Allocation of Runtime Memory and Virtual Memory

Question 1

Which command can we use to disassemble object code?

Answer 1

objdump -d ‘myFile.h’

Question 2

What is a Single Task OS? Give a model of one…

Answer 2

An OS that only allows 1 user to perform 1 task at a time.
MS-DOS.

Question 3

What is MS-DOS? What process does it follow to boot up?

Answer 3

A single task OS.
RAM BIOS is activated to load initial IO systems. Then Software BIOS launches the rest of the system.

Question 4

What is a multi-task OS?

Answer 4

An OS that allows multiple users to perform multiple tasks at once. E.g Linux.

Question 5

How does Linux utilise Virtual Memory for processing?

Answer 5

Linux uses Virtual Memory to create more memory than physically available in order to increase memory for processes. Virtual memory is mapped to Physical Memory via the Pages and the Operating Systems Memory Management Unit.

Question 6

What is a Page Table?

Answer 6

A data structure used by Virtual Memory within the OS to map Virtual Memory Addresses to Physical Memory Addresses.

Question 7

Which component is responsible for mapping Virtual Memory Addresses to Physical Memory Addresses?

Answer 7

Memory Manage Unit within the Operating System.

Question 8

What is the Stack?

Answer 8

A structure that holds the memory containing currently executing functions.

Question 9

How and why does the stack grow?

Answer 9

The stack grows downwards from high addresses to low addresses.
The stack grows whenever a function is executed, and shrinks whenever a function is returned.

Question 10

What are the 2 stack pointers? What are their roles?

Answer 10

Frame/Base pointer -> Points to the top frame of the currently executing function.
Stack Pointer -> Points to the bottom frame of the stack (currently executing function).

Question 11

What happens to the pointers when a function is executed or returned?

Answer 11

Frame Pointer -> When executed it moves down to the top frame of the newly executed function. When returned it moves to the top frame of the function above.
Stack Pointer -> When executed, the pointer moves to the bottom frame of the new function. When returned, moves to the bottom frame of the above function.

Question 12

What is encapsulated in the function frames?

Answer 12

Memory address allocated to the function that holds the functions data.

Question 13

What is the Heap?

Answer 13

Memory within the system that is for dynamic allocation.

Question 14

When allocating from the Heap, from what position on the Heap is the memory taken?

Answer 14

From he bottom of the Heap (if possible).

Question 15

Is it only contiguous memory that can be taken from the heap?

Answer 15

Yes, since hence why malloc can only return contiguous memory.

Question 16

What happens if malloc reaches the top of the heap without finding suitable memory?

Answer 16

It cycles back to the bottom and starts searching up the heap again.

Question 17

What are the 3 allocation policies of malloc when searching the heap?

Answer 17

Best Fit -> Searches for the tightest fit. However, leaves little room for contiguous re-allocation.
First Fit -> Searches for the first memory fit, regardless of size. However, can lead to wasted memory allocation.
Next Fit -> Looks for the next fit after the first fit pointer.

Question 18

What does the malloc policies help prevent?

Answer 18

They help prevent wasted memory being caused by fragmentation. The different policies adapt different allocation specifications to pick up fragmented memory.

Question 19

On a 64 bit machine, what format specifier do we use for the Stack Pointer and Base Pointer?

Answer 19

Frame Pointer -> %rbp
Stack Pointer -> %rsp

Question 20

In ANSI C, what does __asm__ perform?

Answer 20

Assembler instructions.