Information sent on a signal-notification channel. These channels are inbound (to an SPE) registers. They can be used by the PPE or other processor to send information to an SPE. Each SPE has two 32-bit signal-notification registers, each of which has a corresponding memory-mapped I/O (MMIO) register into which the signal-notification data is written by the sending processor. Unlike mailboxes, they can be configured for either one-to-one or many-to-one signalling.
These signals are unrelated to UNIX signals. See also channel and mailbox.