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The effect of a software interrupt never takes place immediately. It may occur after only some slight delay if the affected process is currently running, or possibly after a considerable delay if the affected process is suspended and has been swapped out.
The action dictated by the interrupt is always inflicted on the affected process by itself, and hence can only occur when the affected process is active.
Where the effect is to execute a user defined procedure, the kernel mode process adjusts the user mode stack to make it appear that the procedure had been entered and immediately interrupted (hardware style) before executing the first instruction. The system then returns from kernel mode to user mode in the usual manner. The result of all this is that the next user mode instruction which is executed is the first instruction of the designated procedure.
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