BCM1250/BCM1125/BCM1125H
User Manual
10/21/02
B r o a d c o m C o r p o r a t i o n
Page
170
Section 7: DMA
Document
1250_1125-UM100CB-R
Table 102: Unaligned Buffer Format DMA Descriptor First Doubleword
dscr_a
Bits
Name
Description
39:0
addr
The base address of the data buffer.
47:40
unused
Unused. If extra_status is enabled the receive DMA engine will overwrite this field.
48 reserved
Reserved.
49
interrupt
If this bit is set an interrupt will be raised when the DMA engine has finished using this descriptor.
50
reserved
Reserved.
63:51
status
In the first descriptor of a received packet this field will be updated with the packet status at the end
of reception. The flags are described in
Section: “Option and Flag Bits for Ethernet
Table 103: Unaligned Buffer Format DMA Descriptor Second Doubleword
dscr_b
Bits
Name
Description
3:0
options
This field sets the per packet options that are sent to the interface. The options depend on the
channel, and are outlined below.
7:4 reserved
Reserved.
21:8
size
This field specifies the buffer size in bytes. This field must be set to the number of bytes of data
in the buffer plus bits [4:0] of the start address in the dscr_a addr field.
The DMA engine behavior is UNPREDICTABLE if this size is set to be less than the value of bits
[4:0] of the address in dscr_a.
47:22 reserved
Reserved.
49:48
pkt_size_msb
This field provides bits [15:14] of the packet size. (Note that if packets greater than 16K-1 are
used only the low 14 bits of the length is added to the RMON counters.)
63:50
pkt_size
In the first descriptor of a packet this field contains the packet length. In a transmit channel the
length is written by the CPU as part of setting up the descriptor and is passed to the interface. In
a receive channel the length is written back into the descriptor at the end of reception. In unaligned
buffer mode this field is extended to 16 bits using the pkt_size_msb field as the upper two bits.
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