Take a look at this http://en.wikipedia.org/wiki/Translation_lookaside_buffer http://en.wikipedia.org/wiki/Translation_lookaside_buffer Basically when you swit

Take a look at this http://en.wikipedia.org/wiki/Translation_lookaside_buffer http://en.wikipedia.org/wiki/Translation_lookaside_buffer Basically when you switch between processes you have to update this. In 64 bit and some 32 bit operating systems you don't need to update the TLB to make system calls, the kernel shares address space with whatever process is running. With a single process running I don't think the overhead is anything to concern yourself with, but if you have a lot of processes running I think it can measurably affect performance.