BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
200
Section 8: PCI Bus and HyperTransport Fabric
Document
1250_1125-UM100CB-R
H
YPER
T
RANSPORT
R
EAD
R
ESTRICTIONS
The HyperTransport interface maps read requests from the ZBbus into read commands on the HyperTransport
fabric. The interface supports all requests that can be generated by CPU instructions (reads of 1-8 bytes and
32 bytes) and any request that includes an aligned number of 32-bit words. Care must be taken when using
uncached accelerated accesses from the CPU or the Data Mover, since these can generate accesses that are
not supported and will have UNDEFINED results. For best performance the ZBbus read should translate into
a single HyperTransport request, accesses that are 1-4 bytes translate directly to a Read (sized) Byte request,
and accesses that are an aligned number of 32-bit words translate to the Read (sized) Doubleword request.
The interface is able to split ZBbus reads into two HyperTransport requests allowing the 5-7 byte reads that
the load doubleword left (LDL) and load doubleword right (LDR) instructions can generate, but the interface is
not optimized to give these unusual accesses high performance.
There is no constraint on writes, the interface supports arbitrary writes of 1-32 bytes. Any ZBbus write can be
converted into a Write (sized) HyperTransport request.
There are no constraints on inbound accesses. All legal HT read and write requests are supported. (Requests
larger than 32 bytes will be converted into two requests to the ZBbus but this is transparent to the HT fabric.)