# Computer

The word *computer* can be defined in many ways and can also take many different meanings; a somewhat common definition may be this: computer is a machine that automatically performs mathematical computations. We can also see it as a machine for processing [information](information.md), manipulating symbols or, very generally, as any tool that helps computation, in which case one's fingers or even a [mathematical](math.md) formula itself can be considered a computer. Here we are of course mostly concerned with electronic [digital](digital.md) computers.

We can divide computers based on many attributes, e.g.:

- by continuous or discrete **representation of data**: [digital](digital.md) vs [analog](analog.md)
- by **[hardware](hw.md) technology**: [electronic](electronics.md) ("lightning in sand"), [mechanical](mechanical.md), [quantum](quantum.md), biological etc.
- by **purpose**: special purpose vs general purpose, [personal](pc.md), [server](server.md), [embedded](embedded.md), [supercomputers](supercomputer.md), gaming computer etc.
- by **[programmability](programming.md)**: non-programmable, partially or fully programmable
- by the theoretical **[model of computation](model_of_computation.md)** it is based on: [Turing machine](turing_machine.md), [lambda calculus](lambda_calculus.md) etc.
- by **computational power**: how difficult problems the computer is able to solve, i.e. where in the [Chomsky hierarchy](chomsky_hierarchy.md) it stands (typically we want [Turing complete](turing_completeness.md) computers)
- by **other criteria**: price, reliability, durability etc.

Computers are theoretically studied by [computer science](compsci.md). The kind of computer we normally talk about consists of two main parts:

- **[hardware](hw.md)**: physical parts
- **[software](sw.md)**: [programs](program.md) executed by the hardware, made by [programmers](programmer.md)

**The power of computers is limited**, [Alan Turing](turing.md) mathematically proved that there exist problems that can never be completely solved by any [algorithm](algorithm.md), i.e. there are problems a computer (including our [brain](brain.md)) will never be able to solve (even if solution exists). This is related to the fact that the power of mathematics itself is limited in a similar way (see [Godel's theorems](incompleteness_theorems.md)). Turing also invented the theoretical model of a computer called the [Turing machine](turing_machine.md). Besides the mentioned theoretical limitation, many solvable problems may take too long to compute, at least with computers we currently know (see [computational complexity](computational_complexity.md) and [P vs NP](p_vs_np.md)).

## Typical Computer

Computers we normally talk about in daily conversations are [electronic](electronics.md) [digital](digital.md) mostly personal computers such as [desktops](desktop.md) and [laptops](laptop.md), possibly also [cell phones](phone.md), [tablets](tablet.md) etc.

Such a computer consists of some kind of [case](case.md) (chassis), internal [hardware](hardware.md) plus [peripheral devices](peripheral.md) that serve for [input and output](io.md) -- these are for example a [keyboard](keyboard.md) and [mouse](mouse.md) (input devices), a [monitor](monitor.md) (output device) or [harddisk](hdd.md) (input/output device). The internals of the computer normally include:

- **[motherboard](motherboard.md)**: The main electronic circuit of the computer into which other components are plugged and which creates the network and interfaces that interconnect them (a [chipset](chipset.md)). It contains slots for expansion cards as well as connectors for external devices, e.g. [USB](usb.md). In a small memory on the board there is the most basic software ([firmaware](firmware.md)), such as [BIOS](bios.md), to e.g. enable installation of other software. The board also carries the [clock](clock.md) generator for synchronization of all hardware, heat sensors etc. 
- **[CPU](cpu.md)** (central processing unit): Core of the computer, the chip plugged into motherboard that performs general calculations and which runs [programs](program.md), i.e. [software](sw.md).
- **[RAM](ram.md)/working memory/main memory**: Lower capacity volatile (temporary, erased when powered off) working memory of the computer, plugged into motherboard. It is used as a "pen and paper" by the CPU when performing calculations.
- **[disk](disk.md)**: [Non-volatile](volatile.md) (persisting when powered off) large capacity memory for storing [files](file.md) and other [data](data.md), connected to the motherboard via some kind of [bus](bus.md). Different types of disks exist, most commonly [hard disks](hdd.md) and [SSDs](ssd.md).
- **expansion cards ([GPU](gpu.md), sound card, network card, ...)**: Additional hardware cards plugged into motherboard for either enabling or accelerating specific functionality (e.g. GPU for graphics etc.).
- **[PSU](psu.md)** (power supply unit): Converts the input electrical power from the plug to the electrical power needed by the computer.
- other things like fans for [cooling](cooling.md), batteries in laptops etc.

## Notable Computers

Here is a list of notable computers.

{ Some nice list of ancient computers is here: https://xnumber.com/xnumber/frame_malbum.htm. ~drummyfish }

