"Software interrupts are delivered using signals"
Is this always true, if not then
a) what is the difference between two
If yes, is there some other mechanism, by which software interrupts are raised, other than delivering signals.
The author of that quote appears to be using "Software interrupt" in a very general sense (i.e. "anything that causes a program to diverge from it's usual flow of operation and does not originate in hardware") and "signals" in the sense of a particular mechanism used by a particular operating system (probably unix derived).
Yes its always true that software interrupts are delivered using signals to the operating system. In fact even hardware interrupts are themselves delivered using signals to operating system.
Signal is an IPC(inter process communication) mechanism that is used by 'interrupt mechanism' to notify that something has occurred.
来源：https://stackoverflow.com/questions/3993479/difference-between-software-interrupt-and-signal