Definitions Flashcards
Foreground processes
Those that interact with and perform work for users
Multiprogramming
The way the CPU rapidly switches between processes to give the illusion they are happening parallel to each other (pseudoparallelism)
Process
Instance of executing program
Program
Collection of instructions that perform a task when executed
Time-sharing
Multiprogramming with simultaneous terminal access to the machine by several users
Daemons
Processes that stay in the background to handle background activity (eg. Email)
Architecture
Set of rules and methods that describe functionality, organisation and implementation of computer systems
Embedded Systems
Dedicated systems within a larger mechanical or electrical system. These may not have a kernel mode.
Interpreted Systems
Systems that use interpretation, not hardware, to separate components (eg. Java-based operating systems)
System Call
Programmatic way in which a computer program requests a service from the kernel of the operating system it is executed on
Abstraction
Concealing complex ideas and systems with a simpler one that the user or programmer interacts with
Multiplexing
Sharing of resources
Program Counter
Memory address of next instruction to be fetched
Stack Pointer
Points to top of current stack in memory
Register
A quickly accessible location available to a CPU
PSW
Program status word.
Contains condition code bits which are set by comparison instructions.
Plays important role in I/O.
User progs can generally read entire thing but can only write parts.
A bit in the PSW controls whether CPU is in user or kernel mode.
Pipeline CPU
Executing one instruction, while decoding the next instruction, while fetching the next instruction
Superscalar CPU
Multiple execution units are present. Multiple instructions are fetched and decoded at once, then stored in a holding buffer until the relevant execution unit is free.
Redirection
When output from terminally is redirected from terminal to a file (func > file)
Batch Job
Totally automated
Multicore Processor
Has multiple CPUs
Process Group
A process and all of its descendants in UNIX form a process group
Handle (token)
A token given to a parent process in Windows, allowing it to control a child. Can be given to other processes to allow them to control each other.
Multithreading
A single process having multiple threads
Lightweight Process
This another name for thread
Thread
(aka thread of execution or lightweight process) Sequential execution stream within process
These are used by CPU.
Cache
Collection of heavily accessed pages in main memory for easy access
FSM
Finite state machine. Machine exists in a finite state that can altered by external events. Allows efficiency with a single thread.
Nonblocking System Call
Returns almost immediately. Lengthy calls will give output later in the form of an event, message or signal
Thread Table
Each thread has its own thread table, containing properties unique to the thread. When a thread is moved to ready or to blocked, the information required to restart the thread is stored here
Process Table
Data structure maintained by the OS to facilitate context switching and scheduling,
Context Switching
Switching of CPU from one thread or process to another
Process Control Block
One entry within a process table. Contains information required to restart program if moved to blocked or ready states. This includes program counter, stack pointer, memory allocation and status of open files, accounting and scheduling info
Interrupt Vector
Address of interrupt handler
Area of memory with replacements for SP, PSW and PC. Essentially table of functions or function pointers to deal with interrupts. There’s a function corresponding to each task we want to deal with
IVT
Interrupt vector table. Associates interrupt handlers (ISR/interrupt service routine) with interrupt requests in a table of interrupt vectors
Interrupt
Alerts processor of higher priority process that requires immediate CPU attention
These have different priorities
Upcall
A call from a lower-level subsystem (kernel) to a higher-level subsystem (user)
Jacket
(aka wrapper) Code placed around a system call to determine if it will read or block, before the call is made (prevent blocking in user-level threads)
Page Fault
If program calls or jumps to an instruction that is not in main memory, a page fault occurs. OS will go get instruction from disk
Popup Thread
When a message arrives, the system creates a new thread to handle it. This thread is a popup thread.
IPC
Interprocess Communication. The communication between processes.
