Hitachi H8S/2646 Hardware Manual page 600

• Setting for reception
Transmit/receive mailbox setting (mailboxes 1 to 15)
Setting a bit to 1 in the mailbox configuration register (MBCR) designates the corresponding
mailbox for reception use. When setting mailboxes for reception, to improve message
transmission efficiency, high-priority messages should be set in low-to-high mailbox order
(priority order: mailbox 1 > mailbox 15).
• Receive-only mailbox (mailbox 0)
No setting is necessary, as this mailbox is always used for reception.
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