GDT Tutorial: Difference between revisions

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)

{

target[6] |= (source.flags << 4);

}

 

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.

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 ==