<< Chapter < Page Chapter >> Page >
  • A control signal that opens gates, allowing the contents of the MAR onto the address bus
  • A memory read control signal on the control bus
  • A control signal that opens the gates, allowing the contents of the data bus to be stored in the MBR
  • Control signals to logic that add 1 to the contents of the PC and store the result back to the PC

Following this, the control unit sends a control signal that opens gates between the MBR and the IR.

This completes the fetch cycle except for one thing: The control unit must decide whether to perform an indirect cycle or an execute cycle next. To decide this, it examines the IR to see if an indirect memory reference is made.

The indirect and interrupt cycles work similarly. For the execute cycle, the control unit begins by examining the opcode and. on the basis of that, decides which sequence of micro-operations to perform for the execute cycle.

A Control Signals Example

To illustrate the functioning of the control unit, let us examine a simple example. Figure 6.5 illustrates the example.

Figure 6.5 Data Paths and Control Signals

This is a simple processor with a single accumulator. The data paths between elements are indicated. The control paths for signals emanating from the control unit are not shown, but the terminations of control signals are labeled Ci and indicated by a circle. The control unit receives inputs from the clock, the instruction register, and flags. With each clock cycle, the control unit reads all of its inputs and emits a set of control signals. Control signals go to three separate destinations:

  • Data paths: The control unit controls the internal How of data. For example, on instruction fetch, the contents of the memory buffer register are transferred to the instruction register. For each path to be controlled, there is a gate (indicated by a circle in the figure). A control signal from the control unit temporarily opens the gate to let data pass.
  • ALU: The control unit controls the operation of the ALU by a set of control signals. These signals activate various logic devices and gates within the ALU.
  • System bus: The control unit sends control signals out onto the control lines of the system bus (e.g., memory READ).

The control unit must maintain knowledge of where it is in the instruction cycle. Using this knowledge, and by reading all of its inputs, the control unit emits a sequence of control signals that causes micro-operations to occur.

2.3 internal processor organization

Figure 6.5 indicates the use of a variety of data paths. The complexity of this type of organization should be clear. More typically, some sort of internal bus arrangement. Using an internal processor bus, Figure 6.5 can be rearranged as shown in Figure 6.6.

Figure 6.6 CPU with Internal Bus

A single internal bus connects the ALU and all processor registers. Gates and control signals are provided for movement of data onto and off the bus from each register. Additional control signals control data transfer to and from the system (external) bus and the operation of the ALU.

Two new registers, labeled Y and Z have been added to the organization. These are needed for the proper operation of the ALU. When an operation involving two operands is performed, one can be obtained from the internal bus, but the other must be obtained from another source. The AC could be used for this purpose, but this limits the flexibility of the system and would not work with a processor with multiple general-purpose registers. Register Y provides temporary storage for the other input. The ALU is a combinatorial circuit with no internal storage, Thus, when control signals activate an ALU function, the input to the ALU is transformed to the output. Thus, the output of the ALU cannot be directly connected to the bus, because this output would feed back to the input. Register Z provides temporary output storage, with this arrangement, an operation to add a value from memory to the AC would have the following steps:

t1: MAR<= (IR(address))

t2: MBR<= Memory

t3: Y<= (MBR)

t4: Z<= (AC0 + (Y)

t5: ac<= (z)

Other organizations are possible, but, in general, some sort of internal bus or set of internal buses is used. The use of common data paths simplifies the interconnection layout and the control of the processor. Another practical reason for the use of an internal bus is to save space. Especially for microprocessors, which may occupy only a 1/4-inch square piece of silicon, space occupied by internal connections must be minimized.

