BrickOS is an alternative RCX operating system which offers preemptive multitasking, dynamic memory management, POSIX semaphores, critical sections and native access to display, buttons, IR communication, motors and sensors. Additional features include support for the LEGO Remote control, support for a number of third party sensors and access to the kernel source code (for those so inclined).
BrickOS allows you to unleash the full potential of the RCX by providing a lightweight, comprehensive native mode kernel and standard library. Also known as replacement firmware, native mode busts the bytecode barrier and allows you to program in assembler, standard C and C++.
BrickOS is supported on most Linux platforms and most Windows platforms (via CYGWIN). BrickOS programs and kernel are compiled using the GNU C/C++ cross-compiler and the Hitachi H8 toolchain.
BrickOS was originally written by Markus L. Noga who released it under the name of LegOS. The name was later changed in response to a request from the LEGO Company. They were gracious in letting the project change names at its own pace.