Control Instructions - Week 7

Question 1

What does CISC stand for?

Answer 1

Complex Instruction Set Computer

Question 2

What does RISC stand for?

Answer 2

Reduced Instruction Set Computer

Question 3

Why did CISC instruction sets become large and complex?

Answer 3

To simplify compilers and improve performance

Question 4

What are the advantages of smaller CISC programs?

Answer 4

They use less memory and require fewer instruction bytes to fetch; more instructions fit in cache

Question 5

What characterises CISC instructions?

Answer 5

Powerful, complex instructions with complicated decoding

Question 6

What memory model does CISC arithmetic often use?

Answer 6

Register–memory architecture

Question 7

What characterises RISC instructions?

Answer 7

Simple, uniform instructions typically executed in one cycle

Question 8

What memory model does RISC arithmetic use?

Answer 8

Load/store architecture

Question 9

Why CISC instruction sets became large

Answer 9

To simplify compilers
To improve performance

Question 10

Advantages of smaller CISC programs

Answer 10

Use less memory
Fewer instruction bytes to fetch
More instructions fit in cache

Question 11

CISC characteristics

Answer 11

Powerful multi‑step instructions
Complex decoding
Arithmetic may access memory directly
Fewer general‑purpose registers

Question 12

RISC characteristics

Answer 12

Simple, uniform instructions
Usually one cycle per instruction
Load/store architecture
More general‑purpose registers

Question 13

RISC trade‑offs

Answer 13

More instructions needed
More memory required
Compiler must do more work

Question 14

What are the four stages of the fetch–decode–execute cycle?

Answer 14

Fetch, increment PC, decode, execute

Question 15

What are data processing instructions used for?

Answer 15

Arithmetic and logical operations

Question 16

What are data storage instructions used for?

Answer 16

Moving data into or out of registers or memory

Question 17

What do control instructions do?

Answer 17

Change program flow by altering the program counter

Question 18

What do I/O instructions do?

Answer 18

Transfer data between CPU/memory and external devices

Question 19

What is an unconditional branch?

Answer 19

A branch that always changes the PC

Question 20

What is a conditional branch?

Answer 20

A branch taken only if a condition is true

Question 21

What is the purpose of flags?

Answer 21

To record ALU results for conditional branching

Question 22

What does the Zero Flag (ZF) indicate?

Answer 22

The result of an operation is zero

Question 23

What does the Sign Flag (SF) indicate?

Answer 23

The sign of the result (positive or negative)

Question 24

What does the Carry Flag (CF) indicate?

Answer 24

A carry out occurred in an arithmetic operation

Question 25

What does the Overflow Flag (OF) indicate?

Answer 25

A signed arithmetic overflow occurred

Question 26

What control structures do conditional branches enable?

Answer 26

Decisions and loops

Question 27

What is a subroutine?

Answer 27

A reusable block of instructions called from elsewhere in a program

Question 28

What must be saved when calling a subroutine?

Answer 28

The return address

Question 29

Where is the return address stored during a subroutine call?

Answer 29

On the call stack

Question 30

What principle does a stack follow?

Answer 30

LIFO (Last In, First Out)

Question 31

Why are stacks used in subroutines?

Answer 31

To store return addresses and support nested/recursive calls

Question 32

How are subroutines called using a stack?

Answer 32

To call a subroutine, the contents of the program counter are pushed onto the stack and the PC is reset to the starting address of the subroutine.

Question 33

How does a program return from a subroutine?

Answer 33

To return, the program counter is reset to the value popped from the stack.

Question 34

Why can subroutines be nested to any depth?

Answer 34

Each call pushes its own return address, so calls can nest as long as there is enough stack space.

Question 35

What is a stack frame?

Answer 35

A block of data stored on the stack that holds all information needed for a single subroutine call.

Question 36

What does the stack frame generally include

Answer 36

The return address Argument variables passed on the stack Local variables (in high level languages) Saved copies of any registers modified by the subprogram that need to be restored

Question 37

How do we push a value onto the stack

Answer 37

decrement the address in the stack pointer and store it there.

Question 38

How do we pop a value from the stack

Answer 38

dereference (get the content of) the address stored in the stack pointer, which is then incremented by one.