User Manual
BCM1250/BCM1125/BCM1125H
10/21/02
B r o a d c o m C o r p o r a t i o n
Document
1250_1125-UM100CB-R
Section 7: DMA Page
169
E
THERNET
AND
S
ERIAL
DMA D
ESCRIPTORS
Table 100: DMA Descriptor First Doubleword
dscr_a
Bits
Name
Description
4:0
offset
The offset in the buffer that the header should start at.
39:5
a_addr
The base address of the data buffer (cache block aligned). In ring mode this is the base address of
the first data buffer.
48:40
a_size
The size of the data buffer (in cache blocks). If this field is zero the buffer is 512 cache blocks. In a
transmit channel which has the tbx_en configuration bit set bit [40] must be zero.
49
interrupt
If this bit is set an interrupt will be raised when the DMA engine has finished using this descriptor.
50
offset_b
If this bit is set then the offset field will be applied to the first buffer pointed to by descriptor B in a
packet rather than the first buffer pointed to by descriptor A. The DMA controller behavior is
UNPREDICTABLE if this bit is set in chain mode.
If this bit is set, the b_valid bit must be set in the
decr_b
or the behavior od the DMA engine will be
UNPREDICTABLE.
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 differ between the interfaces and are described in
Bits for Ethernet MACs” on page 171
and
Section: “Control and Flag Bits for Synchronous Serial
Table 101: 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.
4
addr
In chain mode this bit provides the additional address bit that is used with bits 39:5 to form the pointer
to the next descriptor. In ring mode the bit is reserved and should be zero.
39:5
b_addr
In ring mode this gives the base address of the second data buffer (cache block aligned). In chain
mode it and bit 4 contain a pointer to the next descriptor.
48:40
b_size
The size of the second data buffer (in cache blocks). If this field is zero the buffer is 512 cache blocks.
In a transmit channel which has the tbx_en configuration bit set bit [40] must be zero.
49
b_valid
If this bit is set the buffer is valid and will be used. If this bit is clear the B buffer is not used and the
b_addr and b_size fields are ignored. (Ring mode only.)
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.
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