A version of the OS, named NCOS, was used in Oracle's Network Computer and compatible systems.

Meanwhile, Element 14 had also kept a copy of RISC OS 3.8 in house, which they developed into NCOS for use in set-top boxes.

RISC OS Adjust was a culmination of all the Select Scheme updates to date, released as a physical set of replaceable ROMs for the RiscPC and A7000 series of machines.

The following year, Castle Technology bought RISC OS from Pace for an undisclosed sum.

[17] In 2018 RISC OS Developments acquired Castle Technology Ltd including its intellectual property.

[18] While most current desktop OSes use preemptive multitasking (PMT) and multithreading, RISC OS remains with a CMT system.

[20][21] The core of the OS is stored in ROM, giving a fast bootup time and safety from operating system corruption.

RISC OS 4 and 5 are stored in 4 MB of flash memory, or as a ROM image on SD card on single board computers such as the Beagleboard or Raspberry Pi, allowing the operating system to be updated without having to replace the ROM chip.

RISC OS filetypes can be preserved on other systems by appending the hexadecimal type as ',xxx' to filenames.

[22][23] When using cross-platform software, filetypes can be invoked on other systems by naming appending '/[extension]' to the filename under RISC OS.

This allows transparent handling of archives and similar files, which appear as directories with some special properties.

[25] The RISC OS kernel is single-tasking and controls handling of interrupts, DMA services, memory allocation and the video display; the cooperative multi-tasking is provided by the WindowManager module.

The OS differentiates them from normal directories through the use of an exclamation mark (also called a pling or shriek) prefix.

The application's executable files and resources are contained within the directory, but normally they remain hidden from the user.

Acorn's own main bundled applications were not updated to comply with the guide until RISCOS Ltd's Select release in 2001.

[citation needed] Successive OS upgrades have raised more serious issues of backward compatibility for desktop applications and games.

[citation needed] The introduction of the RiscPC in 1994 and its later StrongARM upgrade raised issues of incompatible code sequences and proprietary squeezing (data compression).

Patching of applications for the StrongARM was facilitated and Acorn's UnsqueezeAIF software unsqueezed images according to their AIF header.

The version of the OS for the A9home prevented the running of software without an AIF header (in accord with Application Note 295)[49] to stop "trashing the desktop".

[59] RISC OS has also been used by both Acorn and Pace Micro Technology in various TV connected set-top boxes, sometimes referred to instead as NCOS.

[citation needed] A special cut down RISC OS Pico (for 16MiB cards and larger) styled to start up like a BBC Micro was released for BASIC's 50th anniversary.

[60] SD card images have been released for downloading free of charge to Raspberry Pi 1, 2, 3, & 4 users with a full graphical user interface (GUI) version[61] and a command-line interface only version (RISC OS Pico, at 3.8 MB).