Addressing modes
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The document provides an overview of various addressing modes in computer architecture, including immediate, direct, indirect, register, register indirect, relative, and indexed addressing. Each mode is explained with examples and their respective advantages and limitations regarding access speed and address space. The presentation also discusses auto increment and auto decrement modes as variations in addressing techniques.
![INDIRECT ADDRESSING
Memory cell pointed to by address field contains
the address of (pointer to) the operand
EA = [A]
Look in A, find address (A) and look there for
operand
e.g. ADD (A)
Add contents of cell pointed to by contents of A to
accumulator](https://image.slidesharecdn.com/addressingmodes-161004171531/85/Addressing-modes-8-320.jpg)
![REGISTER INDIRECT ADDRESSING
C.f. indirect addressing
EA = [R]
Operand is in memory cell pointed to by
contents of register R
Large address space (2n)
One fewer memory access than indirect
addressing](https://image.slidesharecdn.com/addressingmodes-161004171531/85/Addressing-modes-14-320.jpg)
![INDEX ADDRESSING
EA = X + [R]
Address field hold two values
X = constant value (offset)
R = register that holds address of memory
locations
or vice versa
(Offset given as constant or in the index register)
Add 20(R1),R2 or Add 1000(R1),R2](https://image.slidesharecdn.com/addressingmodes-161004171531/85/Addressing-modes-16-320.jpg)
![AUTO INCREMENT MODE
The effective address of the operand is the
contents of a register specified in the
instruction.
After accessing the operand, the contents of
this register are automatically incremented to
point to the next item in the list
EA=[Ri]; Increment Ri ---- (Ri)+
Eg: Add (R2)+,R0](https://image.slidesharecdn.com/addressingmodes-161004171531/85/Addressing-modes-19-320.jpg)
![AUTO DECREMENT MODE
The contents of a register specified in the
instruction are first automatically
decremented and are then used as the
effective address of the operand
Decrement Ri; EA= [Ri] ----- -(Ri)](https://image.slidesharecdn.com/addressingmodes-161004171531/85/Addressing-modes-20-320.jpg)
Addressing modes
- 1. CLASS PRESENTATION UNIT-1 Mohammad Asif Iqbal Assistant Professor, Deptt of ECE, JETGI, Barabanki
- 3. LETS BEGAIN WITH AS EXAMPLE.. Suppose we have a two operands instruction ADD X, Y X in register Y in memory Then, after taking this particular example, we were in fact working out the series of actions we must take Next step is to look the various possible ways in which the operands can be referred to. In other words, what we are talking about is the different modes in which we can address the data. That is, modes of addressing the data or referring to the data will usually be called addressing modes
- 4. DIFFERENT TYPES OF ADDRESSING MODE? Immediate Addressing Direct Addressing Indirect Addressing Register Addressing Register Indirect Addressing Relative Addressing Indexed Addressing
- 5. IMMEDIATE ADDRESSING Operand is given explicitly in the instruction Operand = Value e.g. ADD 5 Add 5 to contents of accumulator 5 is operand No memory reference to fetch data Fast Limited range Instruction opcode operand
- 6. DIRECT ADDRESSING Address field contains address of operand Effective address (EA) = address field (A) e.g. ADD A Add contents of cell A to accumulator Look in memory at address A for operand Single memory reference to access data No additional calculations to work out effective address Limited address space
- 8. INDIRECT ADDRESSING Memory cell pointed to by address field contains the address of (pointer to) the operand EA = [A] Look in A, find address (A) and look there for operand e.g. ADD (A) Add contents of cell pointed to by contents of A to accumulator
- 9. INDIRECT ADDRESSING Large address space 2n where n = word length May be nested, multilevel, cascaded e.g. EA = (((A))) Draw the diagram yourself Multiple memory accesses to find operand Hence slower
- 11. REGISTER ADDRESSING Operand is held in register named in address field EA = R Limited number of registers Very small address field needed Shorter instructions Faster instruction fetch
- 12. REGISTER ADDRESSING No memory access Very fast execution Very limited address space Multiple registers helps performance Requires good assembly programming or compiler writing
- 14. REGISTER INDIRECT ADDRESSING C.f. indirect addressing EA = [R] Operand is in memory cell pointed to by contents of register R Large address space (2n) One fewer memory access than indirect addressing
- 15. REGISTER INDIRECT ADDRESSING DIAGRAM
- 16. INDEX ADDRESSING EA = X + [R] Address field hold two values X = constant value (offset) R = register that holds address of memory locations or vice versa (Offset given as constant or in the index register) Add 20(R1),R2 or Add 1000(R1),R2
- 18. RELATIVE ADDRESSING A version of displacement addressing R = Program counter, PC EA = X + (PC) i.e. get operand from X bytes away from current location pointed to by PC c.f locality of reference & cache usage
- 19. AUTO INCREMENT MODE The effective address of the operand is the contents of a register specified in the instruction. After accessing the operand, the contents of this register are automatically incremented to point to the next item in the list EA=[Ri]; Increment Ri ---- (Ri)+ Eg: Add (R2)+,R0
- 20. AUTO DECREMENT MODE The contents of a register specified in the instruction are first automatically decremented and are then used as the effective address of the operand Decrement Ri; EA= [Ri] ----- -(Ri)
- 21. THANK YOU!
Editor's Notes
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