Set Up the Interrupt Descriptor Table in Protected Mode

Interrupt Descriptor Table

In protected mode the IDT holds 8-byte gate descriptors pointing to handlers with privilege and type information. LIDT loads the table base and limit; each interrupt index selects an entry. For example, entry 32 (IRQ0 after PIC remap) needs a present interrupt gate targeting your timer ISR at ring 0.

Gate entry

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• 256 entries cover CPU exceptions and IRQs
• IST not used in basic setups; keep gate type 0x8E for 32-bit interrupt gates
• Remap PIC to vectors 0x20-0x2F before unmasking IRQs
• Handlers end with IRETD in 32-bit code

Exception stubs

Divide-by-zero and invalid opcode handlers help catch early PM bugs. Place stubs in low memory or identity-mapped regions so their addresses fit gate offset fields. Use interrupt gates (type 0x8E) not trap gates unless you need nested faults on the same vector. Double-fault requires a valid TSS stack if you run ring 3 later; for ring 0 only labs, a simple halt loop in the handler is enough.

Why for this exercise

You will build an IDT, load it with LIDT, and register at least one ISR. This exercise requires matching gate selectors to your GDT code segment and using IRETD return.