GDT''' doesn't show you how to write entries in the correct format. The actual structure of descriptors is a little messy for backwards compatibility with the 286's '''GDT'''. Base address are split across three different fields and you cannot encode any limit you want.

 

void encodeGdtEntry(uint8_t *target, struct GDT source)

{

// Encode the flags

}

 

In order to fill your table, you will want to use this function once for each entry, with <tt>*target</tt> pointing to the logical address of the '''Segment Descriptor''' and <tt>source</tt> being a struct of your design containing the necessary information.

The linear address should here be computed as <tt>segment * 16 + offset</tt>. <tt>GDT</tt> and <tt>GDT_end</tt> are assumed to be symbols in the current data segment.

 

gdtr DW 0 ; For limit storage

DD 0 ; For base storage

LGDT [gdtr]

RET

 

==== Protected Mode, Flat Model ====

"Flat" meaning the base of your Data Segment is 0 (regardless of whether '''[[Paging]]''' is enabled). This is the case if your code has just been booted by [[GRUB]], for instance. In the '''[[System V ABI]]''', arguments are passed on reverse order in the stack, so a function that can be called as <tt>setGdt(limit, base)</tt> might look like the following example code.

 

gdtr DW 0 ; For limit storage

DD 0 ; For base storage

LGDT [gdtr]

RET

 

==== Protected Mode, Non-Flat Model ====

If your data segment has a non-zero base, you'll have to adjust the instructions of the sequence above to include the ability to add the base offset of your data segment, which should be a known value to you. You can pass it in as an argument and call this function as <tt>setGdt(limit, base, offset)</tt>.

 

gdtr DW 0 ; For limit storage

DD 0 ; For base storage

LGDT [gdtr]

RET

 

==== Long Mode ====

In '''[[Long Mode]]''', the length of the '''Base''' field is 8 bytes, rather than 4. As well, the '''[[System V ABI]]''' passes the first two arguments via the '''RDI''' and '''RSI''' registers. Thus, this example code can be called as <tt>setGdt(limit, base)</tt>. As well, only a flat model is possible in long mode, so no considerations have to be made otherwise.

 

gdtr DW 0 ; For limit storage

DQ 0 ; For base storage

LGDT [gdtr]

RET

 

=== Reload Segment Registers ===

In this case, reloading '''CS''' is as simple as performing a far jump to the required segment, directly after the jump instruction:

 

reloadSegments:

; Reload CS register containing code selector:

MOV SS, AX

RET

 

An explanation of the above code can be found [http://stackoverflow.com/questions/23978486/far-jump-in-gdt-in-bootloader here].

In '''[[Long Mode]]''' the process of changing '''CS''' is not simple as far jumps cannot be used. Using a far return is recommended instead:

 

reloadSegments:

; Reload CS register:

MOV SS, AX

RET

 

== The LDT ==

 

Tool for easily creating GDT entries.

// Used for creating GDT segment descriptors in 64-bit integer form.

}

 

== See Also ==

[[Category:Tutorials]]

[[Category:X86 CPU]]