Computer
From Programmer's Wiki
A computer is a logic machine, almost always driven by electricity, alhough historically similar mechanical devices made of gearwheels and other mechanics have been constructed. Personal computers usually consist of the computer box, containing the mother board, storage devices (hard disks and similar) and not too few adaptors to external hardware, computer screen and input devices, at the very least a keyboard and a mouse.
The mother board contains a CPU (central processing unit), one (or in a few cases a few) VLSI chip which is designed to execute certain machine instructions sequentially. On the mother board there is also primary memory (PM), somewhat dissimilar VLSI chips which is able to store data as long as the electrical power remains on. There are also a lot of other adaptors, such as hardware slots wherein adaption cards can be inserted, such as audio card that generates electrical signals that are turned into sounds when output in a speaker, TV-cards for converting TV and video signals to data files, adaptors to connect to usual devices such as hard drive, CD/DVD readers/burners, monitor and input devices.
All these peripherals come in many brands with varying technical specifications, and the computer universe would be a pure hell if the programmer should be needed to write his/her software for every perceivable brand of peripheral. Luckily the Operating System takes care of this.
[edit] Turn-on process
When the computer is turned power-on, the CPU initializes itself by putting the program counter to point at a certain address in the primary memory and start execution from that address. This memory must therefore, att the power-on time point to a hard coded "BIOS program" (the PC term of the startup firmware), usually an UV-erasable PROM, containing a first program for the comuter to run. That BIOS scans the hard drive for a certain block of a certain sector, and loads this program into the PM starting to execute it. In the beginning of the PC age this was the "DOS-BIOS", which today is replaced by boot loaders that allow the user to select operating system, f.ex. Linux, FreeBSD, Solaris, Syllable OS, ReactOS, BeOS, and the many Windows OS:es.
[edit] Operating system
The OS provides lots of services to the programs that run on the computer, for example allowing the user to run many programs simultaneously, connecting programs to hardware, software and other programs in a standardized way, improving the so called "connectivity" of the program environment vaslty, and thereby diminishing the stress of the programmers or/and the users to manageable levels (usually).
