Messages

A message is an untyped string of bytes of variable but limited size (64 KB), called the message body. Optionally, the sender of the message can join a second byte string to the message body, called the message annex. The message annex has a fixed size of 120 bytes. The message body and the message annex are transferred with copy semantics from the sender address space to the receiver address space.

A current message is associated with each thread. The current message of a thread is a system descriptor of the last message received by the thread. The current message is used when the thread has to reply to the sender of the message or acquire protection information about the sender of the message. This concept of current message allows the most common case, in which threads reply to messages in the order they are received, to be optimized and simplified. However, for other cases, the microkernel provides the facility to save the current message, and restore a previously saved message as the current message.