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Chapter 8 MAC Data Structures and Registers
8.1Frame Format In Buffer Memory
The frame 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 frame itself. By default, the last byte in the receive frame format is followed by the CRC, and the Control byte follows the CRC.
RAM | bit15 | bit0 |
|
| |
OFFSET | 2nd Byte | 1st Byte |
(DECIMAL) |
|
|
0 |
| STATUS WORD |
|
| |
2 | RESERVED | BYTE COUNT (always even) |
4 |
|
|
|
| DATA AREA |
CRC (4 BYTES)
2046 Max | LAST DATA BYTE (if odd) |
CONTROL BYTE | |
Last Byte |
|
| TRANSMIT PACKET | RECEIVE PACKET |
|
|
|
STATUS WORD | Written by CSMA upon transmit | Written by CSMA upon receive |
| completion (see Status Register) | completion (see RX Frame Status |
|
| Word) |
|
|
|
BYTE COUNT | Written by CPU | Written by CSMA |
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|
|
DATA AREA | Written/modified by CPU | Written by CSMA |
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|
|
CONTROL BYTE | Written by CPU to control odd/even | Written by CSMA; also has odd/even |
| data bytes | bit |
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|
|
BYTE COUNT - Divided by two, it defines the total number of words including the STATUS WORD, the BYTE COUNT WORD, the DATA AREA, the CRC, and the CONTROL BYTE. The CRC is not included if the STRIP_CRC bit is set. The maximum number of bytes in a RAM page is 2048 bytes.
SMSC LAN91C111 REV C | 43 | Revision 1.91 |
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