Week 6 (Lect 14-16)

Question 1

How does the program use Code, Stack, Heap? (what are their functions?)

Answer 1

Code: instructions that live in memory

Stack: to keep track of where it is in the function call. used for local variables.

Heap: Dynamically allocated, user managed memory

Question 2

2 types of allocated memory?

Answer 2

1. Stack: managed implicitly (automatic memory)
2. Heap: managed explicitly by the user

Question 3

What does a stack save in a function?

Answer 3

1. return address
2. save main’s pointer
3. arguments and local variables

Question 4

3 goals of a virtual memory system?

Answer 4

1. Transparency (code that is invisible to the running program)
2. Efficiency (in terms of time and space)
3. Protection (protect processes from one another)

Question 5

What is the malloc() call?

Answer 5

Pass a size asking for room in the heap.

Ex: malloc(10 * sizeof())

Question 6

Some sources of segmentation fault?

Answer 6

1. not allocating enough memory
2. forgetting to initialize allocated memory
3. forgetting to free memory
4. double freeing
5. freeing memory before you’re done
6. invalid frees

Question 7

What are stack canaries?

Answer 7

software defense that attempts to stop buffer overflow attacks / stack smashing.

It is a value (buffer) placed between local variables and return addresses. If an attacker attempts to overflow the buffer the overflow will also modify the canary

Question 8

What is stack smashing?

Answer 8

An attack where an attacker attempts to write beyond the bounds of a local variable to modify the return address

Question 9

Pros of a stack being a bump allocator?

Answer 9

Allocations/deallocations are really fast because we only move the stack pointer.

Question 10

Cons of a stack being a bump allocator?

Answer 10

Allocations / deallocations only in the reverse order of which they are allocated

Question 11

3 Ways to load virtual address into physical address?

Answer 11

1. Direct Mapping
2. Dynamic Relocation (whole memory)
3. Segmentation (segments the memory)

Question 12

What is direct mapping?

Answer 12

Give all the physical memory to the process.

Question 13

Pros of direct mapping?

Answer 13

simple to implement, process knows where every byte of memory is

Question 14

Cons of direct mapping?

Answer 14

Where does the OS go and what if you want to run more than one process

Question 15

What is dynamic relocation?

Answer 15

divides the memory into slots, each process gets a slot.

translates virtual -> physical address by:

physical = virtual + base

Implemented by a base and bounds registers with the help of the MMU (memory management unit)

Question 16

What is the base and bounds register?

Answer 16

Base (OFFSET) register: stores the starting address in physical memory (the offset from virtual –> physical)

Bounds (SIZE): defines the size/limit of the programs memory

Question 17

What is internal fragmentation?

Answer 17

The space inside an allocated memory block is not all used. Happens a lot with dynamic relocation.

Question 18

What are some Hardware Requirements for Dynamic Relocation?

Answer 18

1. Privlege mode to update base/bounds
2. Implement base/bound registers
3. Translate virtual address
4. Raise exceptions

Question 19

What is Segmentation?

Answer 19

Splitting up the address space and storing each segment separately

Question 20

What are the 3 main segments?

Answer 20

1. Heap
2. Stack
3. Code

Question 21

Pros of Segmentation?

Answer 21

Better utilization of physical memory, can place independently.

Question 22

Cons of segmentation?

Answer 22

• External Fragmentation
• segments can be different sizes so the OS has to worry about finding space

Question 23

What is external fragmentation?

Answer 23

Physical memory has become full o flittle holes of free space between segments.

Question 24

What is the explicit approach to knowing the offset and specific segment an address is refering to?

Answer 24

USE VIRTUAL ADDRESS:

• top 2 bits to determine the type of segment
• the remaining bits determine the offset

Question 25

Some issues of using the top 2 bits for code, stack, and heap, and how to solve it?

Answer 25

2 bits with 3 segments → one segment of the address space goes unused. - Each segment would be limited to a maximum size.

Question 26

Implicit approach to knowing the offset and segment an address is referring to?

Answer 26

Hardware notices how the address was formed.

Question 27

How is the offset from virtual --> physical found?

Answer 27

Offset = virtual address - base of the segment

Question 28

What is an additional support for segmenting "Stacks?"

Answer 28

Hardware needs to know if a segment is growing backwards to calculate offset properly

Question 29

What is an additional support for code sharing?

Answer 29

Protection bits to indicate whether it can read or write a segment

Question 30

Coarse grained vs fine grained segmentation?

Answer 30

Coarse: few but segments Fine-grained: large number of smaller segments

Question 31

What is paging?

Answer 31

Divide physical and virtual memory into fixed-size chunks called pages / page frames

Question 32

How do we know what page we're on?

Answer 32

Look at the 2 highest order bits (left-most of) of the virtual address.

Question 33

How does the OS keep track of used pages / where each page is placed?

Answer 33

Through a free list of all free pages and a page table.

Question 34

What is the physical page number (PPN) ?

Answer 34

Also known as the “physical frame number” The ACTUAL page number in the physical memory. The mapping from VPN → PPN is decided by a page table.

Question 35

How does the OS help in translating from virtual -> physical address?

Answer 35

- Handles allocation and management - decides WHERE to place process - SAVE and LOAD values on context switches - exception handlers - clean process

Question 36

How is fairness scheduled in lottery scheduling?

Answer 36

Fairness = time Job A finished / time Job B finishes The shorter the job the greater the unfairness.