DP83916 datasheet(17/96 Pages) NSC

17 / 96 page

30 Buffer Management (Continued)

343 Receive Descriptor Area (RDA)

After the SONIC-16 buffers a packet to memory it writes 5

words of status and control information into the RDA reads

the link field to the next receive descriptor and writes to the

in use field of the current discriptor Each receive descriptor

consists of the following sections (

Figure 3-5 )

TLF11722 – 14

FIGURE 3-5 Receive Descriptor Format

receive status

indicates status of the received packet The

SONIC-16 writes the Receive Control register into this field

Figure 3-6 shows the receive status format This field is

loaded from the contents of the Receive Control register

Note that ERR RNT BRD PRO and AMC are configura-

tion bits and are programmed during initialization See Sec-

tion 433 for the description of the Receive Control register

ERR

RNT

BRD

PRO

AMC

LB1

LB0

LPKT

CRS

COL

CRCR

FAER

LBK

PRX

FIGURE 3-6 Receive Status Format

byte count

gives the length of the complete packet from

the start of Destination Address to the end of FCS

packet pointer

a 23-bit pointer that locates the packet in

the RBA

The SONIC-16 writes the contents of the

CRBA01 registers into this field

sequence numbers

this field displays the contents of two

8-bit counters (modulo 256) that sequence the RBAs used

and the packets buffered These counters assist the system

in determining when an RBA has been completely process-

ed The sequence numbers allow the system to tally the

packets that have been processed within a particular RBA

There are two sequence numbers that describe a packet

the RBA Sequence Number and the Packet Sequence

Number When a packet is buffered to memory the SONIC-

16 maintains a single RBA Sequence Number for all pack-

ets in an RBA and sequences the Packet Number for suc-

ceeding packets in the RBA When the SONIC-16 uses the

next RBA it increments the RBA Sequence Number and

clears the Packet Sequence Number The RBA’s sequence

counter is not incremented when the read RRA command is

issued in the Command register The format of the Receive

Sequence Numbers are shown in

Figure 3-7 These coun-

ters are reset during hardware reset or by writing zero to

them

RBA Sequence Number

Packet Sequence Number

(Modulo 256)

FIGURE 3-7 Receive Sequence Number Format

receive link field

a 15-bit pointer (A15 – A1) that locates

the next receive descriptor The LSB of this field is the End

Of List (EOL) bit and indicates the last descriptor in the list

(Initialized by the system)

in use field

this field provides a handshake between the

system and the SONIC-16 to indicate the ownership of the

descriptor When the system avails a descriptor to the SON-

IC-16 it writes a non-zero value into this field The SONIC-

16 in turn sets this field to all ‘‘0’s’’ when it has finished

processing the descriptor (That is when the CRDA register

has advanced to the next receive descriptor) Generally the

SONIC-16 releases control after writing the status and con-

trol information into the RDA If however the SONIC-16 has

reached the last descriptor in the list it maintains ownership

of the descriptor until the system has appended additional

descriptors to the list The SONIC-16 then relinquishes con-

trol after receiving the next packet (See Section 3461 for

details on when the SONIC-16 writes to this field) The re-

ceive packet descriptor format is shown in

Figure 3-5

344 Receive Buffer Management Initialization

The Receive Resource Descriptor and Buffer areas (RRA

RDA RBA) in memory and the appropriate SONIC-16 regis-

ters must be properly initialized before the SONIC-16 begins

buffering packets This section describes the initialization

process

3441 Initializing The Descriptor Page

All descriptor areas (RRA RDA and TDA) used by the

SONIC-16 reside within areas up to 32k (word) pages This

page may be placed anywhere within the 23-bit address

range by loading the upper 8 address lines into the UTDA

URDA and URRA registers

3442 Initializing The RRA

The initialization of the RRA consists of loading the four

SONIC-16 RRA registers and writing the resource descriptor

information to memory

The RRA registers are loaded with the following values

Resource Start Area (RSA) register

The RSA is loaded

with the lower 16-bit address of the beginning of the RRA

Resource End Area (REA) register

The REA is loaded

with the lower 16-bit address of the end of the RRA The

end of the RRA is defined as the address of the last

RXrsrcptr0 field in the RRA plus 4 words (

Figure 3-3 )

Resource Read Pointer (RRP) register

The RRP is load-

ed with the lower 16-bit address of the first resource de-

scriptor the SONIC-16 reads

Resource Write Pointer (RWP) register

The RWP is load-

ed with the lower 16-bit address of the next vacant location

where a resource descriptor will be placed by the system

Note

The RWP register must only point to either (1) the RXrsrcptr0 field of

one of the RRA Descriptors (2) the memory address that the RSA

points to (the start of the RRA) or (3) the memory address that the

REA points to (the end of the RRA) When the RWP e RRP compari-

son is made it is performed after the complete RRA descriptor has

been read and not during the fetch Failure to set the RWP to any of

the above values prevents the RWP e RRP comparison from ever

becoming true

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