SMP support

Released versions of QNX 4 are strictly single-processor, whereas QNX Neutrino, at this time, has support for SMP.

SMP is a great feature, especially in an operating system that supports threads, but it's also a bigger gun that you can shoot yourself in the foot with. For example, on a single-processor box, an ISR will preempt a thread, but never the other way around. On a single-processor box, it's a worthwhile abstraction to "pretend" that threads run simultaneously, when they don't really.

On an SMP box, a thread and ISR can be running simultaneously, and multiple threads can also be running simultaneously. Not only is an SMP system a great workstation, it's also an excellent SQA (Software Quality Assurance) testing tool—if you've made any "bad" assumptions about protection in a multithreaded environment, an SMP system will find them eventually.

Note: To illustrate just how true that statement is, one of the bugs in an early internal version of SMP had a "window" of one machine cycle! On one processor, what was supposedly coded to be an atomic read/modify/write operation could be interfered with by the second processor's compare and exchange instruction.