Advanced CPU Design

Question 1

What is a subroutine?

Answer 1

A self-contained piece of code which can be called from the main program

Question 2

What is the best way to store the return address during a subroutine?

Answer 2

In a stack

Question 3

What is the main problem with using stacks?

Answer 3

Stack-overflow

Question 4

How do we help minimise stack-overflow?

Answer 4

Dedicate memory for any elements that overflow

Question 5

What does the stack pointer register do?

Answer 5

Stores the address of the most recent data element added to the stack

Question 6

What is an interrupt in programming?

Answer 6

A signal to jump to another piece of code outside the main program

Question 7

What are the two CPU access modes?

Answer 7

Privileged mode
Restricted mode

Question 8

What is privileged mode used for?

Answer 8

Used by the kernel and gives it access to memory, real addresses, OS code execution, the stack, and IRQ timer

Question 9

What is restricted mode used for?

Answer 9

Used by user programs and uses pre-processed virtual addresses

Question 10

What swaps from privileged mode to restricted mode?

Answer 10

The IRQ timer callback

Question 11

What is an instruction set?

Answer 11

A complete set of instructions that are understood by a given CPU

Question 12

What do instruction sets depend on?

Answer 12

Data types, instruction types, addressing modes, and instruction format

Question 13

What is the FPU?

Answer 13

Floating point unit

Question 14

Why do we use the FPU?

Answer 14

Floating point operations use so many resources that it is more efficient to have a dedicated unit. However, the FPU is usually built-into the CPU

Question 15

What is pipe lining?

Answer 15

The CPU breaks instructions down into multiple steps and stores them, this allows the CPU to perform multiple operations simultaneously

Question 16

What can cause hazards in pipe lining?

Answer 16

branching

Question 17

How can we avoid branch hazards?

Answer 17

write code to avoid it, pipe line stalling, eager execution, branch prediction

Question 18

How does branch predictions work?

Answer 18

It assumes that all branches are taken as most branches arise from loops. Programmers will leave hints so that these loops can be spotted and dealt with

Question 19

What is operand forwarding?

Answer 19

It lets pipelining stages access results in-between other stages.

Question 20

What is out of order execution?

Answer 20

Executing instructions not in the order they appear in the program

Question 21

What is CISC?

Answer 21

An instruction set. Focuses on hardware and includes complex, multi-clock instructions. Small code sizes where the compiler doesn’t have to do much work, however, the code is very complex. Uses little RAM

Question 22

What is RISC?

Answer 22

An instruction set. Focuses on software, and uses reduced, single clock instructions. Larger code sizes means the compiler has a lot of work to do, however, the code is simple. Uses a lot of RAM.