Message handling unit

A method for controlling passing of messages in an integrated circuit comprising a first power domain and a second power domain;

• the method comprising:
• receiving a message send request from a sender circuit in the first power domain requesting sending of at least one message to the receiver circuit, at a message handling unit for controlling passing of messages sent from the sender circuit in the first power domain to the receiver circuit in the second power domain, wherein the message handling unit comprises message passing circuitry configured to write messages sent from the sender circuit to a message storage area provided in the second power domain;
• in response to receipt of the message send request when at least one circuit in the second power domain is in a quiescent state, wakeup requesting circuitry of the message handling unit transmitting a wakeup request to a second domain power controller circuitry to request that said at least one circuit in the second power domain transitions from the quiescent state to an awake state;
• and
• deferring writing of said at least one message to the message storage area in the second power domain until the message handling unit has indicated that said at least one circuit in the second power domain has transitioned from the quiescent state to the awake state in response to the wakeup request.