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Comp Systems > Summary 2 > Flashcards

Summary 2 Flashcards

1
Q

Memory management

A

the technique for mapping virtual address spaces to

physical memory

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2
Q

Physical memory

A

memory that is directly accessible to the CPU; operates at

high speed, but typically offers lower capacity than storage

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3
Q

Disk

A

a storage device that uses a magnetic storage mechanism. Not all data fits
into physical memory, so the disk holds data until it is pulled into physical
memory

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4
Q

Fixed partition scheme

A

a strategy for translating virtual to physical addresses
in which we divide the memory into partitions such that one process is loaded per
partition and queue processes that are waiting to be loaded into memory. Uses a
fixed offset to convert between physical and virtual addresses

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5
Q

Base and limit registers

A

strategy for translating virtual to physical addresses
in which the base register contains the value that gets added to the virtual
address to determine the physical address, and the limit register contains the
highest address that the program cannot surpass. Requires hardware support to
implement.

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6
Q

Bitmaps

A

in tracking free space in memory, a bitmap is a vector in which each
digit is 1 or 0 depending on whether a given memory chunk is free or not

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7
Q

Linked list

A

in tracking free space in memory, the elements of the linked list
represent blocks of memory that are free

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8
Q

Worst-fit

A

a memory allocation scheme that finds the largest block of free
memory possible and stores the process in it

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9
Q

Best-fit

A

a memory allocation scheme that finds the smallest hole which is large
enough to fit the process and stores the process in it

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10
Q

First-fit

A

a memory allocation scheme that puts the process in the first
discovered hole of free memory that is large enough to fit the process

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11
Q

Internal fragmentation

A

internal to an allocated space, internal fragmentation is

where a process is allocated more bytes than it uses

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12
Q

External fragmentation

A

space that the OS is aware of, but that is too small to fit

new processes

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13
Q

Page

A

A continuous portion of memory that is treated by the operating system as
a single unit of storage

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14
Q

Slab allocation

A

break large chunks of memory given to a process by the Buddy

algorithm into smaller units for allocation

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15
Q

Object caches

A

freed objects are stored in a cache in case they are allocated
again, rather than freeing them for allocation to other processes

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16
Q

Virtual memory

A

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

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17
Q

Segmentation

A

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

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18
Q

Virtual memory page

A

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

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19
Q

Page frames

A

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.

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20
Q

Paging

A

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.

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21
Q

Memory Management Unit (MMU)

A

translates virtual addresses to physical addresses.

Divides the virtual address space into pages

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22
Q

Disk controller

A

enables the CPU to communicate with the disk storage

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23
Q

Swap file

A

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.

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24
Q

Page table

A

a table exists for each process to map from a page number to page frame
○ Validity bit: stored in the page table; 1 if a page is in memory and 0 if it is not
○ Protection bit: stored in the page table; set to read, write, or execute depending
on the segment
○ Reference bit: 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
○ Modified (“dirty”) bit : stored in the page table; set to 1 if a page has been
written to and 0 otherwise (on pure reads)

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25
Q

Multi-level paging

A

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.

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26
Q

Transaction lookaside buffer (TLB):

A

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.

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27
Q

Inverted page table

A

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

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28
Q

Offset

A

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

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29
Q

Page fault

A

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

30
Q

Page replacement

A

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
ut to disk
○ 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
○ Stack property: incrementing the amount of frames from m to m+1 cannot
increase the number of page faults
○ Belady’s Anomaly: some page replacement policies do not respect the stack
algorithm property

31
Q

Denial of service attacks

A

bad attackers try to overwhelm a web server by
making requests at the same time, using up all the resources on the web server;
it can often be difficult to distinguish the fake traffic from legitimate network traffic

32
Q

Event-driven programming

A

rather than using blocking system calls,
event-driven programming allows higher performance and resource utilization by
allowing functionality to continue or sleeping until the desired event occurs
○ Event loop:
○ Event handlers:

33
Q

Network stack

A

○ OSI model: a formal, standard representation of networking that splits the
network functionality into seven layers.
○ Internet model: an informal representation of networking that splits the
network functionality into five layers. This model groups the “application,
presentation, and session” layers of the OSI model together.
■ Physical layer: includes the actual wires and wireless links that
move bits between two physical systems
■ Data link layer: determines how the raw bytes should be
transferred over the physical links. In particular, the link layer puts
raw bytes into messages, performs error detection and correction in
transferred data, includes functions for reliable delivery, and
determines where and when to send the data over the links.
■ Network layer: includes IP and message routing within the network
■ Transport layer: includes the TCP and UDP protocols; creates
network communication channels between different processes on
the same or on different machines through the use of sockets
■ Application layer: includes all the user level processes that
require network communication. OSI model splits this into three
layers: session, presentation, and application.

