PM3350-SW datasheet(77/224 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

shuffling the bits in order to compact the 24-bit offset into a 16-bit quantity. The

resulting format is as follows:

HBOff (bits 15:6)

HBOff (bits 21:16)

HBOff:

24-bit offset of hash bucket or forwarding tag found during search of address

table for given MAC address.

The compaction of the 24-bit offset into 16 bits takes advantage of the fact that all hash

buckets (and consequently forwarding tags) are expected to be aligned on 64-byte

boundaries, and hence the lower 6 bits of the offset will always be zero; also, the hash

bucket array cannot span memory banks, and hence the uppermost 2 bits of the offset

will also be zero. The bit field shuffling is done to minimize the computational

requirements of the index generation. When the switching firmware removes the data

descriptor from the work queue, it will immediately convert the encoded hash bucket

indices into their 24-bit absolute address equivalents (assuming that the SMiss and/or

DMiss bits are not set) and thereafter use only the 24-bit addresses, discarding the

encoded forms.

As previously noted, the above data descriptor format is only used internally during a

normally invisible portion of the firmware processing cycle; hence it is irrelevant to all

but the most specialized applications.

Data descriptors are never cached by the Switch Processor, and hence the contents of

any data descriptor that is not actually being processed by the Switch Processor may

be read at any time via the PCI expansion port. The Switch Processor is usually the

only entity that allocates, creates, manipulates and de-allocates data descriptors. (An

exception is during frame transfer across the PCI bus: in this case, the DMA hardware

is responsible for copying the contents of remote data descriptors to local memory.)

Local Port Descriptor Tables

A set of data structures, collectively referred to as the

Local Port Descriptor Table, is

associated with the eight MAC interfaces within the ELAN 8x10. The local port

descriptor table contains three types of structures: a local port descriptor, which

contains various control/status and packet statistics fields, and is updated whenever a

packet is received or transmitted by that MAC interface; a chain of data descriptors

(attached to the local port descriptor), referred to as the

transmit queue, that holds

Ethernet frames waiting to be transmitted out the MAC interface; and a set of packet

error statistics fields, known as the

port descriptor error counters, which tracks various

error counts that are updated on an infrequent basis during normal operation. A

separate copy of these three data structures is maintained for each of the eight MAC

channel ports that are implemented by the ELAN 8x10. All of these data structures are

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