Addressing mode of 8051 micro controller

1. Nitin Ahire
XIE,Mahim
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2.  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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3. 1)Direct addressing mode
2)Indirect addressing mode
3)Register addressing
4)Register specific addressing (optional)
5)Immediate addressing
6)External Addressing ( Index )
a) Code access b) Data access
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4.  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.
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5.  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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6.  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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7.  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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8.  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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9.  E.g.
MOV R2,A
MOV A, R1
MOV A,R7
Note: MOV R2,R7 is an invalid instruction
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10.  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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11.  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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12.  E.g.
MOV A,#30h
MOV P1,#0FF
MOV DPTR, #1234h
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13. (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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14.  E.g.
MOVC A,@A+DPTR
MOVC A,@A+PC
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15. (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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