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In computer science, bare machine (or bare metal) refers to a computer executing instructions directly on logic hardware without an intervening operating system. Modern operating systems evolved through various stages, from elementary to the present day complex, highly sensitive systems incorporating many services. After the development of programmable computers (which did not require physical changes to run different programs) but prior to the development of operating systems, sequential instructions were executed on the computer hardware directly using machine language without any system software layer. This approach is termed the "bare machine" precursor to modern operating systems. Today it is mostly applicable to embedded systems and firmware generally with time-critical latency requirements, while conventional programs are run by a runtime system overlaid on an operating system.
The PDP-11 machine allowed programmers to load a program, supplied in machine code, to RAM. The resulting operation of the program could be monitored by lights, and output derived from mag tape, print devices, or storage.
Bare machine programming remains in common practice in embedded systems, where microcontrollers or microprocessors often boot directly into monolithic, single-purpose software, without loading a separate operating system. Such embedded software can vary in structure, but the simplest form may consist of an infinite main loop, calling subroutines responsible for checking for inputs, performing actions, and writing outputs.
- Silberschatz, A.; Galvin, Peter; Gagne, Greg. Operating System Concepts.