GDT Tutorial: Difference between revisions

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In the [[:Category:x86|Intel Architecture]], and more precisely in [[protected mode]], most of the [[memory management]] and [[Interrupt Service Routines]] are controlled through tables of descriptors. Each descriptor stores information about a single object (e.g. a service routine, a task, a chunk of code or data, whatever) the CPU might need at some time. If you try, for instance, to load a new value into a [[Segment|segment register]], the CPU needs to perform safety and access control checks to see whether you're actually entitled to access that specific memory area. Once the checks are performed, useful values (such as the lowest and highest addresses) are cached in invisible registers of the CPU.

 

: a memory structure (part of a table) that tells the CPU the attributes of a given segment

 

 

=== Basics ===

That means whatever you load at physical address 4 MiB will appear as code at <tt>CS:0</tt> and what you load at physical address 8 MiB will appear as data at <tt>DS:0</tt>. However it might not be the best design.

 

 

=== Disable interrupts ===

=== Filling the table ===

 

 

<source lang="c">

</source>

 

 

=== Telling the CPU where the table stands ===

==== From real mode ====

 

 

<source lang="asm">

==== From flat, protected mode ====

 

 

<source lang="asm">

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

 

 

Much like the GDT (global descriptor table), the LDT (''local'' descriptor table) contains descriptors for memory segments description, call gates, etc. The good thing with the LDT is that each task can have its own LDT and that the processor will automatically switch to the right LDT when you use hardware task switching.

Note that with 386+ processors, the paging has made LDT almost obsolete, and there's no longer need for multiple LDT descriptors, so you can almost safely ignore the LDT for OS developing, unless you have by design many different segments to store.

 

 

As said above, the IDT (Interrupt Descriptor Table) loads much the same way as the GDT and its structure is roughly the same except that it only contains gates and not segments. Each gate gives a full reference to a piece of code (code segment, privilege level and offset to the code in that segment) that is now bound to a number between 0 and 255 (the slot in the IDT).

 

 

 

Tool for easily creating GDT entries.