PM3351-SW datasheet(82/268 Pages) PMC

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PM3351 ELAN 1X100

DATA SHEET

PMC-970113

ISSUE 3

SINGLE PORT FAST ETHERNET SWITCH

PROPRIETARY AND CONFIDENTIAL TO PMC-SIERRA, INC., AND FOR ITS CUSTOMERS’ INTERNAL USE

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

frame bytes received by the MAC channel of the ELAN 1x100 (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.

Data Descriptors

Ethernet frames, messages and other pieces of data being transferred between devices

within an ELAN 1x100 system, or between tasks on a single ELAN 1x100, are tracked

by means of 16-byte

data descriptors . (When a data descriptor refers to an Ethernet

frame that contains management data to be processed by the ELAN 1x100 system

itself, or to buffers holding control and status data that are exchanged for messaging

purposes within the ELAN 1x100 system, then it is sometimes referred to as a

message

descriptor.)

Data descriptors form the primary queueing elements in the ELAN 1x100 device: in

addition to forming the individual elements of transfer rings (see below), they are used

to hold message information and to create special frame handling queues for

management functions. All data descriptors have the same general format, and contain

pointers to the first and last packet buffer of a packet buffer chain holding an Ethernet

frame or a control/status message, flags fields that indicate the processing to be

performed on the frame or message, and other information required for processing the

payload.

In general, data descriptors are formatted as follows:

PM3351-SW Datasheet(PDF) 82 Page - PMC-Sierra, Inc