91C94 datasheet(66/120 Pages) SMSC

66 / 120 page

THEORY OF OPERATION

The concept of presenting the shared RAM as a

FIFO of packets, with a memory management

unit allocating memory on a per packet basis

responds to the following needs:

Memory allocation for receive vs. transmit - A

fixed partition between receive and transmit area

would

not

efficient.

Being

able

dynamically allocate it to transmit and receive

represents almost the equivalent of duplicating

the memory size for some workstation type of

drivers.

Software overhead - By presenting a FIFO of

packets, the software driver does not have to

waste any time in calculating pointers for the

different buffers that make up different packets.

The driver usually deals with one packet at a

time.

With

this

approach,

packets

are

accessible always at the same fixed address,

and access is provided to any byte of the

packet.

Headers can be analyzed without reading out

the entire packet. The packet can be moved in

or out with a block move operation.

Multiple upper layer support - The LAN91C94

facilitates interfacing to multiple upper layer

protocols

because

the

receive

packet

processing flexibility.

A receive lookahead

scheme like ODI or NDIS drivers is supported

by copying a small part of the received packet

and letting the upper layer provide a pointer for

the rest of the data. If the upper layer indicates

it does not want the packet, it can be removed

upon a single command.

If the upper layer

wants a specific part of the packet, a block

move operation starting at any particular offset

can be done. Out of order receive processing is

also supported: if memory for one packet is not

yet available, receive packet processing can

continue.

Efficiency - Lacking any level of indirection or

linked lists of pointers, virtually all the memory

is used for data.

There are no descriptors,

forward links and pointers at all. This simplicity

and memory efficiency is accomplished without

giving up the benefits of linked lists which is

unlimited

back-to-back

transmission

and

reception without CPU intervention for as long

as memory is available.

91C94 Datasheet(PDF) 66 Page - SMSC Corporation

91C94 Datasheet(HTML) 66 Page - SMSC Corporation

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