LAN91C100FD_0601 Datasheet, PDF (19/78 Pages) SMSC Corporation – FEAST Fast Ethernet Controller with Full Duplex Capability

FEAST Fast Ethernet Controller with Full Duplex Capability
Chapter 5 Data Structures and Registers
5.1
Packet Format in Buffer Memory
The packet format in memory is similar for 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 it is followed by the data
area. The data area holds the packet itself.
bit15
RAM OFFSET
(decimal)
0
STATUS
2
reserved
4
BYTE
bit0
WORD
COUNT
~~~~
DATA AREA
~~~~
2046 max _{CONTROL BYTE}
LAST DATA BYTE if odd
Figure 5.1 - Data Packet Format
STATUS WORD
BYTE COUNT
DATA AREA
CONTROL BYTE
TRANSMIT PACKET
Written by CSMA upon transmit
completion (see Status Register)
Written by CPU
Written/modified by CPU
Written by CPU to control
odd/even data bytes
RECEIVE PACKET
Written by CSMA upon receive
completion (see RX Frame
Status Word)
Written by CSMA
Written by CSMA
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 relevant.
SMSC DS – LAN91C100FD Rev. D
Page 19
DATASHEET
Rev. 01-20-06