Operating systems

Computer Science AS · System software. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The operating system (OS) is the software that sits between the hardware and the user. Hardware cannot be used without it: the OS gives the user an interface, lets application programs run and hides the complex details of the hardware.

The OS manages five things: memory, files, security, hardware (input, output and peripherals) and processes. Learn one or two tasks for each.

Utility software is extra system software that keeps the computer working well, such as a virus checker or back-up software. A program library is a set of routines that are already written and tested, so a developer can call them and does not have to write them again.

Rules to remember

Memory management: gives RAM to each process, keeps processes from using each other's memory, handles paging and virtual memory.
Process management: decides which process gets the processor next (scheduling) and handles interrupts.
Hardware management: uses device drivers to talk to peripherals, and buffers for input and output.
File management: file names, folders, storage space and access rights. Security management: user accounts, passwords, access control.
Disk formatter: prepares a disk for use by setting up a file system. Defragmentation: moves the blocks of each file so they sit next to each other, so a hard disk reads files faster.
Library routines are already tested, so they save development time and reduce errors.
A DLL is linked at run time, loaded only when needed and can be shared by several programs. If the DLL is missing, changed or corrupted, the program may fail.

Common mistake

Students say defragmentation frees up disk space or should be used on a solid state drive. It only rearranges file blocks, and an SSD has no moving read head so it gains no speed and loses some of its limited write cycles.

Worked example

Three programs on one computer all use the same DLL to print documents. Give two benefits of this compared with putting the print code inside each program.

A DLL is not copied into the executable file, so each of the three program files is smaller.
The DLL is loaded into memory only when a program calls it, and one copy can be shared, so less RAM is used.
If the print routine is improved, only the DLL file is replaced. The three programs do not need to be recompiled.

Answer: Smaller executable files and less memory used, and the routine can be updated without recompiling the programs.

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