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
162
Section 7: DMA
Document
1250_1125-UM100CB-R
illustrates the ASIC mode being used to pass only the header of a packet (for example for
classification). The asicxfr_size is set to the number of header cache blocks (typically 2, causing 64 bytes of
header to be sent to the ASIC with no prepended descriptor or 1 causing 56 bytes of header to be sent with
the prepended descriptor). The dscr_a size must be set to the same value.
Figure 32: Sending a Packet Header in ASIC Mode
The SOP address is still used for the first block in the packet, and MOP for subsequent blocks (if the packet is
only two blocks the second will be sent to the EOP address). Once the header size has been sent to the ASIC
address the DMA controller advances to buffer B and sends the rest of the packet into memory in the normal
way, when it is done it will write the length and flags back into the descriptor. The ASIC can perform its header
processing and write results and the header back using the buffer A address. If CPU intervention is required
an ASIC on the HyperTransport fabric could opt to write the results and header back with HyperTransport
Isochronous writes, which causes the data to be allocated in the L2 cache. If the results are greater than 8
bytes (the offset that was specified to allow the dscr_a to be prepended) the actual buffer size may need to be
greater than indicated in the descriptor, since the size given in the descriptor must match the size sent to the
ASIC. The system software must manage this difference by allocating the larger buffer and adjusting the size
in the descriptors.
D
S
C
R
A
8
32
32
32
32
32
32
valid
SOP
Sent to buffer pointed to by DSCR B
Sent to
Base +
256k * Ch +
64k *1
asic_xfr_en=1
pre_addr_en=1
asicxfr_size=2
DSCR A size=2
MOP
Sent to
Base +
256k * Ch +
64k *0
Содержание BCM1125
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