Paging (5+6)

Question 1

Advantages of partial loading

Answer 1

many processes can concurrently be maintained in main mem (only a few pages for each processes)
Allows users to execute many programs and large data structures at once without worrying about filling up main memory
more processes = efficient utilization of CPU

Question 2

Page

Answer 2

small chunk of memory that constitutes a small part of the total memory needed to execute a process - one process may have many pages

Question 3

Page table

Answer 3

look up table used by MMU to decide how many pages should be swapped in and swapped out of main mem

Question 4

name and explain the 2 different parts of the MMU used in memory translation and assignment

Answer 4

Virtual memory manager - retrieves processes in secondary storage for transfer
Page table translates VA to PA - page table hard coded into cache mem and only accessible by OS kernel

Question 5

What are the three things that can happen when a process at a VA is translated to a PA

Answer 5

1. Physical address is assigned to process by page table
2. Memory page is not accessible to MMU as it is still in secondary storage
3. Nothing - VA is invalid

Question 6

Demand paging

Answer 6

pages are allocated (on demand) to free frames in main memory

“on demand” means that when a process is scheduled to run, then the memory pages (needed by process to run) will be allocated to free frames in main mem - no free frames, pages of other process will be eviced to create free space “frames”

Question 7

how does the MMU accomplish the “illusion” that each processes has access to all the required phys mem space?

Answer 7

MMU uses demand paging - pages most in demand are swapped in and least demanded pahes are swapped out. Priority processes are kept in temp mem cache for fast access and retrieval

Question 8

Context switching

Answer 8

process of swapping in and out parts of a process between VM and PMem

Question 9

What does context switching allow the OS to do?

Answer 9

OS can keep active portions of multiples processes in main mem while the rest of the process is in VM

Question 10

when does context switching happen

Answer 10

when we switch between user and kernel operations - when we need to let the OS swap a process into main mem for execution or to suspend it.

Context switch is an OS level software interrupt

Question 11

Each process is allocated memory that is comprised of

Answer 11

code - compiled source code
data - any data e.g. external files used by the process
stack - used for static memory allocation
heap - dynamic mem allocation

this is the process address space

Question 12

Difference between stack and heap regarding position

Answer 12

stack - starts at max logical position and grows down

heap - starts at min logical position and grows up

Question 13

What is stack

Answer 13

region in mem that stores temp variables created by each function in a program - LIFO
When function exits variables are freed - now available for other stack variables

Question 14

advantage of stack

Answer 14

memory is managed for the user - doesnt have to free or allocate it

Question 15

Heap

Answer 15

not managed automatically
larger free-floating region in memory
need to use malloc() or alloc() and free( )

Question 16

Explain issues with swapping chunks

Answer 16

Frequent swapping - OS context switches - during context switch, the CPU is temporarily unused (MMU needs to context switch a few times so that a suitable number of mem chunks have been swapped into main mem)

Question 17

Mem fragmentation

Answer 17

wasted space

Question 18

name and explain the 2 different types of fragmentation

Answer 18

Internal: wasted memory within chunks - too much main memory allocated for corresponding vMem chunk
external: free gaps between mem chunks in phys mem

Question 19

Benefit of paging

Answer 19

MMU can have fine grained control over what parts of a process is currently running and have many processes lloaded into phys mem at one time

Question 20

Solution to fragmentation

Answer 20

paging

Question 21

Paging

Answer 21

Mem management scheme whereby a computer stotes and retrieves pages from secondary storage for use in main memory

Question 22

Suitable page size

Answer 22

debate

larger pages = increase in fragmentation

Question 23

How to implement paging

Answer 23

key data structure - page table
For each process, there is a pt in main mem
1.) Check that a page can fit in the pt - no: MMU executes a page already in pt to make space

Question 24

Offset

Answer 24

offset from the VA is copied to the PA to indicate how large the page is and how much mem of a mem chunk must be allocated at each frame (phys mem address)

Question 25

PTE (page table entry)

Answer 25

ith entry (row) of a page table and is a pointer to an actual mem page - also includes a set of permission bits

Question 26

valid bit 0 vs 1

Answer 26

1 - page is in memory (PTE points to a page in physical memory)
0 - page not in mem and must be swapped in from virt mem

Question 27

dirty bit

Answer 27

tells MMU whether the page has been modified by processor or not

Question 28

relationship between PT size and address space

Answer 28

address space increases = PT size increases