IBM PowerPC 405GP User Manual page 573

The MAL Scroll Descriptor in the configuration register is clear:
• In this case MAL will read the Status word from the COMMAC channel. Then MAL will terminate
the current channel service by resetting the READY bit of the last processed buffer descriptor (the
one in which there was an early termination) and will write the status only to this descriptor. On the
next service of this channel, MAL will fetch the next descriptor in the current packet. In this case,
the software is responsible to monitor the MAL location in the buffer descriptor table.
In the case that the COMMAC requests a re-transmit of the early terminated packet (when the
"backup" bit in the COMMAC status is set), MAL will re-transmit the packet regardless of the MAL
Scroll Descriptor bit.
20.5 Receive Software Interface
MAL uses the RX channel buffer descriptors in a manner similar to that used for transmission. Once
an RX COMMAC channel requests that a new packet be processed, MAL starts processing the
channel's next buffer descriptor in the table. Once a channel is enabled in MAL, the channel may
request MAL service. When it does, MAL accesses the first buffer descriptor (in that channel's buffer
descriptor table) that is pointed to by the COMMAC channel table pointer register. If that descriptor is
ready (empty for RX), MAL will start processing the buffer.
When it begins processing each packet, MAL writes the contents of the status/control field into the
COMMAC. This information (defined by COMMAC's implementation) can be used by the COMMAC
for a per-packet configuration.
Once data is received from the memory, MAL moves the data from the RX channel FIFO into the data
buffer pointed to by the first buffer descriptor. The current buffer descriptor may be closed for two
reasons: there is no more room left in the buffer, or the COMMAC channel indicated that the packet
reception ended. If additional buffering space is needed for the current packet, MAL moves on to the
next buffer descriptor. As each buffer descriptor is closed, MAL updates the length field with the actual
amount of bytes written into the buffer. The maximal length of the buffers for each channel is defined
by a configuration register. The maximal receive buffer length is defined per channel.
Once the COMMAC channel indicates that the packet reception has ended, it is expected to request
that MAL update the received packet status in the SD status/control field. MAL updates the packet
status and notifies the COMMAC. At this point the packet is considered received and the COMMAC
may request that MAL begin the process of receiving a new packet. The first buffer of the next packet
is the buffer in the SD table that followed the last descriptor of the previous packet.
20.5.1 Wrapping the BO Table for Receive
When MAL processes the buffer descriptor, it may encounter a Wrap indication within a buffer
descriptor control field. This causes MAL to go back to the head of the channel's buffer descriptor for
the next buffer descriptor. This also happens when MAL reaches the maximal number of descriptors.
20.5.2 Continuous Mode for Receive
After using a buffer descriptor, MAL sets the buffer descriptor control to the Not-Empty state. In this
way, MAL will not use the same buffer descriptor a second time until the software has processed the
not-empty buffer descriptor and set it to Empty again. MAL will not clear the Empty bit while the
Continuous Mode (CM) bit is set in the status/control field. The Continuous Mode is generally used by
20-12 PPC405GP User's Manual Preliminary