ECE 391 Exam 1, Fall 2010
Tuesday 28 September
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• Be sure that your exam booklet has 11 pages.
• Write your name at the top of each page.
• This is a closed book exam.
• You are allowed one 8.5× 11″ sheet of notes.
• Absolutely no interaction between students is allowed.
• Show all of your work.
• Don’t panic, and good luck!
Problem 1 25 points
Problem 2 25 points
Problem 3 25 points
Problem 4 25 points
Total 100 points
Problem 1 (25 points): Short Answers
Please answer concisely. If you find yourself writing more than a sentence or two, your answer is probably wrong.
Part A (5 points): Recall the user-level test harness provided for your use with MP1. Describe one advantage and
one disadvantage of developing and using such a testing strategy when writing new kernel code, relative to doing all
testing of the new code directly in the kernel.
Part B (5 points): When an interrupt occurs in a generic cascade PIC configuration, both the master and slave
don’t know the correct vector number to send to the CPU. Explain why this is from each of the individual PIC’s
viewpoints and how this issue is addressed in the 8259A PIC design?
Problem 1, continued:
Part C (5 points): After a call returns from a C function, why should you remove its arguments off the stack AND
not reuse those values?
Part D (5 points): Your friend suggests that mapping the 8259A interrupts into the 0x00 to 0x0F range of the
Interrupt Descriptor Table can save time on translations between vector number and IRQ. Explain why such a mapping
will not work as intended.
Problem 1, continued:
Part E (5 points): Several programs call the functions below in an arbitrary order. These programs share a single
copy of the global variables lock and rnum. Can the call to printf ever print a number less than 1000? Explain.
spinlock_t lock = SPIN_LOCK_UNLOCKED;
unsigned int rnum = 0;
void generate()
spin_lock (&lock);
rnum = rand(); // Generate a random number from 0 to (2ˆ32 – 1)
spin_unlock (&lock);
void check()
int cond = 0;
spin_lock (&lock);
if (rnum >= 1000)
spin_unlock (&lock);
if (1 == cond)
printf(“the number is %d\n”, rnum)
Problem 2 (25 points): Calling Conventions and the Stack
Part A (10 points):
To improve the search speed of the mp1 blink struct list, the linked list is to be converted into an array. You
have been provided a skeleton convert to array function that takes in the current number of elements in the
singly-linked list (list length), allocates enough space for an array that will contain all of the current list nodes,
fills in the array, and returns a pointer to the array.
void add_to_array(mp1_blink_struct *array, mp1_blink_struct *current,
int filled_array_elements);
void* mp1_malloc(int size);
mp1_blink_struct* convert_to_array(int list_length);
Complete convert to array by filling in the call to add to array following the appropriate calling convention.
The code is on the following page. If you find yourself writing more than 15 lines of code, you have probably made a
convert to array will iterate through every element starting at mp1 list head and call the add to array
function. add to array takes:
• array – the pointer to the allocated array of mp1 blink structs
• current – the mp1 blink struct to add to array
• filled array elements – the current number of elements copied into array
Problem 2, continued:
.long mp1_list_head
convert_to_array:
pushl %ebp
movl %esp,%ebp
pushl %ebx
pushl %esi
pushl %edi
movl 8(%ebp), %eax
movl mp1_list_head, %edx
imull $STRUCT_SIZE, %eax
pushl %eax
call mp1_malloc
addl $4, %esp
cmpl $0, %eax
je MALLOC_FAIL
movl $0, %ecx
CHECK_NEXT:
cmpl $0, %edx
je DONE_INSERTING
/* Insert call to add_to_array here */
movl %edi, %edx
jmp CHECK_NEXT
DONE_INSERTING:
MALLOC_FAIL:
pushl %eax
call mp1_free
addl $4, %esp
movl $0, %eax
jmp DONE_INSERTING
Problem 2, continued:
Part B (15 points):
The figures below are the state of the stack before the execution of the given code. Please fill in the state of the stack
after execution and also indicate where %ebp and %esp are pointing to. Treat registers whose values you do not know
as variables, otherwise fill in the actual number. We give you multiple figures for scratch work purposes but one must
contain your final answer. Circle the stack you want us to grade!
