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At the beginning of each instruction cycle, the processor fetches an instruction from memory. In a typical processor, a register called the program counter (PC) holds the address of the instruction to be fetched next. Unless told otherwise, the processor always increments the PC after each instruction fetch so that it will fetch the next instruction in sequence. The fetched instruction is loaded into a register in the processor known as the instruction register (IR). The instruction contains bits that specify the action the processor is to take. The processor interprets the instruction and performs the re­quired action.

Execute cycle:

In general, the required actions fall into four categories:

  • Processor-memory: Data may be transferred from processor to memory or from memory to processor.
  • Processor-I/O: Data may be transferred to or from a peripheral device by transferring between the processor and an I/O module.
  • Data processing: The processor may perform some arithmetic or logic opera­tion on data.
  • Control: An instruction may specify that the sequence of execution be altered.
  • An instruction’s execution may involve a combination of these actions.

Instruction cycle state diagram:

Figure 4 provides a more detailed look at the basic instruction cycle. The figure is in the form of a state diagram. For any given instruction cycle, some stales may be null and others may be visited more than once. The states can be described as follows:

Figure 4: Instruction cycle state diagram

  • Instruction address calculation (iac): Determine the address of the next instruction to be executed. Usually, this involves adding a fixed number to the address of the previous instruction. For example, if each instruction is 16 bits long and memory is organized into 16-bit words, then add 1 to the previous address. If, instead, memory is organized as individually addressable 8-bit bytes, then add 2 to the previous address.
  • Instruction fetch (if): Read instruction from its memory location into the processor.
  • Instruction operation decoding (iod): Analyze instruction to determine type of operation to he performed and operand(s) to be used.
  • Operand address calculation (oac): If the operation involves reference to an operand in memory or available via I/O. then determine the address of the operand.
  • Operand fetch (of): Fetch the operand from memory or read it in from I/O,
  • Data operation (do): Perform the operation indicated in the instruction.
  • Operand store (os): Write the result into memory or out to I/O

2.2 interrupts

Virtually all computers provide a mechanism called Interrupt, by which other modules (I/O. mem­ory) may interrupt the normal processing of the processor. Interrupts are provided primarily as a way to improve processing efficiency.

For example, most external devices are much slower than the processor. Suppose that the processor is transferring data to a printer using the instruction cycle scheme of Figure 3. After each write operation, the processor must pause and remain idle until the printer catches up. The length of this pause may be on the order of many hundreds or even thousands of instruction cycles that do not involve memory. Clearly, this is a very wasteful use of the processor. The figure 5a illustrates this state of affairs.

Questions & Answers

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..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
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
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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
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
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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