8051 Tutorial: Addressing Modes
An "addressing mode" refers to how you are addressing a given memory location. In
summary, the addressing modes are as follows, with an example of each:
|Immediate Addressing||MOV A,#20h|
|Direct Addressing||MOV A,30h|
|Indirect Addressing||MOV A,@R0|
|External Direct||MOVX A,@DPTR|
|Code Indirect||MOVC A,@A+DPTR|
Each of these addressing modes provides important flexibility.
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:
This instruction uses Immediate Addressing because the Accumulator will be loaded with
the value that immediately follows; in this case 20 (hexidecimal).
Immediate addressing is very fast since the value to be loaded is included in the
instruction. However, since the value to be loaded is fixed at compile-time it is
not very flexible.
Direct addressing is so-named because the value to be stored in memory is obtained by
directly retrieving it from another memory location. For example:
This instruction will read the data out of Internal RAM address 30 (hexidecimal) and
store it in the Accumulator.
Direct addressing is generally fast since, although the value to be loaded isnt
included in the instruction, it is quickly accessable since it is stored in the 8051s
Internal RAM. It is also much more flexible than Immediate Addressing since the value
to be loaded is whatever is found at the given address--which may be variable.
Also, it is important to note that when using direct addressing any instruction which
refers to an address between 00h and 7Fh is referring to Internal Memory. Any
instruction which refers to an address between 80h and FFh is referring to the SFR
control registers that control the 8051 microcontroller itself.
The obvious question that may arise is, "If direct addressing an address from 80h
through FFh refers to SFRs, how can I access the upper 128 bytes of Internal RAM that
are available on the 8052?" The answer is: You cant access them using direct
addressing. As stated, if you directly refer to an address of 80h through FFh you
will be referring to an SFR. However, you may access the 8052s upper 128 bytes of
RAM by using the next addressing mode, "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:
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 R0.
For example, lets say R0 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 R0.
Since R0 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. Thus, in
a prior example we mentioned that SFR 99h can be used to write a value to the serial
port. Thus one may think that the following would be a valid solution to write the
value 1 to the serial port:
MOV R0,#99h ;Load the address of the serial port
MOV @R0,#01h ;Send 01 to the serial port -- WRONG!!
This is not valid. Since indirect addressing always refers to Internal RAM these two
instructions would write the value 01h to Internal RAM address 99h on an 8052. On an
8051 these two instructions would produce an undefined result since the 8051 only has
128 bytes of Internal RAM.
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:
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 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:
Once again, the value of R0 is first read and the value of the Accumulator is written
to that address in External RAM. Since the value of @R0 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