030070104 Cebrail Yıldırım 11th bonus week

Asynchronous Systems (first&best) [Operation System]

          Asynchronous (or self-timed) hardware systems can be defined broadly as systems whose operation is

not synchronized to a global clock signal. Synchronous systems on the other hand are based on

a global clock. All subsystems of the circuit work in lock step and the signal levels have significance

only when a clock pulse arrives. In synchronous systems one must optimize for the worst case behavior.

It is the worst case that is going to set the clock frequency, irrespective of how frequently the

worst case scenario is encountered. When designing asynchronous circuits, the designer must optimize

for the average case and not the worst case scenario. Asynchronous design styles were engineered before many synchronous techniques, but were left to the way side because of their perceived difficulty of implementation. Components in an Asynchronous system operate as fast as they can, and notify other components (i.e. handshaking) when they have completed their work. Interest in asynchronous circuits has emerged again to overcome some of the design difficulties presented by the sub-micron and sub-nanosecond VLSI technology available today. As the transistor feature size decreases, VLSI designers are now incorporating millions of transistors within a single chip. This increase in density has also been accompanied by a significant reduction of the switching speed at the gate level. All these technological improvements do not come free and designers are now faced with some very difficult design issues. The interconnection delay is becoming a major problem, demanding extensive post layout timing simulation and very complex schemes to distribute the global clock signal. Asynchronous systems have the potential to solve these problems. Some of the possible benefits of an asynchronous design include :

• ·         No clock skew - Clock skew is the difference in arrival times of the clock signal at different

parts of the circuit. Since asynchronous circuits by definition have no global distributed clock, there is no need to worry about clock skew.

• ·         Lower power - Standard synchronous circuits have to toggle clock lines, and possibly pre charge

and discharge signals, even in portions of a circuit unused in the current computation.

For example, even though a floating point unit on a processor might not be used in a given instruction stream, the unit still must be operated- by the clock.

• ·         Average-case instead of worst-case performance.

• ·         Easing of global timing issues - In systems such as a synchronous microprocessor, the system clock, and thus system performance, is dictated by the slowest (critical) path. Thus, most portions of a circuit must be carefully optimized to achieve the highest clock rate, including rarely used portions of the system. 
•         Automatic adaptation to physical properties -

The delay through a circuit can change with variations in fabrication, temperature, and

power-supply voltage. Synchronous circuits must assume that the worst possible combination

of factors is present and clock the system accordingly. Many asynchronous circuits

sense computation completion, and will run as quickly as the current physical properties allow.

Asynchronous circuits have several problems as well. Primarily, asynchronous circuits are more

difficult to design in an ad hoc fashion than synchronous circuits. In a synchronous system, a designer

can simply define the combinational logic necessary to compute the given functions, and surround

it with latches. By setting the clock rate to a long enough period, all worries about hazards (undesired

signal transitions) and the dynamic state of the circuit are removed. In contrast, designers

of asynchronous systems must pay a great deal of attention to the dynamic state of the circuit. Hazards

must also be removed from the circuit, or not introduced in the first place, to avoid incorrect results.

The ordering of operations, which was fixed by the placement of latches in a synchronous system,

must be carefully ensured by the asynchronous control logic. For complex systems, these issues become

too difficult to handle by hand.
         Another problem with asynchronous systems is, during a transition, signals may assume values that are not well defined discrete values. For example, a signal corresponding to a boolean state variable

could have a value that some circuits interpret as true and others as false. In Synchronous

systems such problems are avoided by using the clock signal as a reference point to indicate the correct

value. Unfortunately, asynchronous circuits in general cannot leverage of existing CAD tools

and implementation alternatives for synchronous system. Also rapid prototyping of asynchronous

circuits is difficult as FPGA's normally do not support asynchronous systems.

(Shourya P. Bhattacharya,Asynchronous Systems: An Introduction, 2003)

There is no previous definition. 

Business Function(first&best) [Function]

Business Function is an atomic step within an elementary business activity. Thereby, a business function is always executed ot performed without any external interruption. Thus, each function is either performed by human or by system why business functions are classified into huma functions and system functions.

(Ilia Bider, Enterprise, Business-Process and Information Systems Modeling, Pg. 43)