5.1 Operating Systems

## operating system (OS)

### Need for OS

- The hardware is unusable without an OS // hides complexity of hardware from user
- Acts as an interface/ controls communications between user and hardware / hardware and software // or by example
- Provides software platform / environment on which other programs can be run
#### Provision of a user interface:
- Allows a user to communicate with the hardware // vice-versa 
- ... by making navigation around the system easier 
- Provides facility for user inputting data
- Provides facility for outputting to the user
- Command line / Graphical User Interface

### Key Management Tasks

#### (Main) Memory Management

- Controls the movement of data between RAM, processor.
- Allocates memory to processes
- Ensures fair usage of memory
- Organises memory
- Keep processes separate
- To release memory when a process stops
- Makes use of virtual memory

#### File Management

- defining the file naming conventions
- performing specific tasks such as create, open, close, delete, rename, copy, move.
- maintaining the directory structures
- ensuring memory allocation for a file by reading it from the HDD/SSD and loading into memory

#### Security Management

- Provide /upgrade firewall / anti-malware
- Makes provision for recovery when data is lost
- Provides usernames and passwords / encryption / user accounts
- Prevents unauthorised access
- Ensures privacy of data

#### Hardware (input/output/peripherals) Management

- Use of device drivers
- Installation of appropriate driver software
- Controls access to data being sent to/from hardware/peripherals
- Controls access to hardware/peripherals
- Manages communication between devices / hardware and software

#### Process Management

- Decides which process to run next
- Supports multitasking
- Allocation of processor time
- Scheduling of processes or tasks / multi-tasking / multi-programming etc.
- Resolution of conflict when two or more processes require the same resource

### utility software

- To help users to set-up / configure / analyse / optimise / maintain the computer ... 
- .. by for example, making memory allocation more efficient 
- ... by for example, checking the system for faults

##### Backup software

- Creates a copy of the contents of a disk / partition. Can be set up to automatically backup // schedules backups
- Allows the user to decide what is backed up, e.g. all data // all files that have changed since the last backup
- Allows the user to set up an off-site backup
- May encrypt the backup files
- Restores the data if necessary

##### Virus checker

- Scans files stored on a computer system for malicious code
- Scans files when they enter the system / memory stick inserted / download etc
- Sets up a schedule for virus-checking
- Isolates / quarantines / deletes viruses
- Regularly updates the virus definitions

##### Defragmentation

- Make individual files occupy contiguous blocks // move free space together
- Improve disk access times // Data/files can be loaded faster
- Finds files that are split across the disk
- ...and moves the blocks that make up each file to be contiguous
- Collates free space

##### File compression

- Reduces the file size
- ...by using algorithms to change the data 
- ... which can be either lossy or lossless

##### Disk content analysis

- used to check disk drives for empty space and disk usage by reviewing files and file folders.
- can lead to optimal use of disk space by removal of unwanted files and downloads

##### Disk Formatter

- removing existing data from a disk that has been used previously
- setting up the file system on the disk
- partitioning the disk into logical drives if this is required.

##### Disk repair software

- Allows disk to report errors (e.g. “bad sector”)
- Restore the functionality of a damaged or corrupted disk
- Preventing data loss and ensuring the proper operation of the storage device.

### Program libraries

- Set of pre-written / pre-compiled / pre-tested subroutines 
- ... which can be called in other programs
- by installing/importing the library

##### Benefits

- Saves (programming/testing) time as code does not have to be written/re-written from scratch // code does not have to be tested
- Code is already tested so it is more robust/likely to work
- If there is an improvement in the library routine the program updates automatically
- can perform complex calculations that the programmer may be unable to do

##### Drawbacks

- Compatibility issues
- ...may not work with the other code/may require changing program for it to work
- Not guaranteed thorough testing
- ... may be unknown or unexpected bugs / virus
- Library routine may not meet exact needs 
- .. may give unexpected results //. may need editing
- If library routine is changed
- ... there may be unexpected results / errors

#### Dynamic link library files - DLL

- A collection of self-contained (shared library) programs that are already compiled
- Linked to the main program during execution
- Library program code is separate from the EXE file
- Library file only loaded into memory when required at run time
- A DLL file can be made available to several applications (at the same time)
- If DLL routine is updated the program that uses it will run the update

##### Benefits

- (main) memory requirements for program is reduced 
- .. as dynamic link library is loaded only once / when required 
- the executable file size of the program using the DLL will be smaller 
- ...because the executable does not contain (all) the library routines
- maintenance not needed to be done by the programmer 
- ...because the DLL is separate from program
- no need to recompile the main program when changes are made to DLL 
- ... because changes / improvements/ error correction to the DLL file code are done independently of the main program
- A single DLL file can be made available to several application programs
- ..Saving space in memory / easing the pressure on memory

##### Drawbacks

- The executable code is not self-contained ...
- ... the DLL file(s) needed to be included at run time.
- Appropriate (linking) software must be available at run-time ...
- ... to link/include / import the DLL files.
- The DLL file must be present ...
- ... otherwise (unable to find X.dIl) errors
- Unexpected changes to the DLL file / corrupted DLL file ...
- ... could mean the program stops working as expected
- Malicious changes to the DLL file ...
- ... could install a virus on the user's computer / related files could be corrupted