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RISC OS is an efficient and fantastically insecure operating system for ARM 26 and 32bit processors. Its motto ought to be “it’s probably our fault we got to this point in time without properly testing this(Steve Pampling).

RISC OS 5 is developed by RISC OS Open Ltd

The OS is available for free download, and there’s lots of documentation and an extensive forum. RISC OS 5 is basically a 32bit version of RISC OS 3.6, with some functionality from RISC OS 4/6 retrofitted. As of 2018 it is still being developed and supported. This is the best choice for new ARM hardware such as Raspberry Pi.

RISC OS 4 became RISC OS SIX and is now owned by 3QD Developments

The OS is available as a paid upgrade (over RISC OS 4), but has not been developed since 6.20 was released in 2009. It still features a lot of functionality that has not yet been implemented in 5. RISC OS 4/6 is 26bit. This is the best choice for emulators.

VirtualRPC is a commercial Acorn RiscPC emulator from 3QD Developments

Based on the freeware RedSquirrel emulator, but faster, more reliable and better integrated into Windows or MacOS. It has full networking, and can connect to any Windows printer. Its HostFS can access any number of Windows drives including USB devices. It is a commercial product and ships with RISC OS 4.02 or 4.39.

RPCEmu is an open source Acorn RiscPC emulator for various platforms

Not as well integrated with its host OS as VirtualRPC, it cannot connect to host printers and networking is complex to set up and limited in scope. Its HostFS is limited to a single directory on the host drive, and the emulator is slightly buggy. It is free, but does not come with an operating system. You can download RISC OS 5 for free, or use a copy of the OS from hardware you own.