MMUless Systems
On several embedded devices, there is no Memory Management Unit, which is a requirement for implementing the copy-on-write semantics prescribed by fork. If the system has some other mechanism for per-process address spaces, such as a segment register, copying the entire process memory to the new process achieves the desired effect, however this is a costly operation and most likely unneeded given that the new process almost immediately replaces the process image in most instances.
If all processes share a single address space, then the only way fork could be implemented would be to swap the memory pages along with the rest of the task context switch. Rather than doing that, embedded operating systems such as uClinux usually omit fork and only implement vfork; part of the work porting to such a platform involves rewriting code to use the latter.
Read more about this topic: Fork (operating System)
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—Ivan Sergeevich Turgenev (1818–1883)