"How come there will be problem with ospf process due to use of shared memory??."
I believe that's just meant as an example that a portion of IOS, with a bug, could damage other parts of the IOS. I.e. all code having complete access to all memory can allow some code to impact other code. That said, there are software techniques that can be used to mitigate such issues and having a system that does provide for "protecting" memory, generally comes with some performance cost.
BTW, even when memory is protected, within an OS, there's often levels of code trust, such as in kernels and/or drivers, that still allows exposure (just less of it). For such"trusted" code (including something like an IOS), you just work very hard to try to keep all the code bug free.
"As Cisco IOS is not supporting SMP then how does it actually works ?? on Single CPU ?"
Generally, yes. However, this often isn't as much limiting as you might think. To take advantage of multiple CPUs, there has to be something else for the OS "to do", especially taking into consideration the complexity and overhead of coordination. As a router or switch, generally, provides a single purpose, there's often not much for its OS to gain with multiple CPUs, especially in low end devices. One area where something else "to do" has been found hosting some add on features, not purely related to the main IOS's function (of pure traffic forwarding). IOS-XE addresses this by allowing something "else" to take advantage of other CPUs while the IOS still only uses one.
Also BTW, consider something like using a word processing program to edit some document. Assuming you're only the one using it, how would additional CPUs help improve its performance? Generally, it doesn't. This is somewhat the same issue IOS has.
