Motorola MC68302 User Manual page 182

The first word of the Rx BD contains control and status bits.
E- Empty
1 =The data buffer associated with this BD is empty. This bit signifies
that the BD and its associated buffer are available to the CP. The
M68000 core should not write to any fields of this BD when this bit
is set. The empty bit will remain set while the CP is currently filling
the buffer with received data.
0 =The data buffer associated with this BD has been filled with received
data, or data reception has been aborted due to an error condition.
The M68000 core is free to examine or write to any fields of this BD.
X - External Buffer
1 =The buffer associated with this BD is in external memory.
0 =The buffer associated with this BD is in internal dual-port RAM.
W - Wrap (Final BD in Table)
1 =This is the last BD in the Rx BD table. After this buffer has been used,
the CP will receive incoming data into the first BD in the table. Setting
this bit allows the use of fewer than eight BD to conserve internal
RAM.
0 =This is not the last BD in the Rx BD table.
NOTE
The user is required to set the wrap bit in one of the first eight BDs;
otherwise, errant behavior may occur.
I - Interrupt
1 =The M68000 core will be interrupted when this buffer has been closed
by the BISYNC controller.
0 =No interrupt is generated after this buffer has been used.
The following status bits are written by the CP after the received data has
been placed into the associated data buffer.
C - Control Character
The last byte in the buffer or the byte before the BCS (differentiated by the
B bit) is a user-defined control character.
MOTOROLA
0 =This buffer does not contain a control character.
1 =This buffer contains a control character.
MC68302 USER'S MANUAL
4-79