Bursting Technology in Cogent Adapters

EISA Bursting Benefits Your Entire System

Cogent's EM935 EISA XL takes advantage of bursting, an EISA-
compliant technology that optimizes system performance. In
machines which comply with the 3.12 EISA specification,
burst mode transfers move data across the system bus at
speeds up to 33 MBytes/sec._more than four times faster than
non-bursting transfers which can only achieve 8.32
MBytes/sec. By transferring large amounts of data at a time,
the EM935 EISA XL bursting adapter increases overall system
performance.

Whenever a request is sent to or received from a network,
data is transferred across a computer's data highway, the
system bus, between memory and a LAN adapter. Greater
amounts of data require more transfers to move the data from
one part of the system to the other. For example, graphics
and Windows files typically require more transfers than
smaller word processing or spreadsheet files because they
contain larger amounts of data.

Depending on the amount of data, data is transferred within
one or more DMA (Direct Memory Access) cycles. A DMA cycle
is a series of operations which initiate and complete the
data transfer, an important component of which is timing;
each DMA cycle contains multiple clock cycles (small
increments of time) which together regulate the entire DMA
cycle's timing. The more DMA cycles that must be performed
to transfer multiple units of data, the more time the
transfers require and the slower the system becomes.

Bursting is the most efficient means of transferring large
blocks of data because it reduces much of the overhead
associated with performing multiple data transfers. By
increasing the number of transfers in a single DMA cycle,
bursting minimizes the overall number of DMA cycles and
subsequently reduces the amount of time necessary to move
data across the system bus. Non-bursting adapters require
more time than bursting adapters to transfer data because
they can only transfer data once every DMA cycle (a minimum
of four clock cycles). Bursting adapters can perform
multiple read or write memory transfers, up to one per clock
cycle, without requiring a new DMA cycle to be initiated for
each transfer. With less overhead associated with each
transfer, a bursting adapter can transfer up to four units
of data across the bus in the time that it takes a non-
bursting adapter to transfer one. Figure 1 contrasts
bursting and non-bursting DMA transfers.

A More Efficient Solution

The difference between burst mode transfers and non-bursting
data transfers is like that of a city's mass transit system
and cars. Metro buses (not to be confused with a computer's
system bus!) make more sense than cars in quickly and
efficiently moving large numbers of people from place to
place. Similarly, burst mode transfers which move large
amounts of data at a time are a better system solution than
non-burst mode transfers.

Because more people can be transported at a time by bus than
by car, buses are not only more efficient than cars, they
reduce highway congestion so more vehicles can share the
highway. Like mass transit, bursting allows more devices to
share a computer's "highway," the system bus, to transfer
data. Bursting is particularly advantageous in systems where
Ethernet adapters must compete with devices such as disk
controllers and other adapters for control of the system
bus. A bursting Ethernet adapter can transfer more data in
the short amount of time it has the bus than a non-bursting
adapter, giving other devices in the system more time on the
bus if they need it.

A computer is not just a box full of sophisticated
electronics, it is a system of interrelated components. As
such, the performance of each component effects the
performance of the entire system. By quadrupling the speed
of data transfers across the EISA bus, Cogent's EM935 EISA
XL adapter with bursting helps the entire system to be more
efficient.

USA Headquarters
Cogent Data Technologies, Inc.
175 West St. P.O. Box 926
Friday Harbor, WA   98250
Tel (206) 378-2929  Fax (206) 378-2882
Toll-free in U.S.  1-800-426-4368
Internet E-Mail  sales@cogentdata.com
CompuServe  GO COGENT

European Sales
Rosenstra_e 7
80331 Munich   Germany
Tel [49] 89/231138-88  Fax [49] 89/231138-11

c 1993 Cogent Data Technologies, Inc. All rights reserved.
Printed in U.S.A. Cogent is a registered trademark and eMASTER+  
is a trademark of Cogent Data Technologies, Inc. All other 
trademarks are the property of their respective owners. 
Part no. 842-00239-00