34
Q

Interface

A

accepted form of representing and accessing data such that the user
can decode the information

35
Q

Protocol

A

rules and formats that govern communication between two entities
over the network or network boxes

36
Q

Data link layer

A

○ Data frame : grouping of raw data bits; the recipient can identify the
frames by the particular sequences of bits that represent the start and end
○ MAC address: unique address that identifies your network card; 48-bit
address space

37
Q

Network layer

A

○ Packet: a method of wrapping data with its control information (e.g. its
error detection codes, destination, etc.)
○ Router: a networking device that is connected to multiple network links
and determines where to send a packet based on its
○ Switch: within a given network, switches forward messages between
devices

38
Q

Encapsulation

A

each layer can only see the service provided by the layer

immediately below it

39
Q

Device drivers:

A

a separate device driver exists for each device; the device driver
is software that interacts with the interrupts generated by its respective hardware
component, acting as an interface between the external devices and the OS; the
driver usually runs as a process
○ Interrupt handler : a separate interrupt handler exists for each device that
may generate an interrupt; executes (e.g. as a callback function in C) on
interrupt; the device driver communicates with the OS through interrupts

40
Q

Device controllers

A

a hardware component that receives data from its
respective device, stores it locally, and passes it to the device driver; the
controller makes sense of signals going in and coming out from the CPU
○ Device registers : allow the device driver to read and write status of the
devices like checking whether a printer is ready or not
○ Data buffers : enable one to write data to the controller, which is then
subsequently pushed onto the external devices; any data that we write to
the disk, or read from the disk, is first buffered in the data buffers of the
device controllers, then can be read by the device drivers

41
Q

Port-mapped I/O

A

special assembly language instructions that allow the device

driver to read directly from the external device

42
Q

Memory mapped I/O

A

reading or writing to particular addresses that correspond
to a location in main memory, control register, of address in the data buffer of a
specific I/O device. I/O operations at these addresses are automatically
converted to corresponding device control registers or data buffers, reducing the
burden on the programmer. This technique requires memory caching to be
disabled and the MMU to distinguish the I/O requests from legitimate memory
accesses.

43
Q

Direct memory access (DMA)

A

to avoid involving the CPU in the data transfer
between the physical memory and external device, a separate hardware
co-processor called the DMA is tasked with handling I/O requests

44
Q

Checksum

A

a sequence of characters, generated by applying cryptographic
hash function to data (e.g. in a file). Using the checksum, a system can validate
whether different copies of data match.

45
Q

ioctl system call

A

allows your user application to make requests of devices and
is typically used for the purposes of hardware configuration, command line
interface, and building upon kernel functionality

46
Q

I/O scheduling

A

many processes create I/O requests and the I/O scheduler
determines the order in which to send these requests to the external devices
while preserving interactivity

47
Q

Networking and the OS Endpoint

A

a remote device capable of communication over a TCP/IP network

48
Q

Networking and the OS Socket

A

a network abstraction uniquely defined by an IP address and port
number; a network abstraction used to receive and send data from a give
endpoint identified by the IP address for the network service identified by the port
number; sockets allow inter or intra machine processes to communicate over the
network
○ IP address : 32-bit number (in IPv4) and 64-bit number (IPv6) used to
identify network endpoints and their location in the network
○ Port number : 16-bit unsigned integer; ports are associated with the IP
address of the machine being used in networking and the type of network
protocol being used in the communication; each port is used to handle a
different network service so that the machine can execute multiple
services and sessions of the same service; port numbers 0-1023 are
reserved for well-known services and port numbers 1024+ can be used by
a developer in general applications

