A Tiny Operating System is an operating system that uses minimal resources. In this context, resources are defined as:
Tiny operating system definitions depend upon the processor size, that is, tiny given the hardware constraints. It is easy enough to have a tiny operating system, but in fairness, it must support a broad set of hardware features to be accurately compared to other solutions running on the same hardware.
Bloatware has attacked most operating systems today. By failing to focus on keeping the solution small as features are added and failing to keep the system modular and tailorable to new requirements without adding unneeded size, most operating systems now fail any measure of the tiny operating system tests.
Some tiny operating systems that offer modularity and support open standards are the ones of most interest today. If an operating system does not offer native support for POSIX and Linux today, it is simply obsolete. The basic level of compatibility is POSIX class 1 – multi-threading with communication, synchronization and interrupt handling. The next level is POSIX class 2 which adds I/O to POSIX class 1. Both of these approaches work well on processors without an MMU. When an MMU is added, POSIX class 3, which adds processes to the mix, may be used, but generally a POSIX class 2 system can offer a much smaller size for embedded systems even if the MMU is present.
The commercial tiny operating systems that fall into the open systems category with actual POSIX and Linux compatible code for all major features, smaller embedded targets and have active development include:
Proprietary operating systems which don't fit this criteria include all of the common operating system names excluded here including most of the other well known kernels and operating systems widely used in industry today.
Of these tiny operating systems, DSPnano RTOS and the Unison RTOS offer an open source model found at planetopensource.com.
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