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Assignment 6 Static Control Flow and the Stack

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CPSC 213 – Assignment 6
Static Control Flow and the Stack

Learning Objectives
Here are the learning objectives of this assignment, which will be examined in this week’s quiz.
They are a subset of the unit learning objectives listed on Slide 2 of Unit 1c.
After completing this assignment you should be able to:
1. translate C code containing for loops into SM213 assembly language;
2. translate C code containing if-then-else statements into SM213 assembly language;
3. identify procedure calls and returns in SM213 assembly language and describe their
semantics by writing equivalent C procedure call and return statements; and
4. explain how to use the stack pointer to access local variables and arguments.
Goal
This assignment has four parts.
First, you will extend the SM213 implementation to add the instructions we have discussed in
class that support static control flow, including static procedure calls and procedure return (which
is dynamic, actually).
Second, you’ll examine the assembly code for for-loops and if-statements using snippet files in the
simulator and then translate a simple C file containing these control-follow statements into
assembly.
Third, you will write a fairly substantial assembly-language program that uses all of the language
concepts we have discussed so far.
Finally, you will examine how programs use the runtime stack to store local variables, arguments
and the return address. You will do this using a set of snippets and you will answer questions
about their execution. Then, you will examine two SM213 programs that contain procedure calls
to determine what they do.
Question 1: Extending the ISA [10%]
Implement the following six control-flow instructions in CPU.java. The code provided with this
assignment in www.ugrad.cs.ubc.ca/~cs213/cur/assignments/a6/code.zip includes the file
CPU.java that you can use as the starting point for this assignment. Alternatively you can use the
version you implemented yourself for Assignment 2.
Note that in the “Format” column below, capital letters are hex digit literals and lower-case letters
represent hex values referenced in the “Assembly” and “Semantics” columns.
Note that the indirect-jump offset is unsigned and so for indirect jump, pp ranges from 0 to 255.
On the other hand, the branch pc-relative value is signed and so for branches, pp ranges from
-128 to 127.
Question 2: Assembly code of if and loops [20%]
Example the Execution of Assembly Snippets
The file www.ugrad.cs.ubc.ca/~cs213/cur/assignments/a6/code.zip contains the following files:
• S5-loop.{java,c,s}
• S5a-loop-unrolled.{c,s}
• S6-if.{java,c,s}
Run each of these snippets through the simulator – your version if you completed Question 1
correctly, or the reference implementation if you didn’t. Carefully observe what happens as they
execute so that you could explain this code to someone if they asked. This step is not for marks.
Q1: Translate C to assembly
Translate q2.c into commented assembly code, placing your code in a file named q2.s. Labels
for variables should be the name of the variable. Run your code in the simulator and examine
every variable to be sure they have the correct value when execution halts.
Instruction Assembly Format Semantics
branch br A 8-pp pc ← (A = pc + pp*2)
branch if equal beq rc, A 9cpp pc ← (A = pc + pp*2) if r[c]==0
branch if greater bgt rc, A Acpp pc ← (A = pc + pp*2) if r[c]0
jump j A B— AAAAAAAA pc ← A
get pc gpc $o, rd 6Fpd r[d] ← pc + (o == p*2)
indirect jump j o(rt) Ctpp pc ← r[t] + (o == pp*2)
Question 3: Write a program in assembly [50%]
Implement an assembly-language program that examines a list of student grades and finds the
student with the median average grade; this student’s id should be placed in the variable m.
Your program must correspond to a C program where the input list of students, and the output
student id are given as follows.
struct Student {
int sid;
int grade[4];
int average; // this is computed by your program
};
int n; // number of students
int m; // you store the median student’s id here
struct Student* s; // a dynamic array of n students
Your assembly file must format student records exactly as C would; i.e., a struct with 6 integers.
You must use the following assembly declarations for this input and output data. Note: this
example is for an array with one student. You should obviously test your code on larger arrays.
n: .long 1 # just one student
m: .long 0 # put the answer here
s: .long base # address of the array
base: .long 1234 # student ID
.long 80 # grade 0
.long 60 # grade 1
.long 78 # grade 2
.long 90 # grade 3
.long 0 # computed average
Put your solution in a file named q3.s.
Work Incrementally – Do each of these steps separately!
