DP83916 datasheet(13/96 Pages) NSC

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30 Buffer Management

31 BUFFER MANAGEMENT OVERVIEW

The SONIC-16’s buffer management scheme is based on

separate buffers and descriptors (

Figures 3-2 and 3-11 )

Packets that are received or transmitted are placed in buff-

ers called the Receive Buffer Area (RBA) and the Transmit

Buffer Area (TBA) The system keeps track of packets in

these buffers using the information in the Receive Descrip-

tor Area (RDA) and the Transmit Descriptor Area (TDA) A

single (TDA) points to a single TBA but multiple RDAs can

point to a single RBA (one RDA per packet in the buffer)

The Receive Resource Area (RRA) which is another form

of descriptor is used to keep track of the actual buffer

When packets are transmitted the system sets up the pack-

ets in one or more TBAs with a TDA pointing to each TBA

There can only be one packet per TBATDA pair A single

packet however may be made up of several fragments of

data dispersed in memory There is one TDA pointing to

each packet which specifies information about the packet’s

size location in memory number of fragments and status

after transmission The TDAs are linked together in a linked

list The system causes the SONIC-16 to transmit the pack-

ets by passing the first TDA to the SONIC-16 and issuing

the transmit command

Before a packet can be received an RBA and RDA must be

set up by the system RDAs are made up as a linked list

similar to TDAs An RDA is not linked to a particular RBA

though Instead an RDA is linked specifically to a packet

after it has been buffered into an RBA More than one pack-

et can be buffered into the same RBA but each packet gets

its own RDA A received packet can not be scattered into

fragments The system only needs to tell the SONIC-16

where the first RDA and where the RBAs are Since an RDA

never specifically points to an RBA the RRA is used to

keep track of the RBAs The RRA is a circular queue of

pointers and buffer sizes (not a linked list) When the SON-

IC-16 receives a packet it is buffered into a RBA and a RDA

is written to so that it points to and describes the new pack-

et If the RBA does not have enough space to buffer the

next packet a new RBA is obtained from the RRA

32 DESCRIPTOR AREAS

Descriptors are the basis of the buffer management scheme

used by the SONIC-16 A RDA points to a received packet

within a RBA a RRA points to a RBA and a TDA points to a

TBA which contains a packet to be transmitted The con-

ventions and registers used to describe these descriptors

are discussed in the next three sections

321 Naming Convention for Descriptors

The fields which make up the descriptors are named in a

consistent manner to assist in remembering the usage of

each descriptor Each descriptor name consists of three

components in the following format

RXTX descriptor name

field

The first two capital letters indicate whether the descriptor is

used for transmission (TX) or reception (RX) and is then

followed by the descriptor name having one of two names

rsrc e Resource descriptor

pkt e Packet descriptor

The last component consists of a field name to distinguish it

from the other fields of a descriptor The field name is sepa-

rated from the descriptor name by a period (‘‘’’) An exam-

ple of a descriptor is shown below

rsrc

buff

ptr

Descriptor consists of two

fields ‘‘0’’ and ‘‘1’’

respectively indicate the

least and most significant

portions of the descriptor

The ‘‘pointer’’ field of the

descriptor

A descriptor for a buffer

resource

A descriptor used for

reception

322 Abbreviations

The abbreviations in Table 3-1 are used to describe the

SONIC-16 registers and data structures in memory The ‘‘0’’

and ‘‘1’’ in the abbreviations indicate the least and most

significant portions of the registers or descriptors Table 3-1

lists the naming convention abbreviations for descriptors

323 Buffer Management Base Addresses

The SONIC-16 uses three areas in memory to store descrip-

tor information the Transmit Descriptor Area (TDA) Re-

ceive Descriptor Area (RDA) and the Receive Resource

Area (RRA) The SONIC-16 accesses these areas by con-

catenating a 16-bit base address register with a 16-bit offset

bits of address and the offset registers provide the lower 16

bits of address The base address registers are the Upper

Transmit Descriptor Address (UTDA) Upper Receive De-

scriptor Address (URDA) and the Upper Receive Resource

Address (URRA) registers The corresponding offset regis-

ters are shown below

Upper Address Registers

Offset Registers

URRA

RSAREARWPRRP

URDA

CRDA

UTDA

CTDA

See Table 3-1 for definition of register mnemonics

Figure 3-1 shows an example of the Transmit Descriptor

Area and the Receive Descriptor Area being located by the

UTDA and URDA registers The descriptor areas RDA

TDA and RRA are allowed to have the same base address

ie URRAeURDAeUTDA Care however must be taken

to prevent these areas from overwriting each other

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