Basic Registers

The Accumulator

If you've worked with any other assembly languages you will be familiar with the concept of an Accumulator register.

The Accumulator, as its name suggests, is used as a general register to accumulate the results of a large number of instructions. It can hold an 8-bit (1-byte) value and is the most versatile register the 8051 has due to the shear number of instructions that make use of the accumulator. More than half of the 8051s 255 instructions manipulate or use the accumulator in some way.

For example, if you want to add the number 10 and 20, the resulting 30 will be stored in the Accumulator. Once you have a value in the Accumulator you may continue processing the value or you may store it in another register or in memory.

The "R" registers

The "R" registers are a set of eight registers that are named R0, R1, etc. up to and including R7.

These registers are used as auxillary registers in many operations. To continue with the above example, perhaps you are adding 10 and 20. The original number 10 may be stored in the Accumulator whereas the value 20 may be stored in, say, register R4. To process the addition you would execute the command:

    ADD A,R4

After executing this instruction the Accumulator will contain the value 30.

You may think of the "R" registers as very important auxillary, or "helper", registers. The Accumulator alone would not be very useful if it were not for these "R" registers.

The "R" registers are also used to temporarily store values. For example, lets say you want to add the values in R1 and R2 together and then subtract the values of R3 and R4. One way to do this would be:

    MOV A,R3 ;Move the value of R3 into the accumulator
    ADD A,R4 ;Add the value of R4
    MOV R5,A ;Store the resulting value temporarily in R5
    MOV A,R1 ;Move the value of R1 into the accumulator
    ADD A,R2 ;Add the value of R2
    SUBB A,R5 ;Subtract the value of R5 (which now contains R3 + R4)

As you can see, we used R5 to temporarily hold the sum of R3 and R4. Of course, this isnt the most efficient way to calculate (R1+R2) - (R3 +R4) but it does illustrate the use of the "R" registers as a way to store values temporarily.

The "B" Register

The "B" register is very similar to the Accumulator in the sense that it may hold an 8-bit (1-byte) value.

The "B" register is only used by two 8051 instructions: MUL AB and DIV AB. Thus, if you want to quickly and easily multiply or divide A by another number, you may store the other number in "B" and make use of these two instructions.

Aside from the MUL and DIV instructions, the "B" register is often used as yet another temporary storage register much like a ninth "R" register.

The Data Pointer (DPTR)

The Data Pointer (DPTR) is the 8051s only user-accessable 16-bit (2-byte) register. The Accumulator, "R" registers, and "B" register are all 1-byte values.

DPTR, as the name suggests, is used to point to data. It is used by a number of commands which allow the 8051 to access external memory. When the 8051 accesses external memory it will access external memory at the address indicated by DPTR.

While DPTR is most often used to point to data in external memory, many programmers often take advantge of the fact that its the only true 16-bit register available. It is often used to store 2-byte values which have nothing to do with memory locations.

The Program Counter (PC)

The Program Counter (PC) is a 2-byte address which tells the 8051 where the next instruction to execute is found in memory. When the 8051 is initialized PC always starts at 0000h and is incremented each time an instruction is executed. It is important to note that PC isnt always incremented by one. Since some instructions require 2 or 3 bytes the PC will be incremented by 2 or 3 in these cases.

The Program Counter is special in that there is no way to directly modify its value. That is to say, you cant do something like PC=2430h. On the other hand, if you execute LJMP 2430h youve effectively accomplished the same thing.

The Stack Pointer (SP)

The Stack Pointer, like all registers except DPTR and PC, may hold an 8-bit (1-byte) value. The Stack Pointer is used to indicate where the next value to be removed from the stack should be taken from.

When you push a value onto the stack, the 8051 first increments the value of SP and then stores the value at the resulting memory location.

When you pop a value off the stack, the 8051 returns the value from the memory location indicated by SP, and then decrements the value of SP.

This order of operation is important. When the 8051 is initialized SP will be initialized to 07h. If you immediately push a value onto the stack, the value will be stored in Internal RAM address 08h. This makes sense taking into account what was mentioned two paragraphs above: First the 8051 will increment the value of SP (from 07h to 08h) and then will store the pushed value at that memory address (08h).

SP is modified directly by the 8051 by six instructions: PUSH, POP, ACALL, LCALL, RET, and RETI. It is also used intrinsically whenever an interrupt is triggered (more on interrupts later. Dont worry about them for now!).

Instruction Set and Timing
Special Function Registers

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