PM3350-SW datasheet(122/224 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

120

descriptor pool to hold a copy of the original data descriptor in the source

device.

12. The firmware will then set up the DMA Controller to transfer the head of the

appropriate transfer queue (both the data descriptor as well as the packet buffer

chain that it points to) from the source device into local memory. It will pass the

DMA Controller a pointer to the newly assigned data descriptor, into which the

DMA Controller is expected to place the contents of the copied data descriptor;

packet buffers must, however, be allocated dynamically by the DMA Controller.

At this point, the request counter interrupt handler firmware has completed its

processing; it decrements the request counter that triggered the interrupt

(signifying that the request has been handled) and exits.

13. The DMA Controller will begin copying data across the PCI bus from the source

device to the remote device's memory space. The first item to be copied is the

data descriptor at the head of the transfer queue in the source device; this is

written into the pre-allocated data descriptor passed to the DMA Controller in

step 12. (Note that only the first 16 bytes of the data descriptor are read as the

last four are only used internally.)The DMA Controller will then start transferring

the actual frame data (i.e., the packet buffers) from the source device. It begins

by allocating a packet buffer to hold the data. (When this packet buffer is filled, it

will allocate the next buffer and chain it to the first one, thus building up a linked-

list of buffers in local memory.)

14. The FirstPB field of the copied data descriptor provides a pointer to the packet

buffer chain associated with it; the DMA Controller continues to copy data

across the PCI bus from the packet buffers at the source device end into the

locally allocated packet buffers, traversing the linked list of source packet

buffers to build a virtually identical list of packet buffers in the remote device's

memory. (The only difference between the two sets of packet buffers, once the

DMA Controller has completed the copy, will be that the next pointers in the

NextPB fields will be different, as the two linked lists lie in different memory

spaces.) Note that the DMA Controller automatically writes a pointer to the first

packet buffer in the local memory space into the FirstPB field of the copied

version of the data descriptor, obliterating the original contents of the FirstPB

field as originally copied.

15. When the transfer is done (the data descriptor and all of the packet buffers have

been copied), the DMA Controller halts the transfer and interrupts the Switch

Processor.

16. The Switch Processor responds to the interrupt by activating the PCI read

completed interrupt service routine. The interrupt service routine firmware will

then read the necessary information out of the DMA Controller channel control

registers, and then decrement the MaxBuffers field of the proper expansion port

descriptor by the number of copied packet buffers to signify that the source

device has used up some of its local memory allocation at the remote device.

PM3350-SW Datasheet(PDF) 122 Page - PMC-Sierra, Inc

PM3350-SW Datasheet(HTML) 122 Page - PMC-Sierra, Inc

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