BYTE COUNT - Divided by two, it defines the
total number of words, including the STATUS
WORD, the BYTE COUNT WORD, the DATA
AREA and the CONTROL BYTE.
The data area contains six bytes of
DESTINATION ADDRESS followed by six bytes
of SOURCE ADDRESS, followed by a variable
length number of bytes. On transmit, all bytes
are provided by the CPU, including the source
address. The LAN91C100 does not insert its
own source address. On receive, all bytes are
provided by the CSMA side.
The receive byte count always appears as even,
the ODDFRM bit of the receive status word
indicates if the low byte of the last word is
relevant.
The 802.3 Frame Length word (Frame Type in
Ethernet) is not interpreted by the LAN91C100.
It is treated transparently as data both for
transmit and receive operations.
The transmit byte count least significant bit will
be assumed 0 by the controller regardless of the
value written in memory.
DATA AREA - The data area starts at offset 4
of the packet structure, and it can extend for up
to 2043 bytes.
CONTROL BYTE - For transmit packets the
CONTROL BYTE is written by the CPU as:
X
X
ODD
CRC
0
0
0
0
ODD If set, indicates an odd number of bytes,
with the last byte being right before the
CONTROL BYTE. If clear, the number of data
bytes is even and the byte before the CONTROL
BYTE is not transmitted.
CRC When set, CRC will be appended to the
frame. This bit has only meaning if the NOCRC
bit in the TCR is set.
For receive packets the CONTROL BYTE is
written by the controller as:
0
1
ODD
0
0
0
0
0
ODD If set, indicates an odd number of bytes,
with the last byte being right before the
CONTROL BYTE. If clear, the number of data
bytes is even and the byte before the CONTROL
BYTE should be ignored.
19
SMSC [ SMSC CORPORATION ]
