Interrupt Descriptor Table: Difference between revisions

The location of the '''IDT''' is kept in the '''IDTR''' ('''IDT''' register). This is loaded using the '''LIDT''' assembly instruction, whose argument is a pointer to an '''IDTRIDT Descriptor''' structure:

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* '''Selector:''' A 16-bit'''[[Segment valueSelector]]''' with multiple fields which must point to a valid code segment in your '''[[GDT]]'''. For more information, see the [[Segment selector|Segment selector]] page or '''Section 3.4.2: Segment Selectors''' and '''Figure 3-6: Segment Selector''' of the Intel Software Developer Manual, Volume 3-A.

For more information, see '''Section 6.11: IDT Descriptors''' and '''Figure 6-2: IDT Gate Descriptors''' of the [https://www.intel.com/content/www/us/en/architecture-and-technology/64-ia-32-architectures-software-developer-vol-3a-part-1-manual.html Intel Software Developer Manual, Volume 3-A].

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* '''Selector:''' A 16-bit'''[[Segment valueSelector]]''' with multiple fields which must point to a valid 64-bit code segment in your '''[[GDT]]'''. For more information, see '''Section 3.4.2: Segment Selectors''' and '''Figure 3-6: Segment Selector''' of the Intel Software Developer Manual, Volume 3-A.

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An '''Interrupt Gate''' is used to specify an '''[[Interrupt Service Routines|Interrupt Service Routine]]'''. For example, when the assembly instruction '''INT 50''' is performed while running in protected mode, the CPU looks up the 50th entry (located at 50 * 8) in the '''IDT'''. Then the Interrupt Gate's '''Selector''' and '''Offset''' values are loaded. The '''Selector''' and '''Offset''' are used to call the '''Interrupt Service Routine'''. When the '''IRET''' instruction is performed, the CPU returns from the interrupt. If the CPU was running in 32-bit mode and the specified selector is a 16-bit gate, then the CPU will go in 16-bit '''Protected Mode''' after calling the '''ISR'''. To return in this case, the '''O32 IRET''' instruction should be used, or else the CPU will not know that it should do a 32-bit return (reading 32-bit values off the [[stack]] instead of 16 bit).