
MOTOROLA
Chapter 11. Ethernet Module
11-33
Buffer Descriptors
11.6.1.1 Ethernet Receive Buffer Descriptor (RxBD)
In the RxBD, the user initializes the E and W bits in the rst word and the pointer in the
second word. When the buffer has been sent as a DMA, the FEC will modify the E, L, M,
LG, NO, SH, CR, and OV bits and write the length of the used portion of the buffer in the
rst word. The M, LG, NO, SH, CR, and OV bits in the rst word of the buffer descriptor
are modied by the FEC only when the L bit is set.
The rst word of the RxBD contains control and status bits. Its format is detailed below.
15
14
13
12
11
10
9876543210
+0
E
RO1
W
RO2
L
—
M
BC
MC
LG
NO
SH
CR
OV
TR
+2
DATA LENGTH
+4
Rx Data Buffer Pointer A[31–16]
+6
Rx Data Buffer Pointer A[15–0]
Figure 11-26. Receive Buffer Descriptor (RxBD)
Table 11-34. RxBD Field Descriptions
Bits
Name
Description
15
E
Empty. Written by the FEC (= 0) and user (= 1).
0 The data buffer associated with this BD has been lled with received data, or data reception
has been aborted due to an error condition. The status and length elds have been updated as
required.
1 The data buffer associated with this BD is empty, or reception is currently in progress.
14
RO1
Receive software ownership. This eld is reserved for use by software. This read/write bit is not
modied by hardware, nor does its value affect hardware.
13
W
Wrap. Written by the user.
0 =The next buffer descriptor is found in the consecutive location.
1 =The next buffer descriptor is found at the location dened in ERDSR.
12
RO2
Receive software ownership. Reserved for use by software. This read/write bit is not modied
by hardware, nor does its value affect hardware.
11
L
Last in frame. Written by the FEC.
0 =The buffer is not the last in a frame.
1 =The buffer is the last in a frame.
10–9
—
Reserved, should be cleared.
8
M
Miss. Written by the FEC. This bit is set by the FEC for frames that were accepted in
promiscuous mode, but were agged as a miss by the internal address recognition. Thus, while
in promiscuous mode, the user can use the M-bit to quickly determine whether the frame was
destined to this station. Valid only if the L-bit and the PROM bit are set.
0 The frame was received because of an address recognition hit.
1 The frame was received because of promiscuous mode.
7
BC
Broadcast. Written by the FEC. Will be set if DA is broadcast. (FF-FF-FF-FF-FF-FF)
6
MC
Mulitcast. Written by the FEC. Is set if DA is multicast and not BC.
5
LG
Rx frame length violation. Written by the FEC. A frame length greater than MAX_FL was
recognized. Frames exceeding 2047 bytes are truncated to prevent wrapping hardware length
counters. Valid only if the L-bit is set.