Addressing Modes of 8051 microcontroller

Addressing Modes of 8051 microcontroller

An "addressing mode" refers to how you are addressing a given memory location.

•  Immediate Addressing MOV A,#20h 
• Direct Addressing MOV A,30h 
• Indirect Addressing MOV A,@RO 
• External Direct MOVX A,@ DPTR 
• Code Indirect MOVC A,@A+DPTR 

Each of these addressing modes provides important flexibility.

Immediate Addressing:

Immediate addressing is so-named because the value to be stored in memory immediately follows the operation code in memory. That is to say, the instruction itself dictates what. value will be stored in memory.

For example, the instruction:

MOV A,#20h

This instruction uses Immediate Addressing because the Accumulator will be loaded with the value that immediately follows. Immediate addressing is very fast since the value to be loaded is included in the instruction. Direct Addressing However, since the value to be loaded is fixed at compile-time it is not very flexible.

Indirect Addressing

Indirect addressing is a very powerful addressing mode which in many cases provides an exceptional level of flexibility. Indirect addressing is also the only way to access the extra 128 bytes of Internal RAM found on an 8052. Indirect addressing appears as follows:

MOV A,@RO

This instruction causes the 8051 to analyze the value of the R0 register. The 8051 will then load the accumulator with the value from Internal RAM which is found at the address indicated by RO:

For example, let's say RO holds the value 40h and Internal RAM address 40h holds the value 67h. When the above instruction is executed the 8051 will check the value of RO. Since RO holds 40h the 8051 will get the value out of Internal RAM address 40h (which holds 67h) and store it in the Accumulator. Thus, the Accumulator ends up holding 67h. Indirect addressing always refers to Internal RAM; it never refers to an SFR.

External Direct

External Memory is accessed using a suite of instructions which use what I call "External. Direct" addressing. I call it this because it appears to be direct addressing, but it is used to access external memory rather than internal memory. There are only two commands that use External Direct addressing mode:

MOVX A,@DPTR

MOVX @DPTR,A

 As you can see, both commands utilize DPTR. In these instructions, DPTR must first be loaded with the address of external memory that you wish to read or write. Once DPTR holds the correct external memory address, the first command will move the contents of that external memory address into the Accumulator. The second command will do the opposite: it will allow you to write the value of the Accumulator to the external memory address pointed to by DPTR.

External Indirect

External memory can also be accessed using a form of indirect addressing which I call External Indirect addressing. This form of addressing is usually only used in relatively

small projects that have a very small amount of external RAM. An example of this addressing mode is:

MOVX @RO,A

Once again, the valuc of RO is first read and the value of the Accumulator is written to that address in External RAM. Since the value of @RO can only be 00h through FFh the project would effectively be limited to 256 bytes of External RAM. There are relatively simple hardware/software tricks that can be implemented to access more than 256 bytes of memory using External Indirect addressing; however, it is usually easier to use External Direct addressing if your project has more than 256 bytes of External RAM,