PM3350-SW datasheet(143/224 Pages) PMC

143 / 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

141

Byte Ordering

The ELAN 8x10 internally uses a little-endian byte ordering scheme to store frames

and data structures in local memory, and to transfer frames and communication

messages across the little-endian PCI bus. In this scheme, the least significant bit of

any 32-bit data word appears on the least-significant bit of the memory and PCI data

buses, and is also placed within the least-significant byte address in memory. If less

than four bytes are being transferred over a data bus, they are right-aligned on the bus,

and placed in the memory in increasing address order.

The Ethernet standard, however, specifies a big-endian byte ordering scheme, which

is the reverse of the ordering used by the ELAN 8x10 device. To properly align the

various fields within a received Ethernet frame for handling by the Switch Processor,

therefore, the ELAN 8x10 performs a big-endian to little-endian conversion in the MAC

interface block during receive, and a little-endian to big-endian conversion during

transmit.

The following diagram illustrates the default mapping between an Ethernet frame and

packet buffers transmitted across the PCI bus or stored in local memory:

PM3350-SW Datasheet(PDF) 143 Page - PMC-Sierra, Inc