PM3350-SW datasheet(88/224 Pages) PMC

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ELAN 8X10

PM3350 ELAN 8 X10

DATA SHEET

PMC-970109

ISSUE 3

8 PORT ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND PMC-SIERRA, INC. CUSTOMERS ONLY

head of the transfer queue, while the LastDD field points to the data descriptor at the

tail of the queue. The fields define a linked-list of data descriptors that in turn point to

the chains of packet buffers containing the Ethernet frame data that must be transferred

to the remote device. They are handled in a special manner, as follows.

Initially, the FirstDD and LastDD fields are set to point to a single empty data descriptor,

i.e., one containing a NULL NextDD field and a NULL FirstPB field. When a frame

needs to be transferred to the remote device, the following steps are performed:

The blank data descriptor in the transfer queue is updated to point to the frame,

i.e., the FirstPB, LastPB, Flags, SrcPort, SrcChip and HashBkt fields are set up

properly.

A new empty data descriptor is allocated and attached to the former (by setting

the NextDD field of the former to point to the new empty data descriptor).

The LastDD field in the expansion port descriptor is updated to point to the new

empty data descriptor.

The process is repeated as additional frames are queued for transfer to the remote

device. (Note that the LastDD field always points to an empty data descriptor, which will

be filled in when the next frame is queued.)

When the remote device acknowledges that it has completed the transfer of a given

frame, the data descriptor at the head of the transfer queue (pointed to by FirstDD) is

removed and freed, and the FirstDD pointer is advanced to the next data descriptor in

the queue. It is illegal for either the FirstDD or the LastDD fields to be NULL.

The ChipNum and CtrMask fields are statically configured during initialization. ChipNum

should be loaded with the index of the remote device, and is used by the firmware

during consistency checks. The CtrMask field contains an 8-bit mask with the bit

corresponding to the numeric index of the remote device (i.e., the least significant bit

corresponds to device zero, and so on) set, and all of the other bits cleared. The mask

is loaded into the DMA Controller when the firmware desires to increment either the

request or acknowledge counter in the remote device during expansion port frame

transfers.

MaxBuffers serves to impose a limit on the number of packet buffers that can be

consumed by frame transfers from the given remote device into local memory. This field

is set up during initialization to the desired (configurable) value, and is decremented for

each packet buffer used to hold frames copied over the PCI bus from the remote

device. (It is incremented as these packet buffers are freed after the frames have been

completely transmitted, or discarded.) If this field becomes equal to or less than zero,

the firmware will halt further transfers from the specific remote device, preventing it from

unfairly using up all of the local buffer space. All subsequent requests from the same

device, will be ignored. A special 'holdoff' mechanism is implemented in the expansion

PM3350-SW Datasheet(PDF) 88 Page - PMC-Sierra, Inc

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