This is a very challenging programming problem. It will stretch (and strengthen) your ability
manage a large number of programming details, as is required when writing assembly. To be
successful with this you must be very disciplined to program and test incrementally.
Observe that the program breaks down into several subproblems. You should tackle them one at a
time and thoroughly test each step before moving to the next.
Here’s a list of the key subproblems. You might want to further sub-divide things, but do not try to
combine steps.
1. Compute the average grade for a single student and store it in the struct. For simplicity, you
can ignore the fractional part of the average; i.e., you do not need to round.
2. Iterate through the list of students and compute their average grades and store them.
3. Swap the position of two adjacent students in the list.
4. Compare the average grades of two adjacent students and swap their position conditionally,
using your code from Step 3.
5. Now consider creating a procedure to encapsulate either Step 3 or Step 4 as described in the
Use Procedures section below.
6. Sort the list by average grade in ascending order. You are free to use any sort algorithm you
like, but Bubble Sort is the simplest. Here’s a version of bubble (sinking) sort in C that you
might consider.
void sort (int* a, int n) {
for (int i=n-1; i0; i–)
for (int j=1; j<=i; j++)
if (a[j-1] a[j]) {
int t = a[j];
a[j] = a[j-1];
a[j-1] = t;
}
}
Take this step by step and incorporate your work from Step 3-5 above. For Bubble Sort,
here are two subproblems.
a. First, write a loop that iterates through the list once, bubbling the student with the
lowest average to the top (or sinking the lowest student to the bottom). If you created
a procedure in Step 5, then the inner part of this loop is a call to this procedure;
otherwise it is the code from Step 4.
b. Then, add the outer loop that repeats the inner loop on the unsorted sublist repeatedly
until the entire list is sorted.
7. Find the median entry in the sorted list and store that student’s sid in m. For simplicity you
can assume the list contains an odd number of students.
Ordering and Combining Steps
Notice that it is not necessary to do these steps in the order listed above. Steps 1 and 2, for
example, are completely independent from Steps 3-7. Step 7 is independent from the other steps.
You could do Step 6 before Steps 3-5 and then incorporate the conditional swap once the other
parts of Step 6 are working. Similarly, you can do Step 5 before Steps 3 or 4 and incorporate them
into the procedure later. The key thing here is work on each piece independently and then careful
combine steps to eventually produce the program. The program itself will be on the order of 70 to
100 assembly-language instructions. That sounds like a lot … and it is. But if you think of this as
seven steps each with 15 or so instructions (some will have less), its not so bad. Or maybe it will
be bad, but at least not impossible.
Be sure to comment every line of your code and to separate subproblems with comments and
labels so that it is easy for you to see what each part of the code does without having to read the
code.
Multiplying and Dividing by 24
You will have noticed that the variable s is a dynamic array of type “struct Student” and
that “sizeof(struct Student)” is 24. And so you might find that you’d like to multiply or
divide by 24 (i.e., to convert between an array index and a byte-offset), but you’ll recall that you
can’t do this with a single instruction in our ISA. Then you’ll notice that x*24 = x*16 + x*8 and
you’ll see that you can multiple by 24 (or divide) using 3 instructions (or 4 depending on how you
count).
Register Allocation
Keeping track of registers is going to be a challenge. Note that this program is too big for you to
use a distinct register for every value in the program. You are going to have to re-use registers to
store different things in different parts of the program.
This will add complexity, for example, when combining two steps or when adding a step to the
rest of the program. For example, if your code for Step 4 uses register r0 and one of the loops
you write in Step 6 also uses r0, you’re going to have to change one of them to use a different
register (or use a procedure as described in the next section). So, for parts of the code that connect
like this, some pre-planning will help.
Another useful strategy is to group sections of code (one or maybe a few steps) and treat them as
independent stages with respect to registers. At the beginning of a stage you assume nothing about
the current value of registers and you are free to use any registers you like within that stage. At the
end of the stage, any register values that are needed by subsequent stages should be written to
memory. You may want to create some additional variables to store these temporary values.
Using Procedures
One way to divide code into stages is to use procedures. Step 5 suggests that you do this to
encapsulate the code that conditionally swaps two adjacent elements of the list (i.e., Steps 3 and 4).
