Lab #5: Semaphores and Statistics
As the next step in the creation of the YAK kernel, you are to add
functions to your existing code so that you can correctly execute the
application program lab5app.c. You are free to
make changes in the kernel code you wrote for the last lab, but you
should not need to change the application program provided. You are
expected to complete this lab with a partner.
The added functionality comes in two forms: (1) routines to create,
pend on, and post to semaphores; and (2) a support mechanism to track
the number of context switches and the CPU utilization.
You must add these kernel functions to your kernel. You should
implement each function exactly according to the prototype detailed on the YAK
Kernel webpage. Refer to that web
page for additional details about these functions.
- YKSemCreate - Create and initialize a semaphore
- YKSemPend - Obtain a semaphore
- YKSemPost - Release a semaphore
In addition to these functions, your ISRs and interrupt handlers
should function as in the last lab, with one exception: You should
modify your keypress interrupt handler so that it calls
YKSemPost(NSemPtr) if the 'p' key is pressed. Other keypresses
should behave as before.
You also need to make sure that your kernel code defines and maintains
the global variables YKCtxSwCount and YKIdleCount, both unsigned integers
that track information about the current workload that the user might
be interested in (see the YAK web page). YKCtxSwCount should simply
be incremented each time a context switch occurs, and the idle task
should repeatedly increment YKIdleCount in its while(1) loop. These
variables are used to output simple statistics in the application code
provided with this lab.
When the application is run correctly, it should output the text
"Hey, it works!" every 6 ticks. When the 'p' key is
pressed, it should output a long series of prime numbers. The CPU
usage should be fairly low (less than 10%) at a tick rate of 10,000
ticks. Output from a working version of the lab is shown below.
Welcome to the YAK kernel
Determining CPU capacity
"Hey, it works!"
"Hey, it works!"
1009 1013 1019 1021 1031 1033 1039 1049 1051 1061 <- User pressed 'p' key
1063 1069 1087
1091 1093 1097 1103 1109 1117 1123
1129 1151 1153 1163 1171 1181 1187 1193 1201 1213
1229 1231 1237 1249 1259 1277 1279 1283
1289 1291 1297 1301 1303 1307 1319 1321 1327 1361
1369 1373 1381 1399 1409 1423 1427 1429 1433
1439 1447 1451 1453 1459 1471 1481 1483 1487 1489
"Hey, it works!"
Implement the required functions and make sure that the application
program runs correctly. The system should run correctly when key
presses occur in rapid succession and when the timer tick frequency is
increased to every 750 instructions.
When you are confident that your program
works correctly demonstrate the execution of your program to a TA.
Since you must demonstrate working code to a TA on or before the due date, please consider
their lab schedule well in advance.
In addition to the demonstration, you should submit via Learning Suite a .txt or .pdf document with
a written summary of problems you encountered, if any, and the total
number of hours you spent on this lab. (A realistic estimate is
sufficient.) You are also invited to include suggestions on
additional information that could have been included on the web pages,
or additional simulator features that would have cut your debugging
- The operation of the tasks in this lab are intended to give you
some insight into possible uses of semaphores. Note how the word,
space, and punc(tuation) tasks cooperate to output a message, using
semaphores to force a particular execution sequence. Pressing the 'p'
key should fire up (via a semaphore) the task that computes primes, so
you should see substantial differences in CPU utilization during those
periods of time when it is running. Make sure you understand the
simple mechanism used to calculate CPU utilization.
- The file "yakk.h" that the application code includes makes
available to the user code any struct or constant definitions that you
(as the kernel designer) need to provide. Part of your assignment is
to add the YKSEM definition to this file. This approach hides the implementation details of
the semaphores from the user code, while still making them available
to the application program.
- Pacing yourself. Here is a statistical summary of hours
of work reported by each group completing this lab in Fall 2010:
- Low: 1.2 hours
- High: 10.0 hours
- Average: 3.9 hours
- Amount of code to be written. For comparison, here are
the sizes of my source files for my lab5 code for the 8086:
- myinth.c: 26 lines
- myisr.s: 32 lines
- yaku.h: 4 lines
- yakk.h: 37 lines
- yakc.c: 266 lines
- yaks.s: 65 lines
Here are comments from student reports (edited only for length)
describing their problems and how they tracked them down. As always,
take them with a grain of salt.
- YKSemPend was missing the YKEnterMutex and YKExitMutex calls
to make it atomic.
- *semaphore++ incremented the pointer value when we thought it
incremented the actual value for the semaphore. (*semaphore)++ fixed it.
- Remember to change your MAXTASKS #.
- We didn't make sure that your idle task is the lowest priority.
- The only problems we came across were with compilation. Our global
variables were declared in our .h file, so every time we #included yakk.h,
they were redeclared.
- When you move TCB's between lists such as the delayed or sempending
lists make sure you change all of the pointers…prev and next.
- We also had a problem creating our semaphores. We initially didn't
use an array of semaphore structs, so we kept overwriting our semaphores.
- We needed to make sure we didn't call YKScheduler until after we called YKRun,
especially from YKExitISR.
- We had two major bugs. We left some bad code in YKSemPend, and it
was trying to put the task on the blocked list twice. The other bug
was that we weren't cheking YKinterruptLevel in YKSemPost, and so
called the scheduler without doing and EOF.
- The most major bug we had in our code was in our SemPend routine.
We were only taking a semaphore if it was available, else we were
blocking the task on the semaphore. This caused problems in the case
when we block a task on a semaphore, then pend the semaphore from
another task. This unblocks our original task and resumes execution.
