Download CS61C Midterm Exam, UC Berkeley - Electrical Engineering and Computer Sciences and more Exams Structural Analysis in PDF only on Docsity!
University of California, Berkeley – College of Engineering
Department of Electrical Engineering and Computer Sciences
Spring 2005 Instructor: Dan Garcia 2005 - 03 - 07
CS61C Midterm
Last Name
First Name
Student ID Number
Login cs61c-
Login First Letter (please circle) a b c d e f g h i j k l m
Login Second Letter (please circle) a b c d e f g h i j k l m
n o p q r s t u v w z y z
The name of your LAB TA (please circle) Andy Casey Danny Steven
Name of the person to your Left
Name of the person to your Right
All the work is my own. I had no prior knowledge of the exam contents nor will I share the contents with others in CS61C
who have not taken it yet. (please sign)
Instructions (Read Me!)
- This booklet contains 8 numbered pages including the cover page. Put all answers on these pages;
don’t hand in any stray pieces of paper.
- Please turn off all pagers, cell phones & beepers. Remove all hats & headphones. Place your
backpacks, laptops and jackets at the front. Sit in every other seat. Nothing may be placed in the “no fly
zone” spare seat/desk between students.
- Question 0 (1 point) involves filling in the front of this page and putting your name & login on every front
sheet of paper.
- You have 180 minutes to complete this exam. The exam is closed book, no computers, PDAs or
calculators. You may use one page (US Letter, front and back) of notes and the green sheet.
- There may be partial credit for incomplete answers; write as much of the solution as you can. We will
deduct points if your solution is far more complicated than necessary. When we provide a blank, please
fit your answer within the space provided. You have 3 hours...relax.
Problem 0 1 2 3 4 5 6 7 Total Minutes 0 25 30 25 25 25 25 25 180 Points 1 10 14 10 10 10 10 10 75 Score
Question 1: For those about to test, we salute you... (10 pts, 25 min.)
You’re coming straight from a wild pre-midterm toga party and are not quite focused on the exam.
Fear not, this question will get you warmed up and ready to rock, 61C style.
a) You “peek” into a register and find the string “#One”
(from our cheer: Cal is #One! ) as shown here:
What would this be if we interpreted this instead as:
…a single hexadecimal number?
…an instruction? (you may leave arguments in hex)
b) You believe the IEEE 754 Floating Point Standard could use some modification. Instead of 8 bits
for the exponent and 23 bits for the significand, you suggest we give two of the exponent’s bits to the
significand, making it 6 and 25. What are the pros and cons of this proposal?
A pro is that we will have:
A con is that we will have:
c) Suppose we want to add a new Pseudo-Instruction “branch if
float is zero”: bfz. This instruction will take two arguments, a float
(stored in a register) and a label, e.g., bfz $a0 done. It takes the
branch if the floating point number in the register is zero (either
one). To what TAL instruction(s) (from the “core instruction set”)
could an Assembler expand this instruction?
d) For each of the allocation systems on the left, circle the column that describes its fragmentation :
Buddy System causes internal only causes external only causes both types
Slab Allocator causes internal only causes external only causes both types
K&R (Free List Only) causes internal only causes external only causes both types
e) Three sisters are running different programs on identical machines (heap memory size M ) using a
language that supports garbage collection (GC). Each is out of memory and chooses a different
garbage collection scheme. Fill in the table. All answers should be a function of M , e.g., “M/7” or “5M”.
What
is the…
most space their
data could take up
before GCing?
least space their
data could take up
after GCing?
most space their
data could take up
after GCing
most wasted space
that GCing couldn’t
recover?
Reference
counting
Mark and
Sweep
Copying
# O n e bfz $a0 done
Question 2: Three is a magic number! (continued) (14 pts, 30 min)
e) Using the same notion of arbitrary-length
ternary three’s complement numbers stored
in a string, write a C function that returns 1 if
a number is negative and 0 otherwise.
We’ve copied the 3-digit table from the
previous page on the right if that helps.
