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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

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
Ali
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
learn
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
learn
Google
da
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Bhagvanji
hey
Giriraj
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
revolt
da
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
Bhagvanji
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.
Damian
yes that's correct
Professor
I think
Professor
Nasa has use it in the 60's, copper as water purification in the moon travel.
Alexandre
nanocopper obvius
Alexandre
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
Rafiq
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
Damian
How we are making nano material?
LITNING Reply
what is a peer
LITNING Reply
What is meant by 'nano scale'?
LITNING Reply
What is STMs full form?
LITNING
scanning tunneling microscope
Sahil
how nano science is used for hydrophobicity
Santosh
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
Rafiq
what is differents between GO and RGO?
Mahi
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
Rafiq
if virus is killing to make ARTIFICIAL DNA OF GRAPHENE FOR KILLED THE VIRUS .THIS IS OUR ASSUMPTION
Anam
analytical skills graphene is prepared to kill any type viruses .
Anam
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
Hafiz
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
Bob
The nanotechnology is as new science, to scale nanometric
brayan
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Damian
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
Renato
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
?
Kyle
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
Adin
why?
Adin
what school?
Kyle
biomolecules are e building blocks of every organics and inorganic materials.
Joe
When we have data register and address registers in user mode (visible registers) then why we need MAR and MBR
Hassan Reply
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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