PM3350-SW datasheet(83/224 Pages) PMC

83 / 224 page

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

The transmit queue is constructed out of a linked-list of data descriptors, with each

descriptor pointing to an Ethernet frame to be transmitted. The FirstDD and LastDD

pointers in the local port descriptor point to the head and the tail of the transmit queue,

respectively. The transmit queue imposes a First-In-First-Out (FIFO) discipline: new

frames to be transmitted are always added to the tail of the queue, and the actual

transmit process always removes frames from the head of the queue in order to

transmit them. Note that a frame will remain in the transmit queue until its transmission

has been completed successfully, or it is discarded for some reason.

The data descriptors and packet buffers in the transmit queue are never cached, and

hence they can be inspected directly via the PCI bus interface. Note, however, that the

head and tail pointers of the queue are located in the local port descriptor, which is itself

a cached structure, as already mentioned.

Port Descriptor Counter Structure

The eight port descriptor counter structures are considered to be auxiliary data

structures to the local port descriptors. Each local port descriptor is associated on a

one-to-one basis with a 64-byte port descriptor counter structure (previously described),

and contains various error and collision counters that are infrequently updated during

normal operation. The purpose of separating these counters from the local port

descriptor structures is to reduce the amount of data that must be cached in the Switch

Processor data cache: the port descriptor counter structures are not cached by the

Switch Processor.

PM3350-SW Datasheet(PDF) 83 Page - PMC-Sierra, Inc