AM79C850 datasheet(65/97 Pages) AMD

65 / 97 page

P R E L I M I N A R Y

AMD

SUPERNET 3

causes the match and exact lines to be asserted. This

pattern will take much longer to apply than the minimal

set presented in table 2.

Destination address exact/inexact logic:

Since the destination address matching logic is identical

to the source address matching logic, the same test can

be applied as described in the previous section of this

document. These tests can be applied simultaneously

with the source address patterns.

PROGRAMMING METHODS

This section provides details of how the AF is intended

to be used. This section provides a description of the

methods to write entries into the AF, to find entries in the

AF and to invalidate entries in the AF.

Writing Entries into the AF

In order for the AF to perform the function of matching

addresses in network frames, the desired addresses

must be loaded into the CAM portion of the AF. The

following procedure should be followed to accomplish

this operation.

1. Write the comparand value into the NP comparand

registers. Note that the comparand register will

retain any previous value if it is not overwritten.

2. Write the NP mask register if it is desired to mask any

portion of the comparand. Note that the mask

overwritten.

3. Write the NP personality register with the desired

configuration of the SA, SAX, DA and DAX bits. The

VALID bit must be set if this entry is to participate in

any comparisons (either NP or network). If the

VALID bit is not set, this entry may be overwritten

when another entry is written to the CAM. The SKIP

bit should be cleared if this it will be necessary to find

this entry through the NP interface at a later time.

Note that the personality register will retain any

previous value if it is not overwritten.

4. Write the “Write CAM” instruction into the NP

command register.

5. The status register should be read once the DONE

bit is set to ensure that the ERROR bit was not set.

Note: The ERROR bit in the status register will not be

set if a “Write CAM” instruction is used when this bit is

set. The user has to read this bit status before attempt-

ing to write an entry into the CAM.

Finding Entries in the AF

Once a number of entries are resident in the AF, it may

be necessary to find one or more of them. The process

below should be used to perform this operation.

1. Load the value of the comparand that it is desired to

find into the NP comparand register.

2. Write the “Find” instruction into the NP command

NP mask registers.

3. Read the NP status register when the DONE bit is

set. If the FOUND bit is set, there is at least one

matching entry in the AF that does not have its SKIP

bit set. If the MULT bit is also set, there is more than

one entry that matches the comparand that does not

have the SKIP bit set.

Invalidating Entries in the AF

In conjunction with managing the contents of the AF, it

may be required to remove entries. This process is

called “invalidation.” To invalidate an entry in the AF, the

following steps should be followed.

1. Load the NP comparand register with the value of the

AF entry that is to be removed.

2. Write the “Find” instruction into the NP command

3. When the DONE bit is set, read the NP status

3a.If the FOUND bit is not set, there is no entry in the AF

that matches the comparand that does not have its

SKIP bit set.

3b.If the FOUND bit is set and the MULT bit is not set,

there is only one entry in the AF that matches the

comparand that does not have its SKIP bit set. Write

the “Invalidate” instruction into the NP command

3c. If the FOUND and MULT bits are set, there is more

than one entry in the AF that matches the comparand

that does not have its SKIP bit set. If all of these

entries should be invalidated, write the “Invalidate”

instruction followed by the “Find” instruction into the

NP command register repeatedly until the FOUND

bit is not set in the NP status register. If only one of

the multiple matching entries should be invalidated,

write the “Read CAM” instruction to the NP

command register, compare the contents read back

from the CAM to the desired comparand, mask and

personality. If the currently matching AF entry is the

one that should be invalidated, write the “Invalidate”

instruction to the NP command register. If the

currently

matching

entry

should

not

invalidated, write the “Skip” instruction to the NP

command register and repeat the invalidate process

from the beginning.

4. Write the “Clear all SKIP” instruction to the NP

command register.

AM79C850 Datasheet(PDF) 65 Page - Advanced Micro Devices

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