Rainbow Electronics DS31256 - Manual (Page 106)

DS31256
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Table 9-G. Transmit DMA Main Operational Areas
DESCRIPTORS FUNCTION SECTION
Packet
A dedicated area of memory that describes the location and attributes of the
packet data.
9.3.2
Pending Queue
A dedicated area of memory that the host writes to inform the DMA that
packet data is queued and ready for transmission.
9.3.3
Done Queue
A dedicated area of memory that the DMA writes to inform the host that the
packet data has been transmitted.
9.3.4
Host Linking of Data Buffers
As previously mentioned, the data buffers are limited to a length of 8188 Bytes. If an outgoing packet
requires more memory space than the available data buffer contains, the host can link multiple data
buffers together to handle a packet length of any size. The host does this through the end-of-frame (EOF)
bit in the packet descriptor. Each data buffer has a one-to-one association with a packet descriptor. If the
host wants to link multiple data buffers together, the EOF bit is set to 0 in all but the last data buffer.
Figure 9-10 shows an example for HDLC channel 5 where the host has linked three data buffers
together. The transmit DMA knows where to find the next data buffer when the EOF bit is set to 0
through the next descriptor pointer field.
Host Linking of Packets (Packet Chaining)
The host also has the option to link multiple packets together in a chain. Through the chain valid (CV)
bit in the packet descriptor, the host can inform the transmit DMA that the next descriptor pointer field
contains the descriptor of another HDLC packet that is ready for transmission. The transmit DMA
ignores the CV bit until it sees EOF = 1, which indicates the end of a packet. If CV = 1 when EOF = 1,
this indicates to the transmit DMA that it should use the next descriptor pointer field to find the next
packet in the chain.
Figure 9-12 shows an example of packet chaining. Each column represents a separate
packet chain. In column 1, three data buffers have been linked together by the host for packet #1, and the
host has created a packet chain by setting CV = 1 in the last descriptor of packet #1.
DMA Linking of Packets (Horizontal Link Listing)
The transmit DMA also has the ability to link packets together. Internally, the transmit DMA can store
up to two packet chains, but if the host places more packet chains into the pending queue, the transmit
DMA must begin linking these chains together externally. The transmit DMA does this by writing to
packet descriptors (
Figure 9-12 ). If columns 1 and 2 were the only two packet chains queued for
transmission, then the transmit DMA would not need to link packet chains together, but as soon as
column 3 was queued for transmission, the transmit DMA had to store the third chain externally because
it had no more room internally. The transmit DMA links the packet chain in the third column to the one
in the second column by writing the first descriptor of the third chain in the next pending descriptor
pointer field of the first descriptor of the second column (it also sets the PV bit to 1). As shown in the
figure, this chaining was carried one step farther to link the forth column to the third.
Priority Packets
The host has the option to change the order in which packets are transmitted by the DMA. If the host sets
the priority packet (PRI) bit in the pending-queue descriptor to 1, the transmit DMA knows that this
packet is a priority packet and should be transmitted ahead of all standard packets. The rules for packet
transmission are as follows: