DP83916 datasheet(16/96 Pages) NSC

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30 Buffer Management (Continued)

342 Receive Buffer Area (RBA)

The SONIC-16 stores the actual data of a received packet

in the RBA The RBAs are designated by the resource de-

scriptors

the

RRA

described

above

The

RXrsrcbuff

wc01 fields of the RRA indicate the length of

the RBA When the SONIC-16 gets a RBA from the RRA

the RXrsrcbuff

wc01 values are loaded into the Remain-

ing Buffer Word Count registers (RBWC01) These regis-

ters keep track of how much space (in words) is left in the

buffer When a packet is buffered in a RBA it is buffered

contiguously (the SONIC-16 will not scatter a packet into

multiple buffers or fragments) Therefore if there is not

enough space left in a RBA after buffering a packet to buffer

at least one more maximum sized packet (the maximum

legal sized packet expected to be received from the net-

work) a new buffer must be acquired The End of Buffer

Count (EOBC) register is used to tell the SONIC-16 the max-

imum packet size that the SONIC-16 will need to buffer

3421 End of Buffer Count (EOBC)

The EOBC is a boundary in the RBA based from the bottom

of the buffer The value written into the EOBC is the maxi-

mum expected size (in words) of the network packet that

the SONIC-16 will have to buffer This word count creates a

line in the RBA that when crossed causes the SONIC-16 to

fetch a new RBA resource from the RRA

Note

The EOBC is a word count not a byte count

3422 Buffering the Last Packet in an RBA

At the start of reception the SONIC-16 stores the packet

beginning at the Current Receive Buffer Address (CRBA01)

and continues until the reception is complete Concurrent

with reception the SONIC-16 decrements the Remaining

Buffer Word Count (RBWC01) by one At the end of recep-

tion if the packet has crossed the EOBC boundary the

SONIC-16 knows that the next packet might not fit in the

RBA This check is done by comparing the RBWC01 regis-

ters with the EOBC If RBWC01 is less than the EOBC (the

last packet buffered has crossed the EOBC boundary) the

SONIC-16 fetches the next resource descriptor in the RRA

If RBWC01 is greater than or equal to the EOBC (the EOBC

boundary has not been crossed) the next packet reception

continues at the present location pointed to by CRBA01 in

the same RBA

Figure 3-4 illustrates the SONIC-16’s ac-

tions for (1) RBWC01 t EOBC and (2) RBWC01 k EOBC

See Section 3444 for specific information about setting

the EOBC

Note

It is important that the EOBC boundary be ‘‘crossed’’ In other words

case

1in

Figure 3-4 must exist before case

2 exists If case

occurs without case

1 having occurred first the test for RBWC01

EOBC will not work properly and the SONIC-16 will not fetch a new

buffer The result of this will be a buffer overflow (RBAE in the Inter-

rupt Status Register section 436)

TLF11722 – 12

FIGURE 3-3 Receive Resource Area Format

TLF11722 – 13

Case

(RBWC01 t EOBC)

(RBWC01 k EOBC)

Case

1 SONIC-16 buffers next packet in same RBA

Case

2 SONIC-16 detects an exhausted RBA and will buffer the next packet in another RBA

FIGURE 3-4 Receive Buffer Area

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