Emulators are applications which reproduce another, different computer system in software allowing the software from one machine to be run on another. Emulators are used for a wide range of purposes. For example, OS X provides emulation on x86 computers to allow old PowerPC binaries to be run. Microsoft provide an ARM PDA emulator for testing applications without having to repeatedly copy things onto a device.
One of the most popular uses for emulators however is to play video games from older (or in some exceptional circumstances, current) systems on modern hardware. Nintendo sells a number of official emulators through its Virtual Console service for the Wii.
PSP and emulators
One of the most popular uses for the PSP in homebrew is to run emulators of old video games systems (though some old home computers and calculators are also emulated). PSP is suited to this task because it has a fairly standard control layout similar (or almost identical to) the layouts of the input devices of the systems it emulates. Furthermore, it has a 480x272 resolution LCD which is large enough to accommodate most older systems without any downscaling. Of course, it is also portable meaning you can play these games whenever and wherever you want without having to carry a TV, machine and collection of cartridges.
What is needed to run emulators
In most cases, emulators can simply be loaded into a homebrew-enabled PSP however some require a dump of the BIOS ROM stored within the system itself. These dumps cannot be distributed legally without the permission of the company/individual who created them due to them being copyrighted.
An emulator is no use without software to run on it. These normally come in the form of ROM dumps (since they are extracted from the ROM chips that used to be used to distribute software) generally known as just "ROMs". In general, these are illegal under copyright law unless you dump them yourself (and even then you are only allowed to make use of them under very specific circumstances). ROMs for one system will not work on another system's emulator, just as you can't plug Super Nintendo cartridges into a PlayStation and run them. As such, you need ROMs for each system and an emulator for each system (though some special cases exist such as MAME which will run ROMs from multiple arcade systems).
What can the PSP emulate
The PSP, despite its apparent low clock speed when compared to those of modern desktop PCs (PSP uses a MIPS processor, while most desktop computers now will use x86, so MIPS is more efficient and thus will get more done per cycle), is a powerful system.
It is possible that most video game machines from the 32-bit era could be emulated on the PSP with a functional Playstation emulator, GBA (it has a 32 bit CPU) and even N64 showing some promise; concerning 32 bit consoles though, Sega Saturn emulation is highly unlikely, as is Dreamcast.
Here is a list of systems for which emulators are available on the PSP (it may not be complete as new software is being released for the PSP daily). This doesn't take into account how complete the emulators are, merely that they exist.
- BBC Micro | PSPBEEB
- Apple ][ | Apple II Portable emulator
- 130XE | Atari800PSP, AtariPSP
- 800/800XL | Atari800PSP, AtariPSP
- 2600 | PSP2600, StellaPSP
- 5200 | Atari800PSP, AtariPSP
- 7800 | PSP7800
- Jaguar | Virtual Jaguar PSP
- Lynx | PLynx, HandyPSP
- ST | CaSTawayPSP
- Vectrex | PSPVECX
- Odyssey 2 | EMUODD
- Intellivision | PSPInt
Miles Gordon Technology
- SAM Coupé | PSPSIM
- GameBoy (and GameBoy Color) | RIN, PSPGBC
- GameBoy Advance | GPSP, PSPVBA, PSPGBA
- NES | Nester J, InfoNES PSP
- SNES | Snes9xTYL
- N64 | Daedalus, Monkey 64
- DS | DeSmuMEPSP
- Dreamcast | nullDC-PSP
- Game Gear | SMSPSP, PSPMaster
- Master System | SMSPSP, PSPMaster
- Mega CD | Picodrive
- Mega Drive | DGENPSP, Generator PSP, MEGADRIVE, PSP Genesis, MasterBoy
- Sega Saturn| Yabause-PSP
- VMU(Dreamcast) | VMU, VirtuaMUnstaz