UAE was originally called the Unusable Amiga Emulator, due to it not being able to boot. In its early stages, it was known as Unix Amiga Emulator and later with other names as well. Since none of the popular expansions fit any more, the acronym no longer stands for anything, and the software is simply known as UAE -- this occasionally gets backronymed as Universal Amiga Emulator.
UAE is almost a full featured Amiga emulator. It emulates most of its functions:
For software, UAE may use disk images made from original Amiga floppy disks. These images have the file extension of "ADF" (Amiga Disk File). Images of Amiga formatted hard drives can also be made. UAE also supports mapping host operating system directories to Amiga hard drives.
UAE has been ported to many host operating systems, including Linux, Mac OS, FreeBSD, DOS, Microsoft Windows, RISC OS, BeOS, the Xbox console, the PSP handheld, and even AmigaOS and AROS, where it allows software that requires the Amiga chipset to be run on PPC-based AmigaOS machines.
There have been many threads in the past on Usenet and other public forums where people argued about the possibility of writing an Amiga emulator. Some considered UAE to be attempting the impossible; to be demanding that a system read, process and output 100 MB/s of data when the fastest PC was a 66 MHz 486, while keeping various emulated chips (the Amiga chipset) all in sync and appearing as they were supposed to appear to software.
For a long time, UAE was almost entirely unusable but slowly and step by step, it fleshed out its support of the Amiga chipset and by 1998 was able to more-or-less emulate an Amiga 500 at full speed.
Today, UAE is usable, thanks partly to the effort taken to develop it and partly to the big improvements in technology that brought computers many times faster than those UAE was initially expected to run on. Many Amiga games and applications can run smoothly on a Pentium II-era system. The realization that a useful Amiga emulator could be written contributed to an increase in enthusiasm about emulation, which started or sped-up efforts to write emulators for other and often less popular computer and electronic game architectures.
A major improvement was made in 2000 by Bernd Meyer with the use of Just-in-time compilation, which significantly improved the emulation speed, to the extent that average PCs could now emulate some Amiga software faster than any real Amiga could run it. UAE can use as much of the host's power in native mode as possible, or balance it with other requirements of the host OS, or to accurately reflect the original speed, depending on a user's choice. UAE also provides an RTG-compatible "video card" for the Amiga side of the emulation which is tailored for display on the host hardware, so as not to be limited to the emulation of the original Amiga video hardware.
There are currently two main forks of the original program:
Today the most active fork is WinUAE; however, current versions of this still contain some serious bugs and compatibility issues. WinUAE has reasonable compatibility for some software but, just like a "real" Amiga, for some old games it requires careful configuration in order to match the originally-supported hardware. For example, 68000 code could cause an exception on an emulated 68040, just like it would perhaps do on an Amiga 4000/040.