| name                                    | year | specs (max, approx)                               | comment                                           |
| --------------------------------------- | ---- | ------------------------------------------------- | ------------------------------------------------- |
| [brain](brain.md)                       |-500M | 86+ billion neurons                               | biological computer, developed by nature          |
| [abacus](abacus.md)                     |-2500 |                                                   | one of the simplest digital counting tools        |
| Antikythera mechanism                   | -125 | ~30 gears, largest with 223 teeth                 | 1st known analog comp., by Greeks (mechanical)    |
| [slide rule](slide_rule.md)             | 1620 |                                                   | simple tool for multiplication and division       | 
| Shickard's calculating clock            | 1623 | 17 wheels                                         | 1st known calculator, could multiply, add and sub.| 
| [Arithmometer](arithmometer.md)         | 1820 | 6 digit numebrs                                   | 1st commercial calculator (add, sub., mult.)      | 
| Difference Engine                       | 1822 | 8 digit numbers, 24 axles, 96 wheels              | mech. digital comp. of polynomials, by Babbage    | 
| Analytical Engine design                | 1837 | ~16K RAM, 40 digit numbers                        | 1st general purpose comp, not realized, by Babbage| 
| [nomogram](nomogram.md)                 | 1884 |                                                   | graphical/geometrical tools aiding computation    |
| [Z3](z3.md)                             | 1941 | 176B RAM, CPU 10Hz 22bit 2600 relays              | 1st fully programmable electronic digital computer| 
| [ENIAC](eniac.md)                       | 1945 | ~85B RAM, ~5KHz CPU, 18000 vaccum tubes           | 1st general purpose computer                      | 
| [PDP](pdp.md) 11                        | 1970 | 4M RAM, CPU 1.25Mhz 16bit                         | legendary [mini](minicomputer.md)                 |
| [Apple II](apple_ii.md)                 | 1977 | 64K RAM, 1MHz CPU 8bit                            | popular TV-attached home computer by Apple        |
| [Atari](atari.md) 800                   | 1979 | 8K RAM, CPU 1.7MHz 8bit                           | popular TV-attached home computer by Atari        |
| [VIC 20](vic_20.md)                     | 1980 | 32K RAM, 1MHz CPU 8bit, 20K ROM                   | successful TV-connected home computer by Commodore| 
| [IBM PC](ibm_pc.md)                     | 1981 | 256K RAM, CPU 4.7MHz 16bit, BASIC, DOS            | 1st personal computer as we know it now, modular  |
| [Commodore 64](c64.md)                  | 1982 | 64K RAM, 20K ROM, CPU 1MHz 8bit                   | very popular TV-connected home computer           |
| [ZX Spectrum](zx_spectrum.md)           | 1982 | 128K RAM, CPU 3.5MHz 8bit                         | successful UK TV-connected home comp. by Sinclair |
| [NES](nes.md)/Famicom                   | 1983 | 2K RAM, 2K VRAM, CPU 1.7MHz 8bit, PPU             | TV-connected Nintendo game console                |
| [Macintosh](macintosh.md)               | 1984 | 128K RAM, CPU 7MHz 32bit, floppy, 512x342         | very popular personal computer by Apple           |
| [Amiga](amiga.md)                       | 1985 | 256K RAM, 256K ROM, CPU 7MHz 16bit, AmigaOS       | personal compuer by Commodore, ahead of its time  |
| [SNES](snes.md)                         | 1990 | 128K RAM, 64K VRAM, CPU 21MHz 16bit               | game console, NES successor                       |
| [PlayStation](playstation.md)           | 1994 | 2M RAM, 1M VRAM, CPU 33MHz 32bit, CD-ROM          | popular TV-connected game console by Sony         |
| [TI](texas_instruments.md)-80           | 1995 | 7K RAM, CPU 980KHz, 48x64 1bit screen             | famous programmable graphing calculator           | 
| [Deep Blue](deep_blue.md)               | 1995 | 30 128MHz CPUs, ~11 GFLOPS                        | 1st computer to defeat world chess champion       |
| [Nintendo 64](n64.md)                   | 1996 | 8M RAM, CPU 93MHz 64bit, 64M ROM cartr.           | famous TV-connected game console                  |
| [GameBoy Color](gbc.md)                 | 1998 | 32K RAM, 16K VRAM, CPU 2MHz 8bit, 160x144         | handheld gaming console by Ninetendo              |
| [GameBoy Advance](gba.md)               | 2001 | ~256K RAM, 96K VRAM, CPU 16MHz 32bit ARM, 240x160 | successor to GBC                                  |
| [Xbox](xbox.md)                         | 2001 | 64M RAM, CPU 733MHz Pentium III                   | TV-connected game console by Micro$oft            |
| [Nintendo DS](nds.md)                   | 2004 | 4M RAM, 256K ROM, CPU ARM 67MHz, touchscreen      | famous handheld game console by Nintendo          | 
| [iPhone](iphone.md) (aka spyphone)      | 2007 | 128M RAM, CPU ARM 620MHz, GPU, cam., Wifi, 480x320| 1st of the harmful Apple "smartphones"            |
| [ThinkPad](thinkpad.md) X200            | 2008 | 8G RAM, CPU 2.6GHz, Wifi                          | legendary laptop, great constr., freedom friendly |
| [ThinkPad](thinkpad.md) T400            | 2008 | 8G RAM, CPU 2.8GHz, Wifi                          | legendary laptop, great constr., freedom friendly |
| [Raspberry Pi 3](rpi.md)                | 2016 | 1G RAM, CPU 1.4GHz ARM, Wifi                      | very popular tiny inexpensive SBC                 |
| [Arduboy](arduboy.md)                   | 2016 | 2.5K RAM, CPU 16MHz AVR 8bit, 1b display          | tiny Arduino [open console](open_console.md)      |
| [Pokitto](pokitto.md)                   | 2017 | 36K RAM, 256K ROM, CPU 72MHz ARM                  | indie educational [open console](open_console.md) |
| [Raspberry Pi 4](rpi.md)                | 2019 | 8G RAM, CPU 1.5GHz ARM, Wifi                      | tiny inexpensive SBC, usable as desktop           |
| [Deep Thought](deep_thought.md)         |      |                                                   | fictional computer from Hitchhiker's Guide ...    |
| [HAL 9000](hal_9000.md)                 |      |                                                   | fictional AI computer (2001: A Space Oddysey)     |
|[PD computer](public_domain_computer.md) |      |                                                   | planned LRS computer                              |
|[Turing machine](turing_machine.md)      |      |                                                   | important theoretical computer by Alan Turing     |

TODO: mnt reform 2, pinephone, NeXT, 3DO, ti-89, quantum?, wii?