Endian Policies; Figure 38: Little Endian System - Broadcom BCM1250 User Manual

User Manual
10/21/02
E P
NDIAN OLICIES
The system can be run either as a big endian system or as a little endian system. Both the PCI bus and
HyperTransport fabric are little endian. When the part is running big endian there are two policies that are used
for connecting to the interfaces.
Selection of endian policy is made based on address bits for requests going both from the system to the PCI
bus or HyperTransport fabric and from the PCI bus or HyperTransport fabric to the system. Peer-to-peer
accesses between the PCI bus and HyperTransport fabric are always passed directly. In most cases address
bit [29] being clear indicates that the match byte lane policy will be used, and address bit [29] being set
indicates the match bit lane policy will be used. The address bit used to select the endian mode is zeroed as
the request passes through the interfaces so that the target always see the match bytes' address.
L E
ITTLE NDIAN
When the part is run as a little endian system there is no need to do any swapping between the system and
PCI or HyperTransport. If the system configuration register is set for little endian then no swapping is done,
and the two access addresses become aliases.
This is illustrated in
used by the CPU and ZBbus. Below it shows how the bytes of the double-word will appear on the PCI byte
lanes. The PCI uses byte enables to indicate both the transfer size and the low two address bits, these are
shown as the BE#[3:0] signals.
BYTE Address [2:0]
Document 1250_1125-UM100CB-R
S : N S
YSTEM O WAPS
Figure 38. At the top this shows a double-word with the bit numbers and byte addresses
63 56 55
H
111
PCI Bus
A [2] = 0
PCI Bus
A [2] = 1
PCI Byte Enables

Figure 38: Little Endian System

B r oadco m C orp or ati on
BCM1250/BCM1125/BCM1125H
48 47 4039 32 31 24 23 16 15 8 7
G F E D
110 101 100 011
31 24 23 16 15 8 7
D C B
31 24 23 16 15 8 7
H G F
BE#[3] BE#[2] BE#[1] BE#[0]
Section 8: PCI Bus and HyperTransport Fabric Page
0
C B A
010 001 000
0
A
0
E
201