Addressing mode of microcontroller

addressing mode of  8051 micro-controller

what is addressing mode?
CPU access data from memory,register,immediate value.These various  way to access data are called addressing mode.
The 8051 micro-controller are five addressing mode.

1. immediate
2. register
3. direct
4. register indirect
5. index

Immediate:

• in this addressing mode source operand is constant.
• it's come after op code in instruction.
• it must be preceded by "#" .
• its used to load data into register.

• examples:

1. MOV A,#32H                                         ;Load 32h into A register.
2. MOV DPTR,#1234H                              ;Load 1234h into DPTR.

Register:

• In this mode registers are used to hold data to be manipulated.
• Size of destination register and source register are always equal.
• the data between to registers Rn(n=0 to 7) and Rn(n=0 to7) do not transfer.

• examples:

1. MOV A,R0               ;copy content of R0 into A.
2. MOV R0,R1             ;not possible because data to be processed in some memory location of RAM.

  Direct:

• to access data directly form RAM location.
• using this mode we can access 128Byte RAM directly.
• in this data is in a RAM memory location whose address is known.
• the difference between immediate and direct addressing mode is "#"sign.in this mode this sign is absent.

• examples:

1. MOV R0,30H                                                  ;save content of RAM location 30h into R0.
2. MOV 7,2                                                          ;invalid (it means copy content of R2 into R7).

Register indirect:

• in this register used as pointer to the data.
• only register R0 and R1 used in this mode.
• R0 and R1 hold address of RAM location.
• both register must be preceded by "@"sign.
• looping is easily used in this mode.
• drawback of this mode is we can used two register R0 & R1 only.

• examples:

1. MOV A,@R0                ;move content of RAM location whose address stored in R0.
2. MOV @R1,A                ;move content of A into RAM location whose address stored in R1.

 index :

• in this mode data access from look up table entire located in the program memory ROM space of 8051.
• register A and DPTR used in this addressing mode.
• here instruction MOVC used instead of MOV because C indicate Code memory(ROM).

• examples:

1. MOVC A,@A+DPTR;