Motorola MC68824 User Manual page 79

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4.1.1.4 NEXT FRAME DESCRIPTOR POINTER. This 32-bit address points to the next frame de-
scriptor in the queue. This pointer must be initialized by the host if the next FD is valid in both
the transmit and free pool cases.
4.1.1.5 FIRST BUFFER DESCRIPTOR POINTER. This 32-bit address is used by the TBC to address
the first buffer descriptor. When in a transmit queue, this pointer must be initialized by the host.
If this FD resides in the free frame descriptor pool, the TBC will update this pointer as it gets a
BD from the free buffer descriptor pool upon receiving a frame.
4.1.1.6 FRAME DATA LENGTH. During transmit, frame data length is updated by the host; during
receive, it is updated by the TBC. Normally, the frame data length is the length of the data unit
of the frame. The only exception is if RCDS (RX CRC to data storage mode) is enabled, in that
case the CRC is included in frame data length.
4.1.1.7 IMMEDIATE RESPONSE FRAME DESCRIPTOR POINTER. This word contains the 32-bit
address of the immediate response frame descriptor. This pointer is updated by the TBC. This is
used after a station has transmitted a request with response frame and receives a valid response.
Then the IR frame descriptor pointer of the RWR frame points to the frame descriptor of the
response, thus linking the RWR frame with its response.
4.1.1.8 FRAME CONTROL. The frame control field determines the category of frame that is being
sent or received. In the transmit case, the host is responsible for updating this field, while in the
receive case the TBC is. Three frame categories are currently defined by IEEE 802.4: MAC control
and LLC data. Note that the host is forbidden from setting this byte to a MAC control value in the
TX case. See APPENDIX B FRAME FORMATS AND ADDRESSING for more details.
4.1.1.9 MAC DESTINATION ADDRESS. The MAC destination address can be either two or six
bytes. The address length is determined by the host at initialization time by setting appropriately
the address length bit which resides in the initialization table at offset 7 A. This address is updated
by the host in the transmit case, while in the receive case the TBC is responsible for doing so.
Note that this address is stored following the IEEE convention, see APPENDIX B FRAME FORMATS
AND ADDRESSING for more details.
4.1.1.10 MAC SOURCE ADDRESS. The MAC source address can be either two or six bytes in
length. The address length is determined by the host at initialization time by setting appropriately
the address length bit which resides in the initialization table at offset 7 A. This address is updated
by the host in the transmit case, while in the receive case the TBC is responsible for doing so.
Note that this address is stored following the IEEE convention, see APPENDIX B FRAME FORMATS
AND ADDRESSING for more details.
4.1.2 Buffer Descriptors
Buffer descriptors (BD) contain buffer parameters, buffer status, and pointers. Buffer descriptors
are chained together linking data buffers which contain frame data. The format of a buffer de-
scriptor is shown in Figure 4-4. Note that a buffer descriptor must start at an even address.
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MC68824 USER'S MANUAL