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T1040 Ethernet switch - Buffer memory in the queue system

Question asked by Clément Dudognon on Sep 2, 2015
Latest reply on Sep 4, 2015 by alexander.yakovlev



I am studying the T1040 Ethernet switch. I am currently focused on the queue system. I am having some difficulties to understand how the frames are stored in the buffer memory and counted.


The document I am using is : T1040RDB (01/2015).


60-byte words:

- When I send a 100-byte (from MAC dest to FCS) frame, it seems that two 60-byte words are used.

- When I send a 101-byte (from MAC dest to FCS) frame, it seems that three 60-byte words are used.

- The documentation says that "All frames are prepended with a 16-byte header".

With these three points, and having performed further tests, I don't manage to understand how a frame is stored in the 60-byte words.

My questions:

(1) Is there a 16-byte header for each frame or for each 60-byte word ?

(2) Are these really 60-byte words or is the size indicated in the documentation wrong?

(3) Can two frames occupy the same word, or are the bytes, which are not used in a 60-byte word, "lost"?


FREECNT counter:

SYS:MMGT:MMGT register

I am wondering if the FREECNT counter is counting 192-byte words (written in the documentation).

I observed that:

- When two or four 60-byte words are occupied, one FREECNT word is occupied.

- When six or eight 60-byte words are occupied, two FREECNT words are occupied.

- When ten or twelve 60-byte words are occupied, three FREECNT words are occupied.

To me, this is incoherent with the fact that FREECNT counts 192-byte words.

Moreover, the initial value for FREECNT is 0x440 = 1088. 1088*192*8 = 1.671.168 bits which is a lot smaller than the 2Mbits of buffer memory.

The hypothesis that FREECNT counts 240-byte words seems more coherent to me with the points above and with what I observed.

My question:

(1) Does FREECNT count 192-byte words?



Thank you very much in advance.