The non-empty XCMP channels in order of becoming non-empty, and the index of the first
and last outbound message. If the two indices are equal, then it indicates an empty
queue and there must be a non-OkOutboundStatus. We assume queues grow no greater
than 65535 items. Queue indices for normal messages begin at one; zero is reserved in
case of the need to send a high-priority signal message this block.
The bool is true if there is a signal message waiting to be sent.
The non-empty XCMP channels in order of becoming non-empty, and the index of the first and last outbound message. If the two indices are equal, then it indicates an empty queue and there must be a non-
OkOutboundStatus. We assume queues grow no greater than 65535 items. Queue indices for normal messages begin at one; zero is reserved in case of the need to send a high-priority signal message this block. The bool is true if there is a signal message waiting to be sent.