49
Q

Networking and the OS Socket API

A

a common set of functions, data structures, and constants across
many OSes used to maintain and interact with sockets; Key functions include:
○ socket() : creates a socket
○ bind() : defines the socket port
○ listen() : sets the socket to start listening for data after it has been
bound
○ connect() : initiates a connection with another socket
○ accept() : accepts a connection initiated by another socket
○ send() , sendto(), write(): send data flowing out of the socket
○ recv() , recvfrom, read(): receive data flowing into the socket
○ close() , shutdown() : closing a socket

50
Q

Domain name system (DNS)

A

a protocol used to map hostnames to IP

addresses, which is used to identify well-known servers in the network

51
Q

Network interface cards

A

hardware component that connects a machine to the

network

52
Q

Device drivers for networking

A

software interrupt handlers for different Network

Interface Cards

53
Q

Little-endian

A

store lower order bytes in the lowest memory locations

54
Q

Big-endian

A

store higher order bytes in the lowest memory locations

55
Q

Stream socket

A

uses TCP as the underlying protocol; connection-oriented, such
that, providing a two way stream of bytes; guaranteed reliable, at-most-once, and
in-order delivery

56
Q

Datagram socket

A

uses UDP as the underlying protocol; connectionless, sending
individual messages as chunks of bytes; no guarantees of reliable, at-most-once, and
in-order delivery

57
Q

System calls for files

A

○ fopen() : open file for reading and writing
○ fwrite() : writes data to a file
○ fseek() : sets the position of the stream to the given offset, allowing
seeking a specified part of the file
○ fread() : read from the file
○ fclose() : close the file

58
Q

Disk block

A

basic logic unit of storage on disk of a contiguous set of bytes

59
Q

Disk partitions

A

logical split of the actual disk drive. The operating system views
each partition as a distinct disk, and different operating systems may be run in
each split; each partition has at least one directory, in which all the partition’s files
are listed.

60
Q

internal structure of the disk partition

A

○ Boot block: contains information on the disk layout and code for loading
the operating system into the kernel space
○ Superblock: contains basic information about the file system, including
the file system size, list of free blocks, list of allocated blocks, and time of the last modification to the partition; the superblock can be read into
memory at boot time
○ Free space data structure: tracks the free blocks in the disk
○ I-nodes list : stores information about the individual files stored on disk,
and allocatable blocks for directories and file systems

61
Q

Master boot record

A

stores information about the entire drive

62
Q

Partition table

A

specifies the beginning of each partition

63
Q

Disk space allocation

A

managing data in the disk with the aims to ensure fast
sequential access, fast random access, the ability to grow a file system quickly,
and to minimize fragmentation
○ Contiguous allocation : each file occupies a contiguous region of blocks
○ Linked-list allocation : hold a linked list of blocks for each file such that
each block contains a pointer to the next block
○ File allocation table : stores a condensed version of the linked list to be
stored at the beginning of the file system; the file is stored in allocated
clusters
○ Linked list with indexing or i-nodes = supports direct access to the file’s
blocks by storing files in the i-node data structure
■ i-node (index node) : a kernel structure storing information about
each file in the file system; contains a pointer to the disk blocks
containing the file’s data, and information such as the file
permissions, ownership, modification time, and file type

64
Q

Directory

A

a file that holds the list of filenames and their inodes; provides
information needed to find the disk data blocks of a file

65
Q

Hard links

A

to represent two different paths for a file, create a single i-node and
have two directory entries that point to the same i-node; relies on the link-counter
○ Link-counter: counts the number of ways that an i-node can be retrieved
along different paths

66
Q

Symbolic/soft link

A

the original file has its own i-node and for each reference or
path to the file, there is a separate i-node, of type LINK, for a file that just
contains the path name to the original file

67
Q

Free space management

A

tracking unallocated blocks in a portion of the disk
partition
○ Bitmap: maintains for the entire disk partition with one encoding the fact
that the block of a particular number is free and zero otherwise
○ Linked list: list storing the free blocks

68
Q

In-memory cache

A

uses a scheme to store blocks that are likely to be requested
from disk, to shortcut the need for a disk search and propagate the data to the
requesting process
○ LRU scheme : given the limited in-memory cache size, evicts the least
recently used block from the cache when new blocks are being added in

69
Q

Incremental backups

A

every time you synchronize with your server, it will only

backup files that have been changed since the last time

70
Q

Physical backup

A

physically copy block by block from one disk to another

71
Q

Logical backup

A

copies files from one disk to another

Comp Systems (6 decks)

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