Questions & Answers

how do they get the third part x = (32)5/4
kinnecy Reply
can someone help me with some logarithmic and exponential equations.
Jeffrey Reply
sure. what is your question?
okay, so you have 6 raised to the power of 2. what is that part of your answer
I don't understand what the A with approx sign and the boxed x mean
it think it's written 20/(X-6)^2 so it's 20 divided by X-6 squared
I'm not sure why it wrote it the other way
I got X =-6
ok. so take the square root of both sides, now you have plus or minus the square root of 20= x-6
oops. ignore that.
so you not have an equal sign anywhere in the original equation?
Commplementary angles
Idrissa Reply
im all ears I need to learn
right! what he said ⤴⤴⤴
what is a good calculator for all algebra; would a Casio fx 260 work with all algebra equations? please name the cheapest, thanks.
Kevin Reply
a perfect square v²+2v+_
Dearan Reply
kkk nice
Abdirahman Reply
algebra 2 Inequalities:If equation 2 = 0 it is an open set?
Kim Reply
or infinite solutions?
The answer is neither. The function, 2 = 0 cannot exist. Hence, the function is undefined.
Embra Reply
if |A| not equal to 0 and order of A is n prove that adj (adj A = |A|
Nancy Reply
rolling four fair dice and getting an even number an all four dice
ramon Reply
Kristine 2*2*2=8
Bridget Reply
Differences Between Laspeyres and Paasche Indices
Emedobi Reply
No. 7x -4y is simplified from 4x + (3y + 3x) -7y
Mary Reply
is it 3×y ?
Joan Reply
J, combine like terms 7x-4y
Bridget Reply
how do you translate this in Algebraic Expressions
linda Reply
Need to simplify the expresin. 3/7 (x+y)-1/7 (x-1)=
Crystal Reply
. After 3 months on a diet, Lisa had lost 12% of her original weight. She lost 21 pounds. What was Lisa's original weight?
Chris Reply
what's the easiest and fastest way to the synthesize AgNP?
Damian Reply
types of nano material
abeetha Reply
I start with an easy one. carbon nanotubes woven into a long filament like a string
many many of nanotubes
what is the k.e before it land
what is the function of carbon nanotubes?
I'm interested in nanotube
what is nanomaterials​ and their applications of sensors.
Ramkumar Reply
what is nano technology
Sravani Reply
what is system testing?
preparation of nanomaterial
Victor Reply
Yes, Nanotechnology has a very fast field of applications and their is always something new to do with it...
Himanshu Reply
good afternoon madam
what is system testing
what is the application of nanotechnology?
In this morden time nanotechnology used in many field . 1-Electronics-manufacturad IC ,RAM,MRAM,solar panel etc 2-Helth and Medical-Nanomedicine,Drug Dilivery for cancer treatment etc 3- Atomobile -MEMS, Coating on car etc. and may other field for details you can check at Google
anybody can imagine what will be happen after 100 years from now in nano tech world
after 100 year this will be not nanotechnology maybe this technology name will be change . maybe aftet 100 year . we work on electron lable practically about its properties and behaviour by the different instruments
name doesn't matter , whatever it will be change... I'm taking about effect on circumstances of the microscopic world
how hard could it be to apply nanotechnology against viral infections such HIV or Ebola?
silver nanoparticles could handle the job?
not now but maybe in future only AgNP maybe any other nanomaterials
I'm interested in Nanotube
this technology will not going on for the long time , so I'm thinking about femtotechnology 10^-15
can nanotechnology change the direction of the face of the world
Prasenjit Reply
At high concentrations (>0.01 M), the relation between absorptivity coefficient and absorbance is no longer linear. This is due to the electrostatic interactions between the quantum dots in close proximity. If the concentration of the solution is high, another effect that is seen is the scattering of light from the large number of quantum dots. This assumption only works at low concentrations of the analyte. Presence of stray light.
Ali Reply
the Beer law works very well for dilute solutions but fails for very high concentrations. why?
bamidele Reply
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
Privacy Information Security Software Version 1.1a
Got questions? Join the online conversation and get instant answers!
QuizOver.com Reply

Get the best Algebra and trigonometry course in your pocket!

Source:  OpenStax, Computer architecture. OpenStax CNX. Jul 29, 2009 Download for free at http://cnx.org/content/col10761/1.1
Google Play and the Google Play logo are trademarks of Google Inc.

Notification Switch

Would you like to follow the 'Computer architecture' conversation and receive update notifications?