PM3350-SW datasheet(71/224 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

PayloadByte0 - PayloadByte(n-4):

8-bit packet buffer payload bytes: contains Ethernet frame data for the frame

contained within the linked-list of packet buffers.

The NextPB field in the packet buffer header contains a pointer to the next packet

buffer in the chain of buffers that holds the entire Ethernet frame. If this is the last (or

only) buffer in the chain, then this field is set to zero to indicate a NULL pointer. The

Size field holds the number of valid payload bytes (i.e., excluding the 8 packet buffer

header bytes) in the packet buffer payload field; for an 80-byte packet buffer, the value

in this byte can range between 0 and 72. A value of zero indicates a completely empty

packet buffer with no data, which will simply be skipped over by the hardware or

firmware with no ill effects.

The RefCount field holds a reference count indicating the number of ports or devices to

which the packet buffer contents must be transmitted, and is used to determine when

the packet buffer may be freed. The reference count is normally 1 for unicast frames,

and equals the sum of the number of destination ports and devices for broadcasts; it is

only valid for the last packet buffer in a chain (i.e., when the next packet buffer pointer

is NULL), and is ignored in other buffers.

The LastSize field in the header holds the total number of valid bytes, including the

header, in the last packet buffer in a linked list of buffers. It should be placed in the first

packet buffer in the chain, and is principally used to optimize processing of packet

buffers by the hardware. This field is ignored if there is only one packet buffer in the

chain, or if the buffer under consideration is not the first one in the chain.

The remaining bytes in the packet buffer contain the payload, consisting of Ethernet

frame bytes received by one of the MAC channels of the ELAN 8x10 (or created via

software). The bytes are placed into the packet buffer in the order depicted above,

depending on the setting of the byte swap hardware configuration bit. Note that

'PayloadByte0' in the above diagrams refers to the first byte received from the medium

for the payload of the given packet buffer; in the case of the first packet buffer in a

chain, this byte will contain the LSB of the 48-bit MAC destination address in the

Ethernet frame header.

Packet buffer chains may also be used by the various chips in the system to contain

message data that is to be exchanged between devices interfaced to the PCI bus. If a

packet buffer holds message data rather than Ethernet frame data, then the format of

the header is unchanged, but the payload field is formatted in a message-specific

manner.

Data contained within packet buffers is never cached by the Switch Processor, and

hence the contents of any packet buffer may be read at any time via the PCI expansion

port. In normal operation, packet buffers are primarily manipulated by the DMA

hardware; the Switch Processor usually writes only the RefCount and LastSize fields.

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