
SMSC DS – FDC37N958FR
Page 181
Rev. 09/01/99
Mailbox Register Interface
The FDC37N958FR provides a set of 16 8 bit
registers, called mailbox registers, by which the
Host CPU may communicate with the 8051.
These registers are accessible to the host in
configuration Mode or through the open mode
Index and Data Registers also described in
Configuration section. At the same time these
registers are accessible to the 8051 through 16
memory mapped control registers. Fourteen of
these mailbox registers are general purpose and
are
typically
used
to
parameters. The remaining two mailbox registers
(mbox-0: System-to-8051, and mbox-1: 8051-to-
System) are specifically designed to pass
commands and to provide a means for each to
interrupt the other assuming interrupts are
unmasked. These registers are not “dual-
ported” meaning that the system BIOS and
keyboard BIOS must be designed to properly
share these registers.
pass
status
and
Note: When the Host CPU performs a write of
the System-to-8051 mailbox register an 8051
INT1 will be generated and seen by the 8051 if
unmasked. When the 8051 writes to the
System-to-8051 mailbox register the data is
blocked but the write forces the System-to-8051
register to clear to “0”, providing a means for the
8051 to inform that Host that an operation has
been completed.
When the 8051 performs a write of the 8051-to-
System mailbox register an SMI may be
generated and seen by the Host if unmasked.
When the host CPU writes to the 8051-to-
System mailbox register the data is blocked but
the write forces the 8051-to-System register to
clear to zero, providing a means for the host to
inform that 8051 that an operation has been
completed.
The protocol used to pass commands back and
forth through the mailbox interface is left to the
system designer. SMSC can provide an
application example of working code in which the
host uses the mailboxes to gain access to all of
the 8051 access only registers.