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).
- 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.
(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"?
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.
(1) Does FREECNT count 192-byte words?
Thank you very much in advance.