Editing OS-9 (section)

===Modern and archaic design===
OS-9 (especially the 68k version and thereafter) clearly distinguishes itself from the prior generation of embedded operating systems in many aspects.
* Runs on 8-bit, 16-bit, and [[32-bit CPU]]s.
* Clear separation between [[user mode]] and supervisor (kernel) mode.
* Dynamic use of individually and separately built software components (executable program images and [[kernel module]]s) rather than a [[Static library|statically linked]] single monolithic image.
* Unix-like process name-space model (not [[Memory model (computing)|memory model]]) and user shell program.
* Clear separation between hardware independent (e.g. file managers) and hardware dependent (e.g. [[device driver]]s) layers.

When compared with more modern operating systems:
* The kernel is written entirely in [[assembly language]] (OS-9/68K version only) and [[C (programming language)|C]] (portable version to other architectures) using simple internal data structures, reducing flexibility and improvement scope while improving determinability required for [[real-time operating system]]s.
* Performance was also affected for some operations, but assembly language helped with the speed issue.
* Systems without a [[memory management unit]] (MMU) have no memory protection against illegal access, nor per-process memory protection, while systems with an MMU can have memory protection enabled. The module controlling the MMU can be included or omitted by the system integrator to enable or disable memory protection. This allows OS-9 to run on older systems which do not include an MMU.
* Older versions of OS-9 do not support [[POSIX threads]], while all OS-9 supported processors support POSIX threads.
* No [[Symmetric multiprocessing|SMP]] support for multiple sockets, cores, or hardware threads in the same OS-9 instance (can run as a RTOS on one of the cores of dual core processors like [[Core Duo]] and [[Core 2 Duo]], when [[Linux]] is running on the other core doing general purpose tasks).