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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 is variations in raman spectra for nanomaterials
Jyoti Reply
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
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
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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with the given example MOV BX,AX describe the sequence that will be followed using instruction state diagram
Gireesh Reply

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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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