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3.7 transfer of control

For all of the operation types discussed so far. The next instruction to be performed is the one that immediately follows, in memory, the current instruction. However, a significant fraction of the instructions in any program have as their function chang­ing the sequence of instruction execution. For these instructions, the operation per­formed by the CPU is to update the program counter to contain the address of some instruction in memory.

There are a number of reasons why transfer-of-control operations are re­quired. Among the most important are the following:

1. In the practical use of computers, it is essential to be able to execute each instruction more than once and perhaps many thousands of times. It may require thousands or perhaps millions of instructions to implement an appli­cation. This would be unthinkable if each instruction had to be written out sep­arately. If a table or a list of items is to be processed, a program loop is needed. One sequence of instructions is executed repeatedly to process all the data.

2. Virtually all programs involve some decision making. We would like the com­puter to do one thing if one condition holds, and another thing if another con­dition holds. For example, a sequence of instructions computes the square root of a number. At the start of the sequence, the sign of the number is tested, If the number is negative, the computation is not performed, but an error con­dition is reported.

3. To compose correctly a large or even medium-size computer program is an exceedingly difficult task. It helps if there are mechanisms for breaking the task up into smaller pieces that can be worked on one at a time.

We now turn to a discussion of the most common transfer-of-control opera­tions found in instruction sets: branch, skip, and procedure call.

Branch instruction

A branch instruction, also called a jump instruction, has as one of its operands the address of the next instruction to be executed. Most often, the instruction is a conditional branch instruction. That is, the branch is made (update program counter to equal address specified in operand) only if a certain condition is met. Otherwise, the next instruction in sequence is executed (increment program counter as usual).

Skip instructions

Another common form of transfer-of-control instruction is the skip instruc­tion. The skip instruction includes an implied address. Typically, the skip implies that one instruction be skipped; thus, the implied address equals the address of the next instruction plus one instruction-length.

Procedure call instructions

Perhaps the most important innovation in the development of programming languages is the procedure, a procedure is a self-contained computer program that is incorporated into a larger program. At any point in the program the procedure may he invoked, or called. The processor is instructed to go and execute the entire procedure and then return to the point from which the call took place.

The two principal reasons for the use of procedures are economy and modu­larity. A procedure allows the same piece of code to be used many times. This is important for economy in programming effort and for making the most efficient use of storage space in the system (the program must be stored). Procedures also allow large programming tasks to be subdivided into smaller units. This use of modularity greatly eases the programming task.

Questions & Answers

what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
Bharti
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
Daniel
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
Maciej
characteristics of micro business
Abigail
for teaching engĺish at school how nano technology help us
Anassong
Do somebody tell me a best nano engineering book for beginners?
s. Reply
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
s.
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
Tarell
what is the actual application of fullerenes nowadays?
Damian
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
Tarell
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
Virgil
is Bucky paper clear?
CYNTHIA
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Harper
Do you know which machine is used to that process?
s.
how to fabricate graphene ink ?
SUYASH Reply
for screen printed electrodes ?
SUYASH
What is lattice structure?
s. Reply
of graphene you mean?
Ebrahim
or in general
Ebrahim
in general
s.
Graphene has a hexagonal structure
tahir
On having this app for quite a bit time, Haven't realised there's a chat room in it.
Cied
what is biological synthesis of nanoparticles
Sanket Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
China
Cied
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
Porter
many many of nanotubes
Porter
what is the k.e before it land
Yasmin
what is the function of carbon nanotubes?
Cesar
I'm interested in nanotube
Uday
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
AMJAD
preparation of nanomaterial
Victor Reply
how to synthesize TiO2 nanoparticles by chemical methods
Zubear
how did you get the value of 2000N.What calculations are needed to arrive at it
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Source:  OpenStax, Computer architecture. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10761/1.1
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