Motorola MCF5281, MCF5282 Hash Algorithm, Ethernet Address Recognitionq-Microcode Decisions

Models: MCF5282 MCF5281

1 816
Download 816 pages 28.97 Kb
Page 369
Image 369

Functional Description

Receive Address

 

 

 

 

 

 

 

 

 

 

 

 

Recognition

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Group

 

 

 

 

 

 

 

Individual

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

I/G Address

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

True

 

 

?

 

 

 

 

 

False

 

 

True

 

FCE

 

 

 

 

 

 

 

 

 

Exact Match

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

False

 

 

False

Pause Address

 

True

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?

 

 

 

 

 

 

 

Hash Search

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Individual Table

 

Receive Frame

Hash Search

 

 

 

 

Receive Frame

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Group Table

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

True

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

True

 

 

 

 

 

 

 

 

Match

 

 

 

 

 

Match

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Receive Frame

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

False

 

 

 

 

 

 

 

 

 

 

 

False

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reject Frame

 

 

 

 

 

Receive Frame

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Flush from FIFO

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Reject Frame

NOTES:

Flush from FIFO

 

FCE - field in RCR register (Flow Control Enable)

 

I/G - Individual/Group bit in Destination Address (least significant bit in first byte received in MAC frame)

Figure 17-3. Ethernet Address Recognitionq—Microcode Decisions

17.4.9 Hash Algorithm

The hash table algorithm used in the group and individual hash filtering operates as follows. The 48-bit destination address is mapped into one of 64 bits, which are represented by 64 bits stored in GAUR, GALR (group address hash match) or IAUR, IALR (individual address hash match). This mapping is performed by passing the 48-bit address through the on-chip 32-bit CRC generator and selecting the 6 most significant bits of the CRC-encoded result to generate a number between 0 and 63. The MSB of the CRC result selects GAUR (MSB = 1) or GALR (MSB = 0). The least significant 5 bits of the hash result select the bit within the selected register. If the CRC generator selects a bit that is set in the hash table, the frame is accepted; otherwise, it is rejected.

For example, if eight group addresses are stored in the hash table and random group addresses are received, the hash table prevents roughly 56/64 (or 87.5%) of the group address frames from reaching memory. Those that do reach memory must be further filtered by the processor to determine if they truly contain one of the eight desired addresses.

MOTOROLA

Chapter 17. Fast Ethernet Controller (FEC)

17-13

Page 369
Image 369
Motorola MCF5281, MCF5282 user manual Hash Algorithm, Ethernet Address Recognitionq-Microcode Decisions