Hitachi H8S/2646 Hardware Manual page 582

Bit 1—Unread Interrupt Flag: Status flag indicating that a receive message has been overwritten
while still unread.
Bit 1: IRR9
0
1
Bit 0—Mailbox Empty Interrupt Flag: Status flag indicating that the next transmit message can
be stored in the mailbox.
Bit 0: IRR8
0
1
550
Description
[Clearing condition]
Clearing of all bits in UMSR (unread message status register) (Initial value)
Unread message overwrite
[Setting condition]
When UMSR (unread message status register) is set
Description
[Clearing condition]
Writing 1
Transmit message has been transmitted or aborted, and new message can
be stored
[Setting condition]
When TXPR (transmit wait register) is cleared by completion of transmission
or completion of transmission abort
(Initial value)