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All of the micro-operations needed to perform one instruction cycle, including all of the micro-operations to execute every instruction in the instruction set, fall into one of these categories.

We can now be somewhat more explicit about the way in which the control unit functions. The control unit performs two basic tasks:

  • Sequencing: The control unit causes the processor lo step through a series of micro-operations in the proper sequence, based on the program being executed.
  • Execution: The control unit causes each micro-operation to be performed.

The preceding is a functional description of what the control unit does. The key to how the control unit operates is the use of control signals.

2.2 control signals

We have defined the elements that make up the processor (ALU, registers, data paths) and the micro-operations that are performed. For the control unit to perform its function, it must have inputs that allow it to determine the slate of the system and outputs that allow it to control the behavior of the system. These are the exter­nal specifications of the control unit. Internally, the control unit must have the logic required lo perform its sequencing and execution functions.

Figure 6.4 is a general model of the control unit, showing all of its inputs and outputs.

Figure 6.4 Model of Control Unit

The inputs are as follows:

  • Clock: This is how the control unit "keeps time." The control unit causes one micro-operation (or a set of simultaneous micro-operations) to be performed for each clock pulse. This is sometimes referred to as the processor cycle time. or the clock cycle lime.
  • Instruction register: The opcode of the current instruction is used lo determine which micro-operations lo perform during the execute cycle.
  • Flags: These are needed by the control unit to determine the status of the processor and the outcome of previous ALU operations. For example, for the increment-and-skip-if-zero (ISZ) instruction, the control until will increment the PC if the zero flag is set.
  • Control signals from control bus: The control bus portion of the system bus pro-vides signals to the control unit, such as interrupt signals and acknowledgments.

The outputs are as follows:

  • Control signals within the processor: These are two types: those that cause data to be moved from one register to another, and those that activate specific ALU functions.
  • Control signals to control bus: These are also of two types: control signals lo memory, and control signals lo the I/O modules.

The new element that has been introduced in this figure is the control signal. Three types of control signals are used: those that activate an ALU function, those that activate a data path, and those that are signals on the external system bus or other external interface. All of these signals are ultimately applied directly as binary inputs lo individual logic gates.

Let us consider again the fetch cycle to see how the control unit maintains control. The control unit keeps track of where it is in the instruction cycle. At a given point, it knows that the fetch cycle is to be performed next. The first step is to transfer the contents of the PC to the MAR. The control unit does this by activating the control signal that opens the gates between the bits of the PC and the bits of the MAR. The next step is to read a word from memory into the MBR and increment the PC. The control unit does this by sending the following control signals simultaneously:

Questions & Answers

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 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
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
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.
what is the actual application of fullerenes nowadays?
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.
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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