Reentrant
Reentrant procedures can be entered again before a return from the first call to the procedure has been returned from
Race Conditions
Situations with two or more processes reading or writing to the same shared data where final result depends on who runs at what time
Mutual Exclusion
Ensuring that if one process is using a shared data file, no other process can access it at that time
Critical Region
(aka critical section) Part of program where shared memory is accessed (shared being by multiple processes)
Busy Waiting
Continually testing a variable until some value appears
(eg. this isn’t needed in definition - this is for understanding
while (True):
while (var != value):
loop
)
Spin Lock
A lock that uses busy waiting
Priority Inversion Problem
In the case of achieving mutual exclusion, priority inversion problem is when a low priority process is in the critical region and a high priority process wants to enter. The preference should be given to the high priority task, but it must wait for low priority to finish.
Atomic Action
An action all completed at once. It cannot be interrupted. Either it all happens or it doesn’t happen at all.
In synchronisation, a test-and-set atomic instruction prevents accesses to the word/value between the read and write
Binary Semaphore
A semaphore initialised to 1 and used by 2 or more processes to ensure only one can enter critical region at given time
Mutex
Variable that can be in one of two states: locked or unlocked
Deadlock
Two processes that both end up and remain blocked forever
Monitor
Collection of procedures, variables and data structures that are grouped together. Only one process can be active in a monitor at any instant.
Condition Variable
Used for blocking in monitors. These variables wait for a condition to become true
Message Passing
Method of IPC that uses 2 primitives, send and receive (system calls)
Starvation
When processes continually do work but make no progress
Compute-Bound Processes
Those that spend most of their time computing (rather than waiting)
I/O-Bound Processes
Those that spend most of their time waiting for I/O (rather than computing)
Non Preemptive Scheduling Algorithm
Picks up process to run and lets it run until it blocks or voluntarily releases CPU
Preemptive Scheduling Algorithm
Allows process to run for a set amount of time. That is, clock interrupt occurs at end of given time interval and CPU control is given back to scheduler
Throughput
Number of jobs per hour that system completes
Turnaround Time
Statistically average time from the
moment that a batch job is submitted until the moment it is completed. How long user has to wait for output.
Degree of Multiprogramming
This occurs in a 3-level scheduling algorithm. It is how many processes are to be stored in memory.
Quantum
Designated time interval
Aging
Estimating the next value in a series by taking the weighted average of the current measured value and the previous estimate (a=1/2 is good)
Main memory
RAM
Memory manager
Part of OS that manages (part of) memory hierarchy
ROM
Read only memory
BIOS
Basic input output system
Memory Compaction
When swapping creates holes in memory, processes can be moved up or down to create consecutive holes that join together to form one big hole.
Overlay
Little pieces that a program has been split into
Virtual address space
Combination of virtual addresses
MMU
(memory management unit) Maps virtual memory pages onto physical memory page frames
Page frames
The units in physical memory that correspond to pages in virtual memory
Page table
Data structure used by virtual memory to store mapping between virtual and physical addresses
Volatile
Storage that only maintains its data while computer is powered
TLB
(translation lookaside buffers aka associative memory) Small hardware device for mapping virtual addresses to physical addresses
Principle of locality
Page replacement algorithms rely on the future following trends of the past. There are two types of locality: spatial and temporal
Spatial locality
If a location was recently referenced, locations NEAR it are likely to be referenced soon
Temporal locality
If a location was recently referenced, it is likely to be referenced again soon
inode
(information node) contains all information kernel has about a file except its name
ACL
(access control list or access matrix)
Access matrix
(aka ACL or access control list)
Link
Each reference to the same inode (file) from more than one directory entry is called a link
Symbolic link
(symlink) Special kind of file that contains filename
File descriptor
Small integer
Network
Interconnected or intersecting configuration or system of components
Internet
network of networks
Packet
chunk of data
Confidentiality
only sender and intended receiver should understand contents of message
Authentication
sender and receiver want to confirm identity of each other
Message integrity
the sender and receiver want to ensure that message is not altered without detection
Access and availability
services must be accessible and available to users
IPL
Interrupt priority level - how important interrupt is
