LSI 53C875A - Manual (Page 144)

4-52 Registers
addition, this bit may be read and written while SCRIPTS
are executing.
Register: 0x16
Mailbox Zero (MBOX0)
Read/Write
MBOX0 Mailbox Zero [7:0]
These are general purpose bits that may be read or
written while SCRIPTS are running. They also may be
read or written by the SCRIPTS processor.
Note: The host and the SCRIPTS processor code could
potentially attempt to access the same mailbox byte at the
same time. Using one mailbox register as a read only and
the other as a write only will prevent this type of conflict.
Register: 0x17
Mailbox One (MBOX1)
Read/Write
MBOX1 Mailbox One [7:0]
These are general purpose bits that may be read or
written while SCRIPTS are running. They also may be
read or written by the SCRIPTS processor.
Note: The host and the SCRIPTS processor code could
potentially attempt to access the same mailbox byte at the
same time. Using one mailbox register as a read only and
the other as a write only will prevent this type of conflict.
7 0
MBOX0
0 0 0 0 0 0 0 0
7 0
MBOX1
0 0 0 0 0 0 0 0