91C100FDREVB Datasheet, PDF (16/68 Pages) SMSC Corporation – FEAST FAST ETHERNET CONTROLLER WITH FULL DUPLEX CAPABILITY

DATA STRUCTURES AND REGISTERS
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
STATUS WORD
BYTE COUNT
DATA AREA
CONTROL BYTE
CONTROL BYTE
LAST DATA BYTE if odd
FIGURE 4 - DATA PACKET FORMAT
TRANSMIT PACKET
RECEIVE PACKET
Written by CSMA upon transmit
completion (see Status Register)
Written by CPU
Written by CSMA upon receive
completion (see RX Frame
Status Word)
Written by CSMA
Written/modified by CPU
Written by CSMA
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 relevant.
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 can extend up to 2043 bytes.
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
LAN91C100FD does not insert its own source address. On receive, all bytes are provided by the CSMA side.
SMSC DS – LAN91C100FD REV. B
Page 16
Rev. 05/31/2000