Adding immediates

Writing to registers is fun and all, but how about we do the thing that computers were invented to do: maths!

We’ll start off from our code before:

.text
main:
    li x5, 42  # Load the value 42 into register x5

stop:
    li a7, 10    # Load the exit system call number into register a7
    ecall         # Make the system call to exit the program

Adding an immediate value

As mentioned before, RISC-V has two types of instructions for many