If you do not circle a stack, we will give you a zero on the problem.
pushl %ebx
pushl %ecx
pushl %ebp
movl %esp, %ebp
pushl %ebx
pushl %esi
pushl %edi
addl $-8, %esp
movl 8(%ebp), %edi
movl 12(%ebp), %esi
xorl %ebx, %ebx
movl %ebx, 4(%esp)
addl $4, %esi
imull %esi, %esi
addl $-4, %esp
movl %esi, (%esp)
Problem 3 (25 points): Synchronization
As part of your first job, you are charged with implementing a more fair lock construct. Specifically, you must
implement a bakery lock (also called a ticket lock) based on the Linux semaphore.
A bakery lock is analogous to the physical mechanism used at many busy bakeries. When a customer enters the bakery,
he/she takes the next paper ticket from the dispenser. Each ticket is marked with a unique number, and consecutive
tickets have have consecutive numbers (1, 2, 3, etc). A display indicates the ticket number currently being served.
After taking a ticket, a customer watches the display for his/her number. When the display shows his/her number, the
customer can order (that is, he/she owns the lock). After the customer orders (that is, when realizing the lock), the
number displayed is incremented.
Using the following data structure and the Linux semaphore API (provided on the last page of the exam), implement
the bakery init, bakery lock, and bakery unlock functions and answer the subsequent questions.
Note: Multiple people should be able to take a ticket while other people are waiting to be served
struct bakery_lock_t {
struct semaphore* sem;
unsigned int dispenser;
volatile unsigned int display;
typedef struct bakery_lock_t bakery_lock_t;
Part A (4 points): Implement bakery init function below such that the display is initially set to 1. Be sure
that the other fields are set properly to work with your answer to Part B.
void bakery_init (bakery_lock_t* b)
Part B (7 points): Implement bakery lock function below. Return only after the bakery lock is owned by the
void bakery_lock (bakery_lock_t* b)
Problem 3, continued:
Part C (4 points): Implement bakery unlock function below.
void bakery_unlock (bakery_lock_t* b)
Part D (5 points): After you implement your lock, a friend in the company confides in you that they are nervous
about the correctness of your implementation and would like to add a second synchronization variable to the bakery
lock. Specifically, the friend suggests adding a reader-writer semaphore to the bakery lock t structure, acquiring
a read lock in bakery lock while waiting for the display field to show the caller’s number, and acquiring a write
lock in bakery unlock before changing the display. Explain why you refuse to adopt your friend’s suggestion.
Part E (5 points): Based on the design of the bakery lock in this problem, is it safe for a program that holds a
bakery lock to acquire a Linux semaphore? Explain briefly why acquiring a semaphore (as opposed to a spin lock)
might be a concern and why that particular problem will or will not show up with bakery lock.
Problem 4 (25 points): x86 Assembly
Your math assignment requires you to perform numerous dot products on sets of two vectors. Being a computer
engineer, you decide to flex your knowledge of x86 assembly to write a dot product calculator and thus make your
math assignment a joke. Write the function dot product that iterates through two equal length singly-linked lists
that represent the two vectors, and returns their dot product. The heads to the linked lists are vect1 head and
vect2 head, which are global variables. The linked list structure is defined below:
struct vect_node_t {
vect_node_t* next;
int value;
How to perform the dot product: given two vectors A and B where A={1,2,3} and B={4,5,6}, their dot product is
(1 ∗ 4) + (2 ∗ 5) + (3 ∗ 6) = 32. If you have any questions on the semantics of the dot product, please ask a TA.
