SH7047 Datasheet, PDF (486/764 Pages) Renesas Technology Corp – Renesas 32-Bit RISC Microcomputer SuperHTMRISC engine Family/SH7000 Series

Mailbox 31 and 0 is a receive-only box, and all the rest of mailboxes (1 to 30) can operate as both
receive and transmit mailboxes depending on the MBC bits.
The following table lists the address map of mailboxes and bit assignment.
Rev. 2.00, 09/04, page 446 of 720