Nitin Ahire

XIE,Mahim

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The CPU can access data various ways The data could be in a register, or

memory or as an immediate value The various ways of accessing data are

called addressing modes.

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1)Direct addressing mode2)Indirect addressing mode3)Register addressing4)Register specific addressing (optional)5)Immediate addressing6)External Addressing ( Index )

a) Code access b) Data access

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In this mode the operand is specified by an

8-bit address field In the instruction.

One can access all the 128 bytes of internal RAM locations and each SFR.

If the MSB bit = 0 then the location is within on chip internal RAM. If MSB bit = 1

then the location is SFR.Prof. Nitin Ahire 4

The location 00h to 7Fh to address the internal RAM .

SFR addresses from 80h to FF h

e.g. MOV A,40h MOV R0,14h

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In this addressing mode, the instruction specifics a register which contains address

of an operand i.e. the register hold the actual address

that will be used in the data move operation.

The R0 and R1 of each bank can be used as an index or pointer to memory location

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The sign @ indicates the register acts as a pointer to memory locations

E.g. MOV A,@ R1 Note : only register R0 and R1 can be

used for indirect addressing mode .

MOV A,@R2 invalid instruction.

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Each bank consist of register R0 to R7 can be used as general purpose register for selection of bank register, the user has to modify the two bits of PSW

These register can be used to stored the address or data in the register addressing mode

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E.g. MOV R2,A MOV A, R1 MOV A,R7

Note: MOV R2,R7 is an invalid instruction

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In this addressing mode the instruction refer to a specific registers such as accumulator or data pointer

Example: DA A SWAP A

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This method is the simplest method to get the data.

In this addressing mode the source operand is a constant rather then a variable.

As the data source is a part of instruction

The # sign indicates that the data followed is immediate operand.

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E.g. MOV A,#30h MOV P1,#0FF MOV DPTR, #1234h

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(a) Code access (ROM access) Using these instructions only program

memory can be accessed.

This addressing mode is preferred for reading look up tables in the program memory.

Either DPTR or PC can be used as pointer.

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E.g. MOVC A,@A+DPTR

MOVC A,@A+PC

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(b) Data access (RAM access)

Using this addressing mode the programmer can access the external Data memory

E.g. MOVX A,@DPTR

MOVX @R0,A

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