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I am developing a 64-bit OS.I wanted to implement multiprocessing but where will the Local and I\O APIC be?
I have read the Intel documents for it's 32-bit APICs,but in 64-bit mode it is the same or it is different?

it will be in same location, usually

• Local APIC: FEE0 0000h
• I/O APIC: FEC0 0000h

but I suggest parse MADT (Multiple APIC Description Table) from ACPI spec and get address from there, because theoretically it can be elsewhere also PC can have several I/O APICs

Thanks for the reply.
The Local APIC has pointers to ISRs(Interrupt Service Routines), but will that still be 32-bit or will it be extended?
You can include references to the AMD64 Architecture Programmers Guide Volume 2.

I think you've misunderstood something
Local APIC isn't related to ISRs
IDT (Interrupt Descriptor Table) it contains pointers to ISRs and IDT structure changes in long mode so it can contain 64-bit pointers
IDTR specifies size and location to IDT


I haven't read AMD manuals, just Intel (because all my PCs have only Intel CPUs)
anyway:

• 82093AA I/O ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (IOAPIC)
• Chapter 6 (INTERRUPT AND EXCEPTION HANDLING)
  and 10 (ADVANCED PROGRAMMABLE INTERRUPT CONTROLLER (APIC)) in Intel® 64 and IA-32 Architectures Software Developer’s Manual Volume 3A: System Programming Guide, Part 1
• Chapter 5.2.12 (Multiple APIC Description Table (MADT)) in Advanced Configuration and Power Interface Specification (Revision 4.0a)

Thanks for that reply also.
So I\O APIC and Local APICs are the same in long mode.

Ask the cpu to tell it's local apic address. It's in MSR 1Bh, and it's a 64 bit address.