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Stýrikerfi Cheat Sheet by

Kafli 1

Instru­ction cycle is fetch, decode, execute
Cache er small, very fast memory on processor for recently used memory content for quick access
Memory Hierarchy. Fast, small, expensive on top, slow, large, cheap on bottom

Kafli 2

Objectives of OS: Conven­iance, effici­ency, evolution
Multip­rog­ram­ming: Processes use the same Processor
Batch proces­sing: One program executed before the next one is started
Processes: A program in execution
Resource manage­ment: Fairness, Differ­ential respon­siv­eness, effiency

Kafli 3

Instru­ction trace : Sequence of instru­ctions executed for a process
Process states : ready, running, block, suspend.
Process control block: Contains inform­ation for OS to control processes
PCB Stack = Identi­fic­ation, State, priority, pc, memory pointers, context data, io status, accounting inform­ation.
Kernel mode has access to all, user should be restricted to protect the os.

Kafli 4

Process can have one or more threads. Threads are execution units within processes while processes are binaries. Process has one process control block while threads gets it's own space on the stack and control block. Threads share the state and resources of that process

kaflar 11 oog 12

Single buffer - process writes to buffer than has to wait for I/O to write and reverse. Double buffer - Process can write to one buffer and read another, don't have to wait. Circular buffer uses more than 2 buffers.
Blocking i/o - process continues in non blocking, os takes over if it is blocking

Kafli 5

Race condition : "­los­er" makes the last change, affects all.
Mutual exclusion: only one process can access code or data.
Producer puts data in buffer, consumer takes data from buffer.
If there is no data consumer sleeps and producer wakes him when he adds data.
If buffer is full, producer sleeps and consumer wakes him when he takes data.
Reader­/Writer problem
The conditions that must be satisfied are as follows
1.Any number of readers may simult­ane­ously read the file
2.Only one writer at a time may write to the file, no reader may read it
Monitors must be invoked with a call to a monitor procedure. uses wait and signal, if no process is waiting the signal is lost.
Message passing: direct and indirect.

Kafli 6

Mutual exclusion, no preemp­tion, hold and wait, circular wait
Eliminate on of the condit­ions. Direct eliminates circular waiting, indirect the other 3. Very conser­vative
Dynamic decision if current resource request will cause deadlock. Bankers algo
Tests for deadlocks, consumes coside­rable cpu time, liberal with resources.
Where process is overlooked by scheduler even though it's ready to run.

Kafli 7

Fixed equal
Program to big, use overlays, ineffi­cient, internal fragme­ntation
Fixed unequal
Chooses the smalles space it can fit in, might lead to swappi­ng,­memory might go unused, internal fragme­ntation
Each process gets exactly what it needs, small holes, external fragme­nta­tion, compact
Best fit
Chooses the block closest in size
first fit
Chooses the next block which can hold the process. scan from beginning
next fit
Starts for current position, finds block next available large enough block.

Kafli 8

Transl­ation lookaside buffer: checks the TLB table and if it's a hit it gets the frame number immedi­ately and can add the offset and find the data it needs else it has to look up the address in the page table.

Page replac­ement: FIFO, LRU, Optimal, Clock.

kafli 9

Levels of scheduling
Long term, medium term, short term.
Long term
Controls what programs are admitted to the system for processing
medium term
if process starts in swap out it is added to medium term scheduler
Short term
when program becomes a process it is added to the short term scheduler
Preemptive and non preemptive
Preemtive can take resources, non preemptive can not


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