*int isNegative(char in) { }
Question 3: Potpourri (10 pts, 25 min)
a) Assemble the following code assuming that the label begin corresponds to the address
0x000E0000. You should fill in the blanks below with the hexadecimal value for the instruction (see
Instruction 13 for an example).
begin: addi $v0, $s1, - 4 #Inst 01 beq $v0, $zero, endif #Inst 02 sw $v1, 4($a0) _#Inst 03
There are 9 more instructions here_
endif: subu $t1, $t2, $t3 #Inst 13
b) Given what you know about the Game of Life 1D from Homework 2,
how many possible rules would exist if at each generation we looked at
six cells from the previous generation? Write your answer in IEC format
(e.g., 128 Kibirules, 2 Gibirules, etc…).
c) We’re designing a quad-length floating point number (16 bytes wide)
with 64 yobi different significand bit patterns. How many different
exponent bit patterns would we have? Don’t worry about the numbers we
represent with these bit patterns. (Hint: this isn’t really a question about floating point, it’s a counting
question. If you’re thinking about bias, NaNs, denorms, etc. you’re heading down the wrong path.)
Ternary Representation Base 10 Number
13: 0x014B
Question 4: Float, float on… (10 pts, 25 min)
The staff of CS61C is constantly investigating new approaches and solutions to old problems. (Either
that or we just don’t know when to leave well enough alone!) Well, we’ve come up with a new floating
point format that obeys the rules of the IEEE 754 Floating Point Standard (denorms, NaNs, ±∞)
but is only 13 bits long. It has 1 sign bit, 3 exponent bits, and 9 bits in the fraction (significand):
Answer the following questions about this new float format. As a sanity-check, if you calculate the
bias of this format, you should get 3.
a) What is the largest non-infinite positive number that can be represented? Leave your answer as a
base 10 mixed fraction (i.e. K ± (N/D)) where either K or N can be 0.
b) What is the smallest positive number that can be stored in this format?
Convert the following floating point values in the format above (expressed in hexadecimal) to their
numerical (base 10) equivalents, if appropriate.
c) 0x
d) 0x1F
What is the representation for the following floating point numbers? (Write your answer in hex).
e) - ∞
f) - 4.
S E E E F F F F F F F F F
Question 6: We’re deep, deep undercover… (10 pts, 25 min)
You've been contracted by top secret government agencies to make a really quick, super-portable
message encoder in MIPS! Go spy go! You have to implement an algorithm for encoding an ASCII
zero-terminated (C-Style) string. Your algorithm should be based on the following C code:
void encryptThis(char* cleartext, int* cypher, int* cyphertext_buffer){ if(*cleartext == ‘\0’){ *cyphertext_buffer = 0; }else{ *cyphertext_buffer = *cypher + *cleartext; encryptThis(++cleartext, ++cypher, ++cyphertext_buffer); } }
Implement the above C function in a non-recursive manner in MIPS. Don't clobber any registers
that you shouldn't, $a0 corresponds to cleartext , $a1 corresponds to cypher , and $a2 corresponds to
cyphertext_buffer. You can assume that you will only be passed the usual types of ASCII values (in
the range 0-127). Use as few lines as possible (you may not need to use every blank).
encryptThis:
Question 7: Meet my friend Andy Anderson… (10 pts, 25 min)
**Main: ......
Set up $a
jal foo ...... foo: li $v0, - 1 lbu $t0, 0($a0) beq $0, $t0, done addi $sp, $sp, - 8 sw $ra, 4($sp) sw $t0, 0($sp) addi $a0, $a0, 1 jal foo lw $t0, 0($sp) and $v0, $v0, $t lw $ra, 4($sp) addi $sp, $sp, 8 done: jr $ra**
a) What does the function foo return?
b) In the box above, fill in the C code for the function foo. Be sure to include arguments and return
values, along with their types.
c) If we call your function foo like this: printf(“%c”, foo(“Cal”)); What will be printed?
d) What would foo do if we changed its first line to read “ li $v0, 0 ”?
foo( ) { return ; }
