MIPS Simulator: Data Cache
Project 5 – CS 3339
In this project, you will further enhance your simulator to model pipeline stalls due to memory latency, and
you will simulate a data cache to study the performance impact of caching. You should begin with a copy of
your Project 3 submission. Additionally, I have provided a class skeleton for a CacheStats class intended to
be instantiated inside your existing CPU class. You may add any functions, function parameters, etc. to it
that you want.
The simulated data cache should store 1 KiB (1024 Bytes) of data in block sizes of 8 words (32 bytes). It
should be 4-way set associative, with a round-robin replacement policy and a write policy of write-back
write-allocate. All blocks in the cache are initially invalid. For simplicity, assume the cache has no write
buffer: a store must be completely finished before the processor can proceed. Since this is a data cache,
only loads and stores access it; instruction fetches should still be assumed to hit in a perfect I-cache with
immediate access (i.e., there is never a stall for an instruction fetch).
Note that since you only have to model the hit/miss/timing behavior of the cache, you do not have to
actually store any data in your cache model. It is sufficient to simulate the valid, tag, and dirty bits as well
as the round-robin replacement policy.
Your cache model will calculate and report the following statistics:
• The total number of accesses, plus the number that were loads vs. stores
• The total number of misses, plus the number caused by loads vs. stores
• The number of writebacks
• The hit ratio
Every time an access is made to the cache model, the cache model should return the number of cycles that
the processor must stall in order for that access to complete. It takes 0 cycles to do a lookup or to hit in the
cache (i.e., data that is hit will be returned or written immediately). A read access to the next level of the
memory hierarchy (e.g., main memory) has a latency of 30 cycles, and a write access has a latency of 10
cycles. Note that an access resulting in the replacement of a dirty line requires both a main memory write
(to write back the dirty block) and a read (to fetch the new block) consecutively. Because the cache has no
write buffer, all stores must stall until the write is complete.
Before computing the cache statistics, be sure to drain all the dirty data from the cache, i.e. write it
back. Count these as writebacks, but do not count any stalls/latency resulting from them.
Inside your Stats class, add a new function similar to the bubble() and flush() functions. This stall()
function should stall the entire pipeline for a specified number of cycles. The Stats class should track the
total number of stall cycles that occur during program execution.
CS3339 Texas State University 2 Copyright 2019 Lee B. Hinkle, all rights reserved
Your simulator will report the following statistics at the end of the program:
• The exact number of clock cycles it would take to execute the program on this CPU
• The CPI (cycle count / instruction count)
• The number of bubble cycles injected due to data dependencies (unchanged from Project 3)
• The number of flush cycles in the shadows of jumps and taken branches (also unchanged from
Project 3 – not the values from Project 4)
• The number of stall cycles due to cache/memory latency, new for Project 5
• The data cache statistics reported by the cache model
I have provided the following new files:
• A CacheStats.h class specification file, to which you’ll need to add member variables
• A CacheStats.cpp class implementation file, which you should enhance with code to model the
described cache and count accesses, misses, and writebacks
• A new Makefile
In addition to enhancing the CacheStats.h/.cpp skeleton, you will need to modify your existing
Stats.h/Stats.cpp in order to implement memory stalls. You will also need to modify CPU.h to
instantiate a CacheStats object, and CPU.cpp to call both CacheStats and Stats class functions
appropriately to model cache behavior and resulting pipeline stalls. You’ll also need to change
CPU::printFinalStats() to match my expected output format (see below).
Here are the steps you should follow for this project:
1) Begin by copying all of your Project 3 files into a new Project 5 directory
2) Untar and add the additional files from TRACS to your project5 directory
3) Write your name in the header of CacheStats.cpp and CacheStats.h
4) Add the #include for CacheStats.h into your CPU.h file
5) Instantiate a CacheStats object named cache in your CPU.h similar to your stats object
6) Complete the functions for stats.stall, cache.access, and others as needed.
