In addition to the synchronous Send/Receive/Reply services, the OS also supports fixed-size, nonblocking messages. These are referred to as pulses and carry a small payload (four bytes of data plus a single byte code).
Pulses pack a relatively small payload—eight bits of code and 32 bits of data. Pulses are often used as a notification mechanism within interrupt handlers. They also allow servers to signal clients without blocking on them.
Figure 1. Pulses pack a small payload.