******************
trace161
******************

.. topic:: Learning Outcome

    Interpret the TLB activity produced by trace161.
    
.. topic:: Introduction

    In assignment 3 of the Operating Systems course, you are required to use functions to fill a TLB.
    trace161 can provide debugging information for each of the TLB operations performed including
    page numbers, frame numbers and TLB flags.


.. topic:: Applicable subjects

    COMP3231

----

Running trace161
=================
To run your os161 kernel you would normally use:
::

    os161 kernel

To run trace161 and have it print out the TLB activity use:
::

    trace161 -t t kernel



Interpreting trace161 Output
==============================

Operations
******************

**tlblookup** -
tlb lookup finds the frame that matches a particular page number.

**tlbp** -
tlb probe finds the index that matches a particular page number.

**tlbwi** -
tlb write index writes a TLB entry into the TLB slot at a particular index.

**tlbwr** -
tlb write random writes a TLB entry into a random TLB slot.

**tlbr** -
tlb read reads a TLB entry at a particular index.

*tlbwi* and *tlbwr* are the two operations we care about, so we can grep the output of trace for these.
See the :ref:`log_files` module for instructions on using grep.


TLB Index
************
The index of the entry within the TLB


Page Number
*************
The virtual address divided by the page size.
Page numbers are mapped to frame numbers via the tlb.


Frame Number
*************
The physical address divided by the page size.


Offset
********
The offset of the address within the page/frame.


TLB Flags
************
**N** - noncached (Not very important for assignment 3)

**D** - dirty (If set, writes are permitted, if not, a TLB Modify exception occurrs)

**V** - valid (If set it is valid to use this TLB entry to translate addresses)

**G** - global (Not very important for assignment 3, can be left as 0)


    
----

Example
=======

An example of a trace161 output is:
::

    trace: 00 tlblookup: 00400/000-> 00050 -V--: [15] - OK
    trace: 00 tlblookup: 0070d/000 -> no match
    trace: 00 tlbwr: [21] 0051d/000 -> 0098 -VD- ==> 0070d/000 ->00114 -VD-

We can analyse this line by line.
From the first line we know that there is a TLB lookup to determine if the address 0x400000 (page number (0x400) * page size (0x1000) + offset (0x000)) is in the TLB:
::

    trace: 00 tlblookup:  00400/    000   ->  00050     -V--:   [15]   - OK
                ^          ^         ^          ^         ^       ^     
            operation     page     offset     frame     flags   index

It is. 

Then, there is another TLB lookup to determine if the address 0x70d000 is in the TLB:
::

    trace: 00 tlblookup:   0070d/  000    ->    no match
                ^            ^      ^               ^
            operation      page   offset   page not found in TLB

It isn't so we perform a TLB write random to add it to the TLB:
::

    trace: 00 tlbwr:    [21]   0051d/   000 ->  0098   -VD- ==> 0070d/  000  -> 00114  -VD-
                ^        ^       ^       ^        ^      ^        ^      ^        ^      ^
            operation  index   page   offset   frame  flags     page   offset  frame   flags
                        \                                  /     \                          /
                         ----------------------------------       --------------------------
                                   ejected page                            new page           

Page 0x51d (the entry at index 21) is randomly chosen to be ejected from the TLB and replaced with page 0x70d.



.. moduleauthor:: Liz Willer <e.willer@unsw.edu.au>

:Date: 2020-02-13