7) Modify CPU.cpp to call cache.access HINT: you probably want to do this in CPU::mem()
8) Also modify CPU.cpp to call cache.printFinalStats() and remove any unnecessary output
9) Check your results using submit_test script
10)Upload to TRACS before the deadline – verify by getting the email confirmation
You can compile and run the simulator program identically to previous projects, and test it using the same
*.mips inputs. Only sssp.mips yields interesting cache behavior; I’ll only grade your code using this
If you examine CacheStats.h, you’ll notice that I’ve already defined constants for you for all of the cache
configuration options you’ll need (e.g., number of sets, number of ways, block size, read miss latency, etc.).
You should not need to change any of these defines. They are defined to be modifiable from the
compilation command line, but for this project, I will not change any of them. You can even get away with
not using these defined constants if you prefer.
CS3339 Texas State University 3 Copyright 2019 Lee B. Hinkle, all rights reserved
The following is the expected result for sssp.mips. Your output must match this format verbatim. Compare
your output to the provided sssp.out file in the tarball using the diff command as in prior projects:
CS 3339 MIPS Simulator
Cache Config: 1024 B (32 bytes/block, 8 sets, 4 ways)
Latencies: Lookup = 0 cycles, Read = 30 cycles, Write = 10 cycles
Program finished at pc = 0x400440 (449513 instructions executed)
Load misses: 8559
Store misses: 3485
Hit Ratio: 93.9%
The CacheStats skeleton already includes code to disable the cache (i.e., all loads result in a read access to
the next level of the memory hierarchy, and all stores result in a write access). To explore the performance
impact of adding a cache, you can compile your simulator with the cache disabled:
$ make clean; make CACHE_EN=0
Re-run the simulator on sssp.mips. What happens to the CPI? How big is the difference?
• Your code must compile with the given Makefile and run on zeus.cs.txstate.edu
• Your code must be well-commented, sufficient to prove you understand its operation
• Make sure your code doesn’t produce unwanted output such as debugging messages. (You can
accomplish this by using the D(x) macro defined in Debug.h)
• Make sure your code’s runtime is not excessive
• Make sure your code is correctly indented and uses a consistent coding style
• Clean up your code before submitting: i.e., make sure there are no unused variables, unreachable
CS3339 Texas State University 4 Copyright 2019 Lee B. Hinkle, all rights reserved
Submit all of the code necessary to compile your simulator (all of the .cpp and .h files as well as the
Makefile) as a compressed tarball. You can do this using the following Linux command:
$ tar czvf yourNetID_project5.tgz *.cpp *.h Makefile
Do not submit the executables (*.mips files). Any special instructions or comments to the grader,
including notes about features you know do not work, should be included in a separate text file (not inside
the tarball) named yourNetID_README.txt.
Use the script file submit_test provided for you on TRACS to confirm the contents of your tar file, ability to
compile and execute on zeus, and check your output format using sssp.mips. The instructions are in the
script itself. Submissions that do not untar correctly and compile on zeus with this script will receive
zero points so you are highly encouraged to check your submission.
All project files are to be submitted using TRACS. You may submit your file(s) as many times as you’d like
before the deadline. Only the last submission will be graded. TRACS will not allow submission after the
deadline, so I strongly recommend that you don’t come down to the final seconds of the assignment
window. Late assignments will not be accepted.
ACADEMIC HONESTY (excerpt from course syllabus)
You are expected to adhere to the Texas State University Honor Code which is described here
All assignments and exams must be done individually, unless otherwise noted in the assignment
handout. Turning in an exam or assignment that is not entirely your own work is cheating and will not be
Group discussion about course content is NOT cheating and is in fact strongly encouraged! You are
welcome to study together and to discuss information and concepts covered in class, as well as to offer (or
receive) help with debugging or understanding course concepts to (or from) other students. However, this
cooperation should never involve students possessing a copy of work done by another student, including
solutions from previous semesters, other course sections, the Internet, or any other sources.
Turning in an assignment any part of which is copied from the Internet, another student’s work, or any
other non-approved source will result in a 0 grade on the assignment and will be reported to the Texas
State Honor Code Council. Should one student copy from another, both the student who copied work and
the student who gave material to be copied will receive 0s and be reported to the Honor Code Council. You
should never grant anyone access to your files or email your programs to anyone (other than the
ACKNOWLEDGEMENT: This assignment, handout, and provided code are based on prior work by Molly
O’Neil and Martin Burtscher. I am grateful for their support.