Computer Organization

1
Computer Organization

• Project 1 - Extra
• Prof. Jerry Breecher
• CSCI 140
• Spring 2002

2
What You Will Do In This Lab.

• The purpose of this lab is to write an exception
  handler to be used to determine if your
  calculator might return incorrect results.
• You will work VERY INDEPENDENTLY I wont give
  you a lot of guidance other than what you get in
  this document.

3
What You Will Do In This Lab.

• You will work VERY INDEPENDENTLY I wont give
  you a lot of guidance other than what you get in
  this document.
• Write an exception handler to assure that your
  calculator correctly handles overflows and
  underflows.
• As a starting point, you have the handler
  currently used by SPIM. It is found at
  /usr/lib/spim/trap_handler. A copy of this code
  is reproduced later in this document.
• Section A.7 in the text gives some information.
  The code referenced above actually contains more
  information than what is given in the text.
• The general concept of what you need to do is
  this
• jal enable_my_trap_handler Override the
  existing trap handler
• add s3,s2,s1 This add MAY cause an
  exception
• jal disable_my_trap_handler Restore the
  original trap handler
• lw t0,did_trap_occur Get flag showing trap
  occurred
• Analyze trap flag

4
What You Will Do In This Lab.

• A vital piece of information
• When an exception occurs, SPIM jumps to, and
  starts executing, the code at location 0x80000080
  note that the sample handler starts its
  .ktext at this location.
• So you will need THREE pieces of code
• enable_my_trap_handler a subroutine that
  removes and stashes away the original
  instruction, currently at 0x80000080, and
  replaces it with a jump to the address where YOUR
  new trap handler, my_trap_handler, is located.
• disable_my_trap_handler a subroutine that
  stores back into location 0x80000080 the
  instruction that was originally there. In effect
  it restores the original trap handler so that any
  surprising traps can be handled.
• my_trap_handler A piece of code you write that
  determines if an exception has occurred. If a
  trap did occur, it sets the word at
  did_trap_occur. Note the location
  did_trap_occur and the routines described on this
  page, all live in code your write. I believe
  they must all live in user space, not in .ktext.