Department of Electrical and Computer Engineering
		  The University of Texas at Austin


Y. N. Patt, D. N. Armstrong
Interrupts and Exceptions for the Project
3 April 2002


* Information on  x86 interrupts and exceptions in  protected mode can
  be found in Chapter 6 of Vol. 1, and Chapter 5 of Vol. 3.

* You must support  at least one external interrupt  triggered from an
  I/O device.

* You must support both general protection exceptions and page faults.
  Note that both instruction and data accesses may throw exceptions.

* Page Faults are caused by trying to  access a page not in the TLB or
  a page not present in physical memory.

* General protection exceptions are caused by:

  1. Writing to a read-only page, or,

  2. When computing a memory operand with an effective address outside
     the  CS, DS,  ES, FS  or GS  segment limit.  Note  that accessing
     memory  outside  the SS,  stack  segment,  limit  causes a  Stack
     Segment Exception which is not required for this project.

* The  procedure  for  transferring   control  to  the  interrupt  and
  exception handler is the following:

  1. Push EFLAGS.

  2. Push CS.

  3. Push EIP.

  4. Determine  the address  of the  Interrupt Descriptor  Table (IDT)
     entry that contains  the far pointer that points  to the relevant
     service routine.

  5. Load the new  CS and EIP values and begin  executing from the new
     location.

* The  IRETD instruction  will  appear  at the  end  of the  interrupt
  service routine and will pop the EIP, CS and EFLAGS registers.

* The far pointer pointing to the service routine is found by indexing
  into the IDT with the interrupt or exception vector.

* The IDTR register contains a pointer to the base address of the IDT.
  The  value in  the IDTR  is a  linear address  The IDTR  register is
  actually a  6-byte register,  where the first  4 bytes are  the base
  address of the IDT  and the final 2 bytes are the  limit of the base
  address.  You  are not required  to implement limit checking  on the
  IDT.  Therefore  the first 4  bytes are all  that you need  to worry
  about for this project.

* The vector  number for the page  faults is 14 and  the vector number
  for general protection exceptions is 13.

* You  are  not required  to  implement  anything  having to  do  with
  changing the priority level for your project.