Revision 1.02 – April 12, 2007
S5920 – PCI Product: Operation Registers
Data Book
This register provides empty/full visibility for each byte
within the mailboxes. The empty/full status for the PCI
Outgoing mailbox is displayed on bits 15 to 12 and the
empty/full status for the PCI Incoming mailbox is pre-
sented on bits 31 to 28. A value of 1 signifies that a
given mailbox has been written by one bus interface
but has not yet been read by the corresponding desti-
nation interface. The PCI bus incoming mailbox
transfers data from the Add-On bus to the PCI bus,
and the PCI outgoing mailbox transfers data from the
PCI bus to the Add-On bus. This register is also
referred to as the Add-On Mailbox Empty/Full Status
Register (AMBEF).
PCI MAILBOX EMPTY/FULL STATUS REG-
ISTER (MBEF)
Mailbox Empty/Full Status
Register Name:
PCI Address Offset:
Power-up value:
PCI Attribute:
Size:
34h
00000000h
Read Only
32 bits
Figure 28. Mailbox Empty/Full Status Register (MBEF)
31
28 27
16 15
12 11
0
0000
0000
Reserved
Reserved
PCI Outgoing
Mailbox Status (RO)
PCI Incoming
Mailbox Status (RO)
Table 40. Mailbox Empty/Full Status Register
Bit
Description
31:28
PCI Incoming Mailbox Status. This field indicates which byte of the incoming mailbox register has been written
by the Add-On interface but has not been read by the PCI bus. Each bit location corresponds to a specific byte
within the incoming mailbox. A value of one for each bit signifies that the specified mailbox byte is full, and a
value of 0 signifies empty. The mapping of these status bits to bytes within the mailbox is as follows:
Bit 31 = Incoming mailbox byte 3
Bit 30 = Incoming mailbox byte 2
Bit 29 = Incoming mailbox byte 1
Bit 28 = Incoming mailbox byte 0
15:12
PCI Outgoing Mailbox Status. This field indicates which byte of the outgoing mailbox register has been written
by the PCI bus interface but has not yet been read by the Add-On bus. Each bit location corresponds to a spe-
cific byte within the outgoing mailbox. A value of one for each bit signifies that the specified mailbox byte is full,
and a value of 0 signifies empty. The mapping of these status bits to bytes is as follows:
Bit 15 = Outgoing mailbox byte 3
Bit 14 = Outgoing mailbox byte 2
Bit 13 = Outgoing mailbox byte 1
Bit 12 = Outgoing mailbox byte 0
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DS1596
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