You can use this approach to simplify register management by having the procedure save registers
to memory before it starts and restore them from memory before it returns (e.g., using temporary
variables). Any register the procedure saves in this way is a register it is free to use without
interfering with the registers used in the two loops in Step 6 that call it.
This swap procedure needs one argument / parameter: i.e., the index of one of the array elements
to swap. The index of the other element is this value plus or minus one, depending on how you do
it. The caller should pass this value in a register; for example if the loop has the index in r4 then
the procedure should just use r4 to get this value.
Use gpc and j to call the procedure and use the indirect jump to return. Do not worry about
creating a stack.
All of this is optional, and you can do this for other parts of your program as well.
Partial Marks — Work Incrementally
Finally, partial marks (if you need them … probably not!) will be awarded based on the number of
the steps listed above that you’ve written correctly. Submitting a big blob of assembly that doesn’t
do any of the steps right won’t be worth anything. So, work incrementally. Test each step
separately. Once you have a step working, save it in an auxiliary file just in case.
Question 4 – The Stack [20%]
The file provided code contains the following files.
• S7-static-call.{java,c} and S7-static-call-{stack,reg}.s
• S8-locals-args.{java,c,s}
• S9-args.{java,c} and S9-args-{stack,regs}.s
Evaluating Snippets using Simulator
Familiarize yourself with snippets 7-9 by running them in the simulator and asking yourself the
following questions. This part is not for marks.
• Carefully examine the execution of S7-static-call-stack in the simulator and compare
it to S7-static-call-regs. Run the snippets and look carefully at what happens. Ask
yourself these questions.
a) What is the difference between the two approaches?
b) What is one benefit the approach followed in stack?
c) What is one benefit of the approach followed in reg?
• Carefully examine the execution of S8-locals-args.s in the simulator. Ask yourself these
questions.
d) What lines of foo and b allocate b’s stack frame?
e) What the lines of foo and b de-allocate b’s stack frame?
The next two questions ask you to consider changing b so that it has 3 arguments and 4 local
variables. Ask yourself these questions.
f) What changes required in b to add the arguments and locals; note that you do not
actually use these new variables in any way?
g) What changes required in foo to call b(0,1,2)?
• Carefully examine both versions of S9-args-stack.s and S9-args-regs in the
simulator. Ask yourself these questions.
h) What memory accesses does stack makes that regs doesn’t make?
i) How many more memory accesses does stack make, compared to regs?
Questions 4a and 4b [20%]
The next two question use these files found in code.zip.
• q4a.s
• q4b.s
Answer the next two questions by modifying the .s files and by writing .c files. The .c files
must compile and execute. When they execute they must perform the same computation as the .s
file and print out the value of its static variables, one per line as a decimal number (nothing other
than that number on each line). This means that q4a.c must print 10 lines of numbers and
q4b.c must print 16.
1. [10%] Examine q4a.s and its execution in the simulator. Add a comment to every line
that explains what that line does in as high-level a way as possible.
Then, write an equivalent C program called q4a.c that is the most likely candidate for the
file that was compiled into q4a.s. Use the same variable and procedure names in this
program that you used in the assembly-file comments. Ensure that there is a
correspondence between lines in the assembly file and lines of the C program, but do not
include the start procedure in q4a.c; this procedure is added automatically by the c
compiler to initialize the stack and call main. The end of your main procedure should
print the value of the program’s ten static variables as described above.
You may notice a register that is used in a way that is best explained by saying that it is a
local variable, even if its value is never read from or written to the stack. Avoiding these
memory accesses is a common optimization compilers make for local variables.
2. [10%] Do the same for q4b.s.
What to Hand In
Use the handin program.
The assignment directory is ~/cs213/a6, it should contain the following plain-text files.
1. PARTNER.txt containing your partner’s CS login id and nothing else (i.e., the 4- or 5-
digit id in the form a0z1). Your partner should not submit anything.
2. CPU.java that contains your implementation of the new instructions listed in Step 3 as
well as a correct implementation of the part of the ISA you implemented previously.
3. q2.s that contain your answer to Question 2.
4. q3.s that contains your answer to Question 3.
5. q4a.s, q4a.c, q4b.s, and q4b.c that contain your answers to Question 4.

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