Adding registers

Just as we can add the value stored in a register to an immediate value (a constant) and then save the result in another register, we can also add the values of two registers together - then save that result in a third register.

Adding two registers

The instruction for adding the values of two registers together is add (not addi, which we used for adding an immediate value).

The syntax for add is:

add <destination register>, <source register 1>, <source register 2>

This means “add the value in <source register 1> to the value in <source register 2>, and store the result in <destination register>”.

Example

If we wanted to:

• Load the value 22 into register x1
• Load the value 8 into register x2
• Add the values in x1 and x2 together, and store the result in x3

We would write:

.text
main:
    li x1, 22        # x1 = 22
    li x2, 8         # x2 = 8
    add x3, x1, x2   # x3 = x1 + x2

stop:
    li a7, 10        # 10 = exit syscall
    ecall            # Make syscall

Try it out in Creatorsim!

Now, when we run this code, the value in register x3 will be 30, or 1E in hex!

The += operation

In many programming languages, there is a shorthand way to add a value to a variable and store the result back in the same variable. This is often done using the += operator.

For example, in C:

int x1 = 10;
int x2 = 5;
x1 += x2;  // equivalent to x1 = x1 + x2;

We can effectively do the same in RISC-V assembly using the add instruction by using the same register for both the destination and one of the source registers.

For this C code, we can instead write:

    li x1, 10       # x1 = 10
    li x2, 5        # x2 = 5
    add x1, x1, x2  # x1 = x1 + x2

This will add the value in x2 to the value in x1, and store the result back in x1. After , x1 will contain the value 15.