CSCI-UA.0201-003
Computer Systems Organization
Midterm Exam Fall 2015 (time: 60 minutes) Last name: First name:
If you perceive any ambiguity in any of the questions, state your assumptions clearly.
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Questions vary in difficulty; it is strongly recommended that you do not spend too much time
on any one question.
1. [2 points] What would have happened if we didn’t have linkers? State 2 consequences.
2. [5 points] Fill in the blanks in the following table, assume the instructions are executed sequentially (i.e. instruction 1 executed, then instruction 2, then instruction 3. Assume rax and rbx are holding signed numbers.
instruction# instruction Initially
1 addq %rbx, %rax
2 testq %rbx, %rax
3 cmpq %rbx, %rax
0xFFFFFFFF 0x00000001
CF ZF SF OF
3. [5 points] For the following assembly code and its corresponding C code, fill-in the blanks in the C code assuming x will go into %rbx and y will go into %rax;
cmpq %rax, %rbx ja L1
subq, %rbx, %rax jmp L2
L1: addq $10, %rax
L2: addq %rbx, %rax
____________ x, y; /* declaration of x and y; if ( _________________ )
____________________________
_____________________________
__________________________
4. [2 points] Can the carry flag (CF) and the overflow flag (OF) be both 1 at the same time? If yes, give an example of an operation that does this (no need for assembly code, just describe the operation). If not, explain why not.
5. [2 points] State two reasons for why do we need an assembler and not making the compiler generate the binary presentation right away.
6. Suppose x is an integer (i.e. 4 bytes). We want to test whether the most significant bit of x is 1 or not (i.e. the left most bit), so we wrote the expression:
if( (x & mask) != 0)
a. [1 point] What is the value of mask, both in binary and hexadecimal?
b. [2 points] Which of the following expressions generate correct mask? Circle ALL correct answers. There may be more than one correct answer, or there may be none!
0x1FFFFFFF << 3
0x1FFFFFFF << 2
two’s complement of 0xFFFFFFFC
two’s complement of (-2)
c. [1 point] Please give the expression that sets the most significant bit of x to 1 and leave all the other bits unchanged.
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