Topic B

Question 1

What is a subroutine?

Answer 1

a section of code that can be called repeatedly as a program executes

Question 2

What are other names for subroutines?

Answer 2

Procedures, functions, methods

Question 3

Advantages of subroutines?

Answer 3

• save effort in programming
• reduce size of programs
• Share code/workload between programmers
• encapsulate/package a specific activity
• provide east access to tried and tested code

Question 4

What do subroutines need to have and why?

Answer 4

• every subroutine needs a return address
• this is because there can be different calls of the same subroutine to different places
• so we need to know where to return to after the subroutine is finished

Question 5

What does RET stand for and what are they used for?

Answer 5

-RET retrieves the stored return address (which is the top of the stack) to the instruction pointer and pops it off the call stack.

Question 6

What instruction is used to call a procedure?

Answer 6

CALL label

Question 7

What does the call instruction do?

Answer 7

• The call operation pushes the current value of the instruction pointer (the return address) to the top of the call stack
• puts the required subroutine address into IP (so the next instruction to be executed is the first instruction of the subroutine)

Question 8

How do we deal with nested subroutine calls?

Answer 8

• we use a call stack to keep track of the return addresses of the subroutines called.
• once a subroutine has finished, we pop it off the stack

Question 9

What is a stack?

Answer 9

A data structure that uses a LIFO structure

Question 10

What is the name of the stack pointer register?

Answer 10

ESP - it holds the address of the item which is currently on top of the stack

Question 11

What do the pop and push instructions do?

Answer 11

• push decrements address in ESP so it points to free space and writes item to the memory location pointed to by the ESP
• pop fetches the item pointed to by the ESP and increments the ESP by a chunk to remove the item from the stack

Question 12

Why is it important to remember that the stack grows down in memory?

Answer 12

Because this means that the top of the stack is the bottom free slot in memory, so to push something to the stack you decrement the stack pointer down a chunk

Question 13

How do we pop off multiple items from the stack at the same time?

Answer 13

If we’re using a 32 bit system, to pop off one item would be 4 bytes so we just add 8 or 12 bytes to the ESP

Question 14

What are the two main forms of parameters?

Answer 14

Value Parameters

Reference Parameters

Question 15

What is the difference between value and reference parameters?

Answer 15

value parameters are numeric values where as reference parameters are the addresses of numeric values such as variables

Question 16

What is the simplest method of passing parameters into a subroutine?

Answer 16

by using a register: cope a value into a register then call the subroutine

Question 17

What does the LEA assembly instruction do?

Answer 17

• LEA loads the effective address of a variable

Question 18

How can we pass more parameters into a subroutine?

Answer 18

• since we don’t have enough registers to store all the variables in a program we can use a stack.

Question 19

What is printf()

Answer 19

• typing printf and then a string as a parameter will take whatever’s in the string and output it to the console
• it expects its parameters to be on the stack
• it doesn’t remove the parameter from the stack so you must do it

Question 20

what are the two external library routine calls we can make?

Answer 20

printf and scanf

Question 21

What is the ECX register used for?

Answer 21

-to count what loop iteration we’re on

Question 22

Why do we need to pop ECX off the stack at the end of our program?

Answer 22

Because ECX will be storing the incorrect loop value of which iteration we’re on, so we pop it and then add the new loop value

Question 23

what are the 4 common format specifiers and what type are they associated with?

Answer 23

%d - integer
%s - string
%c - character
f%f - float

Question 24

What is scanf()

Answer 24

calling scanf() allows you to read values from the keyboard
- it takes two parameters a string including a format specifier and the address to store whatever it reads in from the keyboard

Question 25

What is one of the ways a compiler can turn your high level code into machine code?

Answer 25

By creating stack frames and pushing it to the stack.

Question 26

What are the three things a stack frame holds

Answer 26

• local variables
• return address
• parameters

Question 27

What happens when a HLL subroutine is called?

Answer 27

a new stack frame is created on the stack

Question 28

What is the EBP?

Answer 28

• the base pointer is a register which points to the bottom of the current stack frame
• The compiler uses EPB to store and manipulate local variables

Question 29

how does the compiler know what area of memory it can use to store its local variables?

Answer 29

Because this area is is somewhere between the base pointer and the stack pointer

Question 30

What happens when a subroutine returns (regarding frames)

Answer 30

-As each subroutine returns, the block of the subroutine’s stack frame is cleared from the stack, the old base pointer is restored and the old return address is restored

Question 31

What happens calling a subroutine (regarding frames)

Answer 31

• params are pushed onto stack
• return address is pushed onto stack
• old EBP is pushed onto stack
• current address of top of stack is put into EBP
• local variabls are installed on the stack increasing EPS

Question 32

What is a recursive subroutine?

Answer 32

a subroutine that calls itself

Question 33

What is mutual recursion

Answer 33

where two or more subroutines recursively call each other

Question 34

What are recursive algorithms useful for coding?

Answer 34

Useful for coding data structures, if you want a tree or some other nested structure

Question 35

What does tail recursive mean?

Answer 35

the recursive subroutine does what it needs to do and it calls itself as the very final instruction in its code. So it’s tail recursive because the call happens at the end.

Question 36

What does recursion use to simplify the stack?

Answer 36

uses the stack (of nested calls and frames) to simplify the algorithm

Question 37

What do we use to find the length of a string in C? and what library do we need to include?

Answer 37

strlen

with the