But since we were in the else portion of the if statement, we never
take the newly posted semaphore. This caused the our code to think
there was one more semaphore than there actually was. We fixed this
by replacing the "if(semaphore avail) then get it, else block code"
with "get semaphore, then if(semaphore not avail) block" This fixed
our erratic output.
- The biggest problem I had was my linked list manipulation. In my
data structures for the semaphores, there is a linked list containing
the tasks waiting for the semaphores. When adding new tasks to the
waiting list, I always add them to the back (and remove the highest
priority when it unblocks) but I did not nullify the link to the next
element in the list of the new unit when the initial waiting list
status was NULL. After a few iterations, the list would duplicate
itself several times. Simply correcting the list function corrected
that. The keypressed interrupt would also cause a bug, but it was
very desultory and usually occurred only after I held the key for 30
seconds or more. Disabling interrupts in my YKDelayTask fixed that.
- One problem we ran into was having maximum number of tasks set at
3, from last lab, which cause a lot of problems until we fixed it.
- The biggest problem we had was not with the semaphores, but a bug
that finally came out now. We were restoring context in ISRs then
calling YKExitISR; this was a terrible mistake. We fixed it and there
were no more weird interrupt problems.
- We forgot to disable interrupts in our YKSemPend/Post functions.
- If we used our YKSaveContext function and then manipulated a few
more variables (either local or non-local) before calling the
YKScheduler, then the pointers 'bp' and 'sp' began pointing at all
sorts of odd things. So, we made sure to modify all variables and do
any clean up that needed to be done before calling our YKSaveContext
function, and that was followed immediately by YKScheduler.
- I am used to C++, which allows you to declare 'int' inside of a
for-loop statement -> for(int i = 1;i<10;i++). However, our compiler
does not function properly if you follow this syntax. It does not
declare any errors, but it never enters the loop. To fix this, merely
declare the variable at the beginning of a function and use a simple
statement -> for(i = 1;...etc.).
- We are using an Array for our TCB's, so our scheduler loops
through each of the tasks and finds the highest-priority ready task.
However, the upper limit to our loop was the constant MAXTASKS. When
we called the scheduler before there were MAXTASKS created, then we
would be searching through garbage for some of the loop, which
returned bad values and caused the kernel to crash if we ever set our
tick number below 1530.
- When we moved a task from the delay task list into the running
task list it erased a task already in there.
- The bug that killed us this time was that when the keypress
handler posted a semaphore, it didn't set to ready the highest
priority task blocked on the semaphore, but left that to the
- Another nasty bug was that we signalled in YKExitISR that we need
to restore context of a task in all interrupts, instead of only when
the ISRCallDepth is 0. This caused the context of an interrupt to be
restored instead of the task, when the task was dispatched.
- The problems we had were modifying our semaphore pends and posts.
We also had to hack a bit of code since the tasks weren't created in
priority order. We had to make sure our priority array was in
- Our dispatcher appeared to work in lab4, but it really didn't. We
were trying to save our ip in a different place than our registers and
our sp. This is a bad idea.
- We forgot to set the runstate to ran for the first task ever run.
- The problems we encountered were in our tick handler. We were not
pushing and popping the registers after the interrupt. It was a
pridiculous problem. We spent between 20 and 25 hours combined on the
- The main problems I encountered were from lab 4 code that was not
caught through testing until now. I found that my partner and I had
incorrectly written the ISRs for lab 4. For lab 5 I was using a
priority queue to keep track of all tasks that were blocked on each
semaphore. This made it easy to pop of the highest priority task from
the semaphore when it got posted to but I was forgetting to check for
null pointers so I was adding null pointers to the ready queue which
was causing the program to access invalid code.
- I was inserting new tasks TCBs with an insertion sort, but I
forgot to update the pointer to the running task.
- I was not setting the semaphores to the initial value.
- We had a problem with our kernel working fine up to a certain
number of ticks and then it would stop printing out the "Hey, it
works!" line after every 6 ticks. We checked the semaphores and they
seemed to have the right values and everything and we checked a ton of
stuff. After many, many hours, we realized that out stack was growing
too big and that we were overwriting part of the next stack. (When we
changed the stack sizes to be bigger, it would print out the "Hey..."
line every 6 ticks up to a higher number of ticks...) Our approach
had been to store the context of the current task within the kernel
functions which could potentially cause another higher priority task
to be ready than the one currently running and then at the end of the
task, we would check if it had changed, and update the stack pointer
in the TCB to the appropriate spot. Then our dispatcher simply
reassigned the stack pointer to the value in the TCB of the highest
priority ready task, popped the context, and didn't worry at all about
saving context. One big problem was accounting for parameters passed
to functions (such as in DelayTask and SemPost and SemPend and
NewTask) and that once control returned to them, the next instruction
was to increment the stack pointer to get rid of the space where the
parameters passed would have been. This messed us up, since we were
rearranging the stack to have our context popped on before the stack
frame for the kernel function. In any case, we're re-structuring our
kernel to save context in the Dispatcher rather than in the individual
functions that might change the highest priority ready task.
- One problem was with the function that would take the TCB that is
being delayed and put it in the Suspended List. It would only take the
very first TCB in our YKRdyList and put it in the YKSuspList. So when
the application code finished running it would delay the punctuation
task but put the highest priority task in the YKSuspList, being the
word task. So we made the proper changes in the function to take into
account the fact that the task being delayed is not necessarily the
highest priority task.
- We forgot to save context in Post and Pend.
Last updated 8 November 2017