Texas Instruments TMS320C674X manual Receive Buffer Descriptor Format, Next Descriptor Pointer

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2.5.5Receive Buffer Descriptor Format

A receive (RX) buffer descriptor (Figure 8) is a contiguous block of four 32-bit data words aligned on a 32-bit boundary that describes a packet or a packet fragment. Example 2 shows the receive buffer descriptor described by a C structure.

2.5.5.1Next Descriptor Pointer

This pointer points to the 32–bit word aligned memory address of the next buffer descriptor in the receive queue. This pointer is used to create a linked list of buffer descriptors. If the value of this pointer is zero, then the current buffer is the last buffer in the queue. The software application must set this value prior to adding the descriptor to the active receive list. This pointer is not altered by the EMAC.

The value of pNext should never be altered once the descriptor is in an active receive queue, unless its current value is NULL. If the pNext pointer is initially NULL, and more empty buffers can be added to the pool, the software application may alter this pointer to point to a newly appended descriptor. The EMAC will use the new pointer value and proceed to the next descriptor unless the pNext value has already been read. In this latter case, the receiver will halt the receive channel in question, and the software application may restart it at that time. The software can detect this case by checking for an end of queue (EOQ) condition flag on the updated packet descriptor when it is returned by the EMAC.

2.5.5.2Buffer Pointer

The buffer pointer is the byte-aligned memory address of the memory buffer associated with the buffer descriptor. The software application must set this value prior to adding the descriptor to the active receive list. This pointer is not altered by the EMAC.

		Figure 8. Receive Buffer Descriptor Format
Word 0
31							0
			Next Descriptor Pointer
Word 1
31							0
			Buffer Pointer
Word 2
31			16	15			0
	Buffer Offset				Buffer Length
Word 3
31	30	29	28	27	26	25	24
SOP	EOP	OWNER	EOQ	TDOWNCMPLT	PASSCRC	JABBER	OVERSIZE
23	22	21	20	19	18	17	16
FRAGMENT	UNDERSIZED	CONTROL	OVERRUN	CODEERROR	ALIGNERROR	CRCERROR	NOMATCH
15							0
			Packet Length

SPRUFL5B –April 2011	EMAC/MDIO Module	25
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