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MOTOROLA Chapter 6. Programming Model 6-25
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
The RxBD format is shown in Table 6-27.
0123456789101112131415
+0 E RO1 W RO2L 0 0 M BCMCLGNOSHCROVTR
+2 DATA LENGTH
+4 RX BUFFER POINTER A[0Ð15]
+6 RX BUFFER POINTER A[16Ð31]
Figure 6-23. Receive Buffer Descriptor (RxBD)
Table 6-27. Receive Buffer Descriptor (RxBD) Field Description
Bits Name Description
0 E Empty. Written by the FEC and user. Note that if the software driver sets RxBD[E], it should
then write to R_DES_ACTIVE.
0 The buffer associated with this BD is Þlled with received data, or reception was aborted due
to an error. The status and length Þelds have been updated as required.
1 The buffer associated with this BD is empty, or reception is in progress.
1 RO1 Receive software ownership bit. Software use. This read/write bit is modiÞed by hardware and
does not affect hardware.
2 W Wrap, written by user.
0 The next BD is found in the consecutive location
1 The next BD is found at the location deÞned in RAM.R_DES_START.
3 RO2 Receive software ownership bit. Software use. This read/write bit is not modiÞed by hardware
and does not affect hardware.
4 L Last in frame, written by FEC.
0 The buffer is not the last in a frame.
1 The buffer is the last in a frame.
5Ð6 Ñ Reserved.
7 M Miss, written by FEC.Set by the FEC for frames that were accepted in promiscuous mode but
were ßagged as a miss by the internal address recognition. Thus, while promiscuous mode is
being used, the user can use the M bit to quickly determine whether the frame was destined to
this station. This bit is valid only if both the L bit and PROM bit are set.
0 The frame was received because of an address recognition hit.
1 The frame was received because of promiscuous mode.
8 BC Set if the DA is broadcast.
9 MC Set if the DA is multicast and not broadcast.
10 LG Rx frame length violation, written by FEC. The frame length exceeds the value of
MAX_FRAME_LENGTH in the bytes. The hardware truncates frames exceeding 2047 bytes so
as not to overßow receive buffers This bit is valid only if the L bit is set. (Note that the Þrst
revision of the MPC860T (mask #H56S) must not be given frames in excess of 2047 as it will
not truncate frames.)
11 NO Rx nonoctet-aligned frame, written by FEC. A frame that contained a number of bits not
divisible by 8 was received and the CRC check that occurred at the preceding byte boundary
generated an error. NO is valid only if the L bit is set. If this bit is set the CR bit is not set.
12 SH Short frame, written by FEC. A frame length that was less than the minimum deÞned for this
channel was recognized.Note that the MPC860T does not support SH, which is always zero.
13 CR Rx CRC error, written by FEC. This frame contains a CRC error and is an integral number of
octets in length. This bit is valid only if the L bit is set.
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