Cisco Internetwork Operating System (IOS) is a family of network operating systems used
on many current software engineer cisco switches
ran CatOS. IOS is a package of routing, switching, internetworking
and telecommunications functions integrated into a multitasking operating
system. Although the IOS code base includes a cooperative
multitasking kernel, most IOS features have
been ported to other kernels such as QNX and Linux for use in Cisco products.
Cisco IOS has a monolithic architecture, owing to the
limited hardware resources of routers and switches in the 1980s. This means
that all processes have direct hardware access to conserve CPU processing time.
There is no memory protection between processes and IOS has a run to completion scheduler,
which means that the kernel does not pre-empt a running process. Instead the process must make a kernel call before
other processes get a chance to run. IOS considers each process a single thread and assigns it a priority value, so that high
priority processes are executed on the CPU before queued low priority
processes, but high priority processes cannot interrupt running low priority
processes
The disadvantage of the
IOS architecture is that it increases the complexity of the operating system,
data corruption is possible as one process can write over the data of another,
and one process can destabilize the entire operating system or even cause a
software-forced crash. In the event of an IOS crash, the operating system
automatically reboots and reloads the saved configuration.
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