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OS/161 - Debugging with Fault Addresses
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.. topic:: Learning Outcome

    Able to interpret EPC and vaddr, and use them to locate the offending code.    
    
.. topic:: Introduction

    When an exception occurs, OS/161 often stops and prints out the exception program counter (EPC) and the bad virtual address register (vaddr) related to whatever triggered the exception. It is very useful to be able to take these values, interpret them, and determine the code that triggered the exception.

.. topic:: Applicable subjects

    COMP3231

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Here is more detailed description of these two registers.


EPC Register
============

This is a 32-bit register containing the 32-bit address of the return point
for an exception. The instruction causing (or suffering) the exception is at
EPC, unless BD is set in Cause, in which case EPC points to the previous
(branch) instruction.






BadVaddr Register (vaddr)
=========================

A 32-bit register containing the address whose reference led to an
exception; set on any MMU-related exception, on an attempt by a user
program to access addresses outside kuseg, or if an address is wrongly
aligned for the datum size referenced.

After any other exception this register is undefined.

----

Example
==========

::

    $ sys161 kernel
    sys161: System/161 release 2.0.8, compiled Feb 19 2017 14:31:53

    OS/161 base system version 2.0.3
    (with locks&CVs solution)
    Copyright (c) 2000, 2001-2005, 2008-2011, 2013, 2014
    President and Fellows of Harvard College.  All rights reserved.

    Put-your-group-name-here's system version 0 (ASST0 #5)

    16224k physical memory available
    Device probe...
    lamebus0 (system main bus)
    emu0 at lamebus0
    ltrace0 at lamebus0
    ltimer0 at lamebus0
    beep0 at ltimer0
    rtclock0 at ltimer0
    lrandom0 at lamebus0
    random0 at lrandom0
    lser0 at lamebus0
    con0 at lser0

    cpu0: MIPS/161 (System/161 2.x) features 0x0
    panic: Fatal exception 3 (TLB miss on store) in kernel mode
    panic: EPC 0x8000b504, exception vaddr 0x0
    panic: I can't handle this... I think I'll just die now...
    sys161: trace: software-requested debugger stop
    sys161: Waiting for debugger connection...




::

    panic: EPC 0x8000b504, exception vaddr 0x0