DP83936AVF datasheet(27/104 Pages) NSC

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50 Buffer Management (Continued)

542 Receive Buffer Area (RBA)

The SONIC-T stores the actual data of a received packet in

the RBA The RBAs are designated by the Resource De-

scriptors in the RRA as described above The RXrsrcbuff

wc01 fields of the RRA indicate the length of the RBA

When the SONIC-T gets an RBA from the RRA

the

RXrsrcbuff

wc01 values are loaded into the Remaining

Buffer Word Count registers (RBWC01) These registers

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

When a packet is buffered in a RBA it is buffered contigu-

ously (the SONIC-T 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 network) a new

buffer must be acquired The End of Buffer Count (EOBC)

size that the SONIC-T will need to buffer

5421 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-T will have to buffer This word count creates a

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

fetch a new RBA resource from the RRA

Note

The EOBC is a word count not a byte count Also the value pro-

grammed into EOBC must be a double word (32-bit) quantity when

the SONIC-T is in 32-bit mode (eg in 32-bit mode EOBC should be

set to 760 words not 759 words even though the maximum size of an

IEEE 8023 packet is 759 words for double work boundary align-

ment)

5422 Buffering the Last Packet in an RBA

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

beginning at the Current Receive Buffer Address (CRBA01)

and continues until the reception is complete Concurrent

with reception the SONIC-T decrements the Remaining

Buffer Word Count (RBWC01) by one in 16-bit mode or by

two in 32-bit mode At the end of reception if the packet has

crossed the EOBC boundary the SONIC-T knows that the

next packet might not fit in the RBA This check is done by

comparing the RBWC01 registers with the EOBC

RBWC01 is less than the EOBC (the last packet buffered

has crossed the EOBC boundary) the SONIC-T 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 5-5

illustrates the SONIC-T’s actions for (1)

RBWC01 t EOBC and (2) RBWC01 k EOBC See Sec-

tion 5444 for specific information about setting the EOBC

Note

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

case

1in

Figure 5-5 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-T will not fetch a new

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

rupt Status Register Section 646)

TLF12597 – 17

FIGURE 5-4 Receive Resource Area Format

TLF12597 – 18

Case

(RBWC01 t EOBC)

(RBWC01 k EOBC)

Case

1 SONIC-T buffers next packet in same RBA

Case

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

FIGURE 5-5 Receive Buffer Area

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