The buffer descriptor table forms a circular queue with a programmable length. The last descriptor
in
the table is defined by setting the Wrap bit
in
the status/control field (see "Status/Control Field
Format" on page 20-14).
If there is no Wrap bit set
in
the table, then MAL automatically wraps after
processing 256 descriptors (the maximum number of descriptors allow per channel).
The format of the buffer descriptor (see Figure
20-5) is the same for all COMMACs, and has the same
structure for both transmit and receive. The most significant halfword
in
each buffer descriptor
contains a status/control halfword. This field contains two parts: the first part (6 bits) is
SD handling
information used by the MAL for descriptor processing, the second field
(10 bits) is content specific for
each
COMMAC. The second halfword determines the data length referenced
in
this buffer descriptor.
The second word
in
the buffer descriptor contains a 32 bit data buffer pointer that pOints to the actual
data buffer
in
memory. It is suggested that each data buffer start on a cache line boundary and
be
a
multiple of a cache line
in
size if it resides in cachable memory. (The cache line size
in
the PPC405
processor core is 32 bytes.)
o
15
16 20
31
Offset + 0
Status/Control
I Not in Use I
Data Length
Offset + 4
Data Buffer Pointer
Figure 20-5. Buffer Descriptor Structure
A packet may reside
in
as many buffers as necessary (transmit or receive). Each buffer has a
maximum length of
(4KS - 16) bytes.
In
TX
channels, the buffer descriptor length field is written by the
device driver and defines the number of bytes
in
the data buffer that is identified by the data buffer
pointer.
In
RX channels, the buffer descriptor length field is written by MAL and defines the number of
bytes written by MAL to the buffer that is identified by the data buffer pointer (see
"Receive Software
Interface" on page 20-12).
When processing a packet, MAL does not assume that all buffers of the current packet are already
valid.
It
expects the buffers to be ready
in
due time to be transmitted or received. Failure of the
software to provide the descriptors
in
due time may result
in
.an
error.
20-8 PPC405GP User's Manual Preliminary
To Next Page
Loading...
