CS261 - Midterm 2

Question 1

Elements of an array are next to each other in memory

Answer 1

True

Question 2

Does A[i] = *(A+i)?

Answer 2

True

Question 3

C has always supported variable sized multidimensional arrays?

Answer 3

False

Question 4

How big will int C[5] be if ints are 32 bits?

Answer 4

It will be 20 bytes long.

Question 5

Assume array long B[10] starts at address x. What address will be B[5] be?

Answer 5

x + 40

Question 6

Assume the address of int C[5] is in %rdx, and i is in %rcx. What is the address for C[i]?

Answer 6

(%rdx, %rcx, 4)

%rdx = base register of C[5]
%rcx = index register i
4 = an integer is 4 bytes, so the scale is going to be 4

Question 7

Memory Addressing Mode

Answer 7

D(Rb, Ri, S)
D = Constant "displacment" of 
Rb = Base register
Ri = Index Register: Any, except for %rsp
S = Scale: 1,2,4,8

Question 8

Assume long A[4][6]. What will the size of A be?

Answer 8

8 * 4 * 6

long = 8 bytes
4 * 6 = 24 cells

Question 9

Assuming long A[4][6] has base address x. What will the address of A[1][0] be?

Answer 9

x + 6*8

Question 10

All pointers in C are the same type?

Answer 10

False, like in *p; char *c;

Question 11

Adding 1 to a pointer will always add 1 byte

Answer 11

False

Question 12

Pointers can point to functions

Answer 12

True

Question 13

All pointers are how many bytes?

Answer 13

8

Question 14

To add 2 operands in y86, the must both be

Answer 14

registers

Question 15

y86 instructions are all the same size

Answer 15

False, 1-10 bytes

Question 16

A multiplexor

Answer 16

selects a value from a set of different data values

Question 17

Registers change value

Answer 17

When the clock signal rises

Question 18

A single logic gate implements a boolean function in hardware

Answer 18

boolean function

Question 19

We compute for the value for the next PC in

Answer 19

Fetch

Question 20

The inputs to the ALU are

Answer 20

Register A and B, or register B and a constant

Question 21

Whether we read or write during the memory stage, depends on the value of

Answer 21

the code

Question 22

CPU

Answer 22

• We have the basic components we need to
create a CPU that runs y86 programs
– ALU: Does math/logic
– Registers: Hold values for ALU
– Memory: Holds values over time
– Instruction Memory: Holds instructions
– PC register: Holds pointer to current instruction

Question 23

Instructions in Stages

Answer 23

Fetch: Read instruction from memory
• Decode: Read operands from the register file
• Execute: ALU performs operations
• Memory: Moves to or from memory are
performed if necessary
• Write Back: Results are saved to register
• PC Update: PC is set to address of next
instruction

FDEMW

Question 24

What hardware do we need for

decode?

Answer 24

Register file

Question 25

Decode

Answer 25

Values from register file sent to the ALU

Question 26

What hardware do we need for

execute?

Answer 26

ALU and condition code registers