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The procedure mechanism involves two basic instructions: a call instruction that branches from the present location to the procedure, and a return instruction that returns from the procedure to the place from which it was called. Both of these are forms of branching instructions.

The above figure illustrates the use of procedures to construct a program. In this example, there is a main program starting at location 4000. This program includes a call to procedure PROC1, starting at location 4500. When this call instruction is encountered, the CPU suspends execution of the main program and begins ex­ecution of PROC1 by fetching the next instruction from location 4500. Within PROC1, there are two calls to PR0C2 at location 4800. In each case, the execution of PROC1 is suspended and PROC2 is executed. The RETURN statement causes the CPU to go back to the calling program and continue execution at the instruc­tion after the corresponding CALL instruction. This behavior is illustrated in the right of this figure.

Several points are worth noting:

1. A procedure can be called from more than one location.

2. A procedure call can appear in a procedure. This allows the nesting of proce­dures to an arbitrary depth.

3. Each procedure call is matched by a return in the called program.

Because we would like to be able to call a procedure from a variety of points, the CPU must somehow save the return address so that the return can take place appropriately. There are three common places for storing the return address:

• Register

• Start of called procedure

• Top of stack

4. addressing modes

The address field or fields in a typical instruction format are relatively small. We would like to be able to reference a large range of locations in main memory or for some systems, virtual memory. To achieve this objective, a variety of addressing techniques has been employed. They all involve some trade-off between address range and/or addressing flexibility, on the one hand, and the number of memory ref­erences and/or the complexity of address calculation, on the other. In this section, we examine the most common addressing techniques:

  • Immediate
  • Direct
  • Indirect
  • Register
  • Register indirect
  • Displacement

4.1 immediate addressing

The simplest form of addressing is immediate addressing, in which the operand is actually present in the instruction:

  • Operand is part of instruction
  • Operand = address field
  • e.g. ADD 5 ;Add 5 to contents of accumulator ;5 is operand

The advantage of immediate addressing is that no memory reference other than the instruction fetch is required to obtain the operand, thus saving one mem­ory or cache cycle in the instruction cycle.

The disadvantage is that the size of the number is restricted to the size of the address field, which, in most instruction sets, is small compared with the word length.

4.2 direct addressing

A very simple form of addressing is direct addressing, in which:

  • Address field contains address of operand
  • Effective address (EA) = address field (A)
  • e.g. ADD A ;Add contents of cell A to accumulator

The technique was common in earlier generations of computers but is not common on contemporary architectures. It requires only one memory reference and no spe­cial calculation. The obvious limitation is that it provides only a limited address space.

4.3 indirect addressing

With direct addressing, the length of the address field is usually less than the word length, thus limiting the address range. One solution is to have the address field refer to the address of a word in memory, which in turn contains a full-length address of the operand. This is known as indirect addressing.

4.4 register addressing

Register addressing is similar to direct addressing. The only difference is that the address field refers to a register rather than a main memory address.

The advantages of register addressing are that :

  • Only a small address field is needed in the instruction
  • No memory 'references are required, faster instruction fetch

The disadvantage of register addressing is that the address space is very limited.

4.5 register indirect addressing

Just as register addressing is analogous to direct addressing, register indirect ad­dressing is analogous to indirect addressing. In both cases, the only difference is whether the address field refers to a memory location or a register. Thus, for register indirect address: Operand is in memory cell pointed to by contents of register.

The advantages and limitations of register indirect addressing are basically the same as for indirect addressing. In both cases, the address space limitation (limited range of addresses) of the address field is overcome by having that field refer to a word-length location containing an address. In addition, register indirect addressing uses one less memory reference than indirect addressing.

4.6 displacement addressing

A very powerful mode of addressing combines the capabilities of direct addressing and register indirect addressing. It is known by a variety of names depending on the context of its use but the basic mechanism is the same. We will refer to this as displacement addressing, address field hold two values:

  • A = base value
  • R = register that holds displacement

Questions & Answers

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
Do somebody tell me a best nano engineering book for beginners?
s. Reply
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.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
what is biological synthesis of nanoparticles
Sanket 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
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit 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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