The Amiga computer can be used to emulate several other computer platforms, including legacy platforms such as the Commodore 64, and its contemporary rivals such as the IBM PC and the Macintosh.
MS-DOS on Amiga via Sidecar or Bridgeboard
MS-DOS compatibility was a major issue during the early years of the Amiga's lifespan in order to promote the machine as a serious business machine. In order to run the MS-DOS operating system, Commodore released the Sidecar for the Amiga 1000, basically an 8088 board in a closed case that connected to the side of the Amiga. Clever programming (a library named Janus, after the two-faced Roman god of doorways) made it possible to run PC software in an Amiga window without use of emulation. At the introduction of the Sidecar the crowd was stunned to see the MS-DOS version of Microsoft Flight Simulator running at full speed in an Amiga window on the Workbench.
Later the Sidecar was implemented on an expansion card named "Bridgeboard" and was released as the A2088XT for Amiga 2000+ models. Bridgeboard models based on the Intel 80286 and 80386 CPUs were later released by Commodore as the A2286 and A2386. The Bridgeboard card and the Janus library made the use of PC expansion cards and harddisk/floppydisk drives possible. Later third party cards also appeared for the Amiga 500 and Amiga 600 expansion slot such as the KCS Powerboard, and Vortex released full-length cards for the Amiga 2000+ based on the 80386 and 80486 CPUs called the Golden Gate.
Eventually, full-software emulators, such as PC-Task and PCx allowed Amigas to run MS-DOS programs, including Microsoft Windows, without additional hardware, at the costs of speed and compatibility.
The KCS PowerPC board
Dutch Amiga Kolff Computer Supplies built a similar expansion for the A500. It was later improved so it could emulate VGA. It did not multitask however.
Amiga Transformer
When Commodore introduced the Amiga 1000 in July 1985 it also unexpectedly announced a software-based IBM PC emulator for it. The company demonstrated the emulator by booting IBM PC DOS and running Lotus 1-2-3. Some who attended the demonstration were skeptical that the emulator, while impressive technically, could run with acceptable performance. The application, called Transformer, was indeed extremely slow; The 'Landmark' benchmark rated it as a 300 kHz 286, far slower than the 4.7 MHz of IBM's oldest and slowest PC. In addition, it would only run on Amigas using the 68000 microprocessor, and would not run if the Amiga had more than 512K of RAM.
PCTask
PCTask is a software PC emulator emulating PC Intel hardware with 8088 processor and CGA graphic modes. The latest version of it (4.4) was capable to emulate an 80386 clocked at 12 MHz and features include support for up to 16 MiB RAM (15 MB extended) under MS-DOS, up to two floppy drives and 2 hard drives. The emulator could make use of hardfile devices and then it could handle multiple hard disk files and hard disk partitions. It supported high Density floppies and CD-ROM if the Amiga hardware had mounted those devices.
The graphics mode available were MDA, CGA, EGA, VGA and SVGA emulating Hercules graphic cards with 512 KiB to 2 MiB RAM, and up to 256 colors on Amiga AGA machines, and could make use of Amiga graphic boards (e.g. Cybergraphics, EGS Spectrum, Picasso).
Parallel, Serial and PC speaker emulation, and mouse support, including serial mouse emulation were also granted.
If the Amiga hardware is fast enough (68060 or PPC) and has enough RAM, there could be also the possibility to run multiple PC-Task processes on the same machine, run MS-DOS applications in an Amiga window on a public screen (e.g. on Amiga Workbench GUI).
PCTask could also transfer files between Amiga side and the emulated MS-DOS machine; it could make use of GoldenGate bridge cards which allow the Amiga equipped with expansion slots to get complete control of its silent ISA slots and use PC-ISA cards. And latest version of it (4.4) could run even Microsoft Windows up to 95.
PcTask has an 8088/80286/80486 JITM (Just in Time Machine) capable to map all instructions of these processors, but require 4 megabytes extra of RAM for activating this feature.
PcTask has been re-released as freeware by its author.