CS644 HW2 -- Eric Christiansen   mailto:tericc@mac.com   508-496-1065


1. Summary of the implementation of the pc sampler

PC Sampler provides three routines in its API: PCsamp_on(), PCsamp_off(), 
and PCsamp_histogram().

PCsamp_on() sets up the interrupt gate for TIMER0 to call the interrupt 
handler irq0inthandc(), enables the interrupt controller for TIMER0, and 
finally sets up the timer circuit (PIT) to regularly interrupt at a 
specified microsecond interval.  The startup.c routine modified for this 
test calls PCsamp_on with an interval of 500 microsecs.  (After main() 
is called in the startup.c routine it calls PCsamp_off and 
PCsamp_histogram.)

irq0inthandc() called by the timer interrupt reads the PC off the stack 
by way of StackLayout structure it overlays on top of the register save 
area used by the interrupt mechanism.  It stores the PC in the next 
location of a 10,000 item ring buffer which is later read by the 
histogram routine.

PCsamp_off() simply disables the interrupt controller.

PCsamp_histogram() sorts the list of PCs captured and counts how many 
are in each consecutive group.  It then prints out the results.



2. Results on pctest, interpreted by symbolic values for at least
   the top three locations/regions, incl. library locations

[Note from eoneil: the text-graphics here are "extra"]
From the following output:

     Saw 12181 interrupts.

          PC  | # of ints 
     =========|===========
        5a61b |  304   :oooooo
        5a61d |  298   :oooooo
       100427 | 1416   :oooooooooooooooooooooooooooo
       10042e | 1793   :oooooooooooooooooooooooooooooooooooo
       100434 | 1415   :oooooooooooooooooooooooooooo
       100437 | 1793   :oooooooooooooooooooooooooooooooooooo
       100483 |  723   :oooooooooooooo
       10048a |  626   :ooooooooooooo
       100490 |  796   :oooooooooooooooo
       100493 |  707   :oooooooooooooo
       1005ee |    3   :
       1005f5 |    3   :
       1005fc |  119   :oo
       1005ff |    4   :

most of the interrupts occured in the main() routine.  The addresses 
100427, 10042e, 100434, and 100437 are from the first "for" loop of 
main() (looping for 300*MILLION at line 11).  The addresses 100483, 
10048a, 100490, and 100493 are from the last loop of main() at line 
18.  More interrupts were caught in the first loop.  Some of them 
were probably over written by the ring buffer.



3. Discussion of accuracy and relevance of results

With the sample rate set at 0.5 ms and with a 400 MHz processor 
approximately 20,000 instructions executed between samples.  But 
with the test program set up as it was with three "slow" loops the 
general characteristic of the test program could be captured.