Datasheet

Non-PCI Single-Chip Full Duplex Ethernet Controller with Magic Packet
Datasheet
Revision 1.0 (10-24-08) Page 38 SMSC LAN91C96 5v&3v
DATASHEET
Chapter 6 Frame Format in Buffer Memory for
Ethernet
The frame format in memory is similar to that in the TRANSMIT and RECEIVE areas. The first word is
reserved for the status word, the next word is used to specify the total number of bytes, and that in turn is
followed by the data area. The data area holds the packet itself, and its length is determined by the byte
count. The frame memory format is word oriented.
Figure 6.1 – Data Frame Format
TRANSMIT PACKET RECEIVE PACKET
STATUS WORD
Written by CSMA upon transmit
completion (see Status Register)
Written by CSMA upon receive
completion (see RX Frame
Status Word)
BYTE COUNT
Written by CPU Written by CSMA
DATA AREA
Written/modified by CPU Written by CSMA
CONTROL BYTE
Written by CPU to control
ODD/EVEN data bytes
Written by CSMA. Also has
ODD/EVEN bit
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 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
RESERVED
BYTE COUNT (always even)
STATUS WORD
DATA AREA
LAST DATA BYTE (if odd)
bit0
bit15
RAM
OFFSET
(DECIMAL)
0
2
4
1534 Max
CONTROL BYTE
Last Byte
1st Byte2nd Byte