DP83916 datasheet(15/96 Pages) NSC

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30 Buffer Management (Continued)

33 DESCRIPTOR DATA ALIGNMENT

All fields used by descriptors (RXpktxxx RXrsrcxxx and

TXpktxxx) are word quantities (16-bit) and must be aligned

to word boundaries (A0e0) The Receive Buffer Area

(RBA) must also be aligned to a word boundary The frag-

ments in the Transmit Buffer Area (TBA) however may be

aligned on any arbitrary byte boundary

All descriptor areas follow little endian byte ordering even

when BMODE e 1

34 RECEIVE BUFFER MANAGEMENT

The Receive Buffer Management operates on three areas in

memory into which data status and control information are

written during reception

(Figure 3-2 ) These three areas

must be initialized (section 344) before enabling the receiv-

er (setting the RXEN bit in the Command register) The re-

ceive resource area (RRA) contains descriptors that locate

receive buffer areas in system memory These descriptors

are denoted by R1 R2 etc in

Figure 3-2 Packets (denoted

by P1 P2 etc) can then be buffered into the corresponding

RBAs Depending on the size of each buffer area and the

size of the packet(s) multiple or single packets are buffered

into each RBA The receive descriptor area (RDA) contains

status and control information for each packet (D1 D2 etc

Figure 3-2 ) corresponding to each received packet (D1

goes with P1 D2 with P2 etc)

When a packet arrives the address recognition logic checks

the address for a Physical Multicast or Broadcast match

and if the packet is accepted the SONIC-16 buffers the

packet contiguously into the selected Receive Buffer Area

(RBA) Because of the previous end-of-packet processing

the SONIC-16 assures that the complete packet is written

into a single contiguous block When the packet ends the

SONIC-16 writes the receive status byte count and loca-

tion of the packet into the Receive Descriptor Area (RDA)

The SONIC-16 then updates its pointers to locate the next

available descriptor and checks the remaining words avail-

able in the RBA If sufficient space remains the SONIC-16

buffers the next packet immediately after the previous pack-

et If the current buffer is out of space the SONIC-16 fetches

a Resource descriptor from the Receive Resource Area

(RRA) acquiring an additional buffer that has been previous-

ly allocated by the system

341 Receive Resource Area (RRA)

As buffer memory is consumed by the SONIC-16 for storing

data the Receive Resource Area (RRA) provides a mecha-

nism that allows the system to allocate additional buffer

space for the SONIC-16 The system loads this area with

resource descriptors that the SONIC-16 in turn reads as its

current buffer space is used up Each resource descriptor

consists of a 23-bit buffer pointer locating the starting point

of the RBA and a 32-bit Word Count that indicates the size

of the buffer in words (2 bytes per word) The buffer pointer

and word count are contiguously located using the format

shown in

Figure 3-3 with each component composed of 16-

bit fields The SONIC-16 stores this information internally

and concatenates the corresponding fields to create 23-

and 32-bit long words for the buffer pointer and word count

The SONIC-16 organizes the RRA as a circular queue for

efficient processing of descriptors Four registers define the

RRA The first two the Resource Start Area (RSA) and the

Resource End Area (REA) registers determine the starting

and ending locations of the RRA and the other two regis-

ters update the RRA The system adds descriptors at the

address specified by the Resource Write Pointer (RWP)

and the SONIC-16 reads the next descriptor designated by

the Resource Read Pointer (RRP) The RRP is advanced 4

words after the SONIC-16 finishes reading the RRA and

automatically wraps around to the beginning of the RRA

once the end has been reached When a descriptor in the

RRA is read the RXrscbuff

pt01 is loaded into the

CRBA01 registers and the RXrscbuff

wc01 is loaded into

the RBWC01 registers

The alignment of the RRA is confined to word boundaries

(A0 is always zero)

TLF11722 – 11

FIGURE 3-2 Overview of Receive Buffer Management

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