"In v1 there was a separate system call to catch each of interrupt, quit, and two kinds of machine traps. Floating point hardware (v3) brought another and no end was in sight. To stop the proliferation, all traps were subsumed under a single system call, signal (v4). The various traps were given numbers, by which they were known until symbolic names were assigned in v7. We have not been fully weaned yet: the numbers are still needed in the shell trap command."
Regarding communications it adds:
"Never, however, was the basically unstructured signal-kill mechanism regarded as a significant means of interprocess communication. The research systems therefore declined to adopt the more reliable, but also more complex, Berkeley signals."
"In v1 there was a separate system call to catch each of interrupt, quit, and two kinds of machine traps. Floating point hardware (v3) brought another and no end was in sight. To stop the proliferation, all traps were subsumed under a single system call, signal (v4). The various traps were given numbers, by which they were known until symbolic names were assigned in v7. We have not been fully weaned yet: the numbers are still needed in the shell trap command."
Regarding communications it adds:
"Never, however, was the basically unstructured signal-kill mechanism regarded as a significant means of interprocess communication. The research systems therefore declined to adopt the more reliable, but also more complex, Berkeley signals."