IBM PowerPC 405GP User Manual page 575

• GOMMAG channel configuration information:
- Should the channel generate an interrupt following the end of packet processing.
- Protocol specific configuration.
20.6.2 Information from MAL and COMMAC Directed to Software
• MAL generated status information:
- Buffer Ready/Not Ready (passes the buffer handling to software).
- In receive buffer descriptor - Is the current buffer the first one in the packet?
- In receive buffer descriptor - Is the current buffer the last one in the packet?
• GOMMAG channel generated status information:
- Protocol specific error and status information (transmit and receive).
20.6.3 Status/Control Field Handling
When MAL accesses a new buffer descriptor, the status/control word is written to the GOMMAG
channel. This allows the channel to configure itself for the current packet.
For all "intermediate" buffer descriptors (all descriptors that do not contain the packet's ending), the
status/control field is written by MAL (rather than the GOMMAG). In this case, the status/control field
indicates that the current buffer is not the last one in the current packet.
As MAL finishes processing the last buffer descriptor in a given packet, it reads the channel's status
(via an OPB transaction) and writes it into the buffer descriptor's status/control field.
In effect, since all of the various control and status fields do not overlap, the status/control halfword is
read/written as a whole. Each agent (MAL, GOMMAG channel, and software) reads the entire
status/control halfword, relates to specific fields of interest, and updates another subset of fields
within the same halfword. While an agent modifies its related fields, all other fields remain unchanged.
20.6.4 Status/Control Field Format
The status/control halfword is divided into GOMMAG channel data and MAL related data. As
explained above, the MAL related fields are either aimed at controlling MAL or written by MAL for use
by the software. The MAL fields are of no interest to the GOMMAG (except the Ready and Empty
bits).
The same applies to the GOMMAG channel fields. The GOMMAG related fields are either aimed at
controlling the GOMMAG or written by GOMMAG for use by the software. These fields are of no
interest to MAL.
MAL will not manipulate the GOMMAG related fields, and GOMMAG is not allowed to manipulate the
MAL related fields.
20-14 PPC405GP User's Manual Preliminary