Write your x86 assembly code on the next page. You may use the space provided on the current page to write the code
in C, but your C code will not be graded. Assume equal length vectors and the result will not overflow a 32 bit
Problem 4, continued:
.global vect1_head
.global vect2_head
dot_product:
(scratch paper)
You may tear off this page to use as a reference
Synchronization API reference
spinlock_t lock; Declare an uninitialized spinlock
spinlock_t lock1 = SPIN_LOCK_UNLOCKED; Declare a spinlock and initialize it
spinlock_t lock2 = SPIN_LOCK_LOCKED;
void spin_lock_init(spinlock_t* lock); Initialize a dynamically-allocated spin lock
(set to unlocked)
void spin_lock(spinlock_t *lock); Obtain a spin lock; waits until available
void spin_unlock(spinlock_t *lock); Release a spin lock
void spin_lock_irqsave(spinlock_t *lock, Save processor status in flags,
unsigned long& flags); mask interrupts and obtain spin lock
(note: flags passed by name (macro))
void spin_lock_irqrestore(spinlock_t *lock, Release a spin lock, then set
unsigned long flags); processar status to flags
struct semaphore sem; Declare an uninitialized semaphore
static DECLARE_SEMAPHORE_GENERIC (sem, val); Allocate statically and initialize to val
DECLARE_MUTEX (mutex); Allocate on stack and initialize to one
DECLARE_MUTEX_LOCKED (mutex); Allocate on stack and initialize to zero
void sema_init(struct semaphore *sem, int val); Initialize a dynamically allocated semaphore to val
void init_MUTEX(struct semaphore *sem); Initialize a dynamically allocated semaphore to one.
void init_MUTEX_LOCKED(struct semaphore *sem); Initialize a dynamically allocated semaphore to zero.
void down(struct semaphore *sem); Wait until semaphore is available and decrement (P)
vod up(struct semaphore *sem); Increment the semaphore
You may tear off this page to use as a reference
x86 reference
8 016 15 7
jb below CF is set
jbe below or CF or ZF
equal is set
je equal ZF is set
jl less SF 6= OF
jle less or (SF 6= OF) or
equal ZF is set
jo overflow OF is set
jp parity PF is set
(even parity)
js sign SF is set
(negative)
movb (%ebp),%al # AL ← M[EBP]
movb -4(%esp),%al # AL ← M[ESP – 4]
movb (%ebx,%edx),%al # AL ← M[EBX + EDX]
movb 13(%ecx,%ebp),%al # AL ← M[ECX + EBP + 13]
movb (,%ecx,4),%al # AL ← M[ECX * 4]
movb -6(,%edx,2),%al # AL ← M[EDX * 2 – 6]
movb (%esi,%eax,2),%al # AL ← M[ESI + EAX * 2]
movb 24(%eax,%esi,8),%al # AL ← M[EAX + ESI * 8 + 24]
movb 100,%al # AL ← M[100]
movb label,%al # AL ← M[label]
movb label+10,%al # AL ← M[label+10]
movb 10(label),%al # NOT LEGAL!
movb label(%eax),%al # AL ← M[EAX + label]
movb 7*6+label(%edx),%al # AL ← M[EDX + label + 42]
movw $label,%eax # EAX ← label
movw $label+10,%eax # EAX ← label+10
movw $label(%eax),%eax # NOT LEGAL!
call printf # (push EIP), EIP ← printf
call *%eax # (push EIP), EIP ← EAX
call *(%eax) # (push EIP), EIP ← M[EAX]
call *fptr # (push EIP), EIP ← M[fptr]
call *10(%eax,%edx,2) # (push EIP), EIP ←
# M[EAX + EDX*2 + 10]
Conditional branch sense is inverted by inserting an “N” after initial “J,” e.g., JNB. Preferred forms
in table below are those used by debugger in disassembly. Table use: after a comparison such as
cmp %ebx,%esi # set flags based on (ESI – EBX)
choose the operator to place between ESI and EBX, based on the data type. For example, if ESI and EBX hold unsigned values,
and the branch should be taken if ESI ≤ EBX, use either JBE or JNA. For branches other than JE/JNE based on instructions
other than CMP, check the branch conditions above instead.
preferred form
unsigned comparisons
6= < ≤ = ≥ >
preferred form jne
signed comparisons
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