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Addressing Modes

The way in which data in the main memory of a  Computer is addressed by a Processor is termed as the Addressing Mode used by the Processor. Such address of data is contained within the instruction word. It should be noted that data in memory is referenced in different ways.

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Instruction Word is a small  memory area that can hold  ( i ) an opcode ( ii ) and an entity of relevance to the opcode.

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The referee of data is the Processor. A location in main memory is referred to by the Processor.

There are different ways in which reference to a piece of data is made. These ways of referring to data are:

( 1 ) Direct Addressing: When the address of data is given in binary form, a direct reference to data is made. 

Address in binary form is a state that matches the deployed address standard at the native level. Direct Addressing requires us to do away with any kind of translation of  address. We are talking  of the data address.

( 2 ) Immediate Addressing: The operand itself appears in the instruction word in memory. The memory reference performed by the Processor results in arrival of the required data.

The data that is immediately available is instantly used as instructed by the Processor instruction.

( 3 ) Relative Addressing:

( 3 ) ( a ) Self-Relative Addressing: The address part of the instruction word is added to the current contents of the Program Counter.

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Note: Program Counter is a counting register. It is used to maintain the sequence of  execution of a Computer Program.

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In fact, the address part of the instruction word contains a value indicating displacement. 

The displacement value and Program Counter contents are added to yield a new Program location. This new location of instruction execution is at a displacement. 

( 3 ) ( b ) Base Addressing: In this scheme the address part of the instruction word is added to the contents of the Base Register. The addition yields the direct address of the required operand. Here the content of the  Base Register contains the base address.

( 4 ) Indirect Addressing: In this scheme the address contained in the instruction word gives another address. This newly obtained address is the address of the operand. 

We can see that two memory references are needed to obtain the data.

( 5 ) Indexed Addressing: This scheme is used to identify a section of data in a storage area. The contents of the Index Register are modified by adding the specified address in the instruction word. 

Based on  Index Register contents a series of direct addresses are generated one by one. These addresses help to  reference successive instructions in a step by step manner. 

After some time instruction execution stops as dictated by the contents of the Index Register. 


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