M10: Memory Management Architecture

Question 1

Virtual memory

Answer 1

virtual memory is the memory that is allocated to a process for storing its in-memory state. It is considered “virtual” because at creation, it is not yet mapped
into physical memory locations. Virtual memory is broken down into the code, data, stack, and heap segments, creating a standardized memory representation between all processes. Virtual memory offers the OS a complete separation of logical and physical addresses

Question 2

Segmentation

Answer 2

divides the address space into variable sized segments or logical addressable units. These include the code, stack, heap, and data segments.

Question 3

Virtual memory page

Answer 3

a contiguous block of virtual memory addresses of a fixed range, which is the same as page frames. A page is the smallest unit of addressability to virtual
memory available to the OS for memory management. Pages will not contain addresses from different segments of virtual memory.

Question 4

Page frames:

Answer 4

a contiguous block of physical memory addresses of a fixed range, which is the same as the size of virtual memory pages. Virtual memory pages swapped into
main memory from storage are mapped to page frames.

Question 5

Paging:

Answer 5

Paging is the technique by which the OS swaps in various virtual memory
pages as they are needed as virtual memory is typically much larger than physical
memory.

Question 6

Memory Management Unit (MMU)

Answer 6

translates virtual addresses to physical addresses.

Divides the virtual address space into pages.

Question 7

Disk controller :

Answer 7

enables the CPU to communicate with the disk storage

Question 8

Swap file:

Answer 8

space in the hard disk that operates as an extension of the RAM for portions of the process virtual memory that do not fit in RAM. Pages can be stored in the swap
file to create room for other processes to execute in RAM

Question 9

Page table:

Answer 9

table exists for each process to map from a page number to page frame

Question 10

Validity bit:

Answer 10

stored in the page table; 1 if a page is in memory and 0 if it is not

Question 11

Protection bit:

Answer 11

stored in the page table; set to read, write, or execute depending on the segment

Question 12

Reference bit:

Answer 12

stored in the page table; allows us to detect pages that are frequently used by setting this bit to 1 when a page is accessed

Question 13

Modified (“dirty”) bit :

Answer 13

stored in the page table; set to 1 if a page has been

written to and 0 otherwise (on pure reads)

Question 14

Multi-level paging:

Answer 14

divide up the virtual addresses such that the highest order group indexes into a top level page table, the second highest order group indexes into the next
level page table, and so on. When looking up in a multi-level page table, the corresponding bits of the page number are used to index into each level until we get to the page table of interest. This strategy is used because virtual memory is often larger than physical memory and each process has its own page table, thus the total amount of memory required to store page tables needs to be managed.

Question 15

Transaction lookaside buffer (TLB):

Answer 15

small cache maintained within the MMU hardware

to lookup pages before we go to main memory to retrieve the page table. The TLB is associative memory.

Question 16

Inverted page table:

Answer 16

stores entries in the form of (page-frame, process ID, and page number), which is like a hash table, to help find the desired page frames and avoid a linear search of the page table

Question 17

Offset:

Answer 17

in the offset number, the lower order bits correspond to the offset at which we want to access addresses within a given page, and the higher order bits represent the
page we want to access

Question 18

Page fault:

Answer 18

when a page is requested but is not currently in physical memory; causes a trap to the kernel

Question 19

Page replacement:

Answer 19

when a page is required, but physical memory is full, the page replacement process selects one of the current pages in physical memory to be written
out to disk

Question 20

Page replacement strategies

Answer 20

○ FIFO: replace the longest resident page that is already in memory
○ Other policies include: second-chance algorithm, clock algorithm, least recently used algorithm (LRU), approximate LRU

Question 21

Stack property:

Answer 21

incrementing the amount of frames from m to m+1 cannot increase the number of page faults

Question 22

○ Belady’s Anomaly:

Answer 22

some page replacement policies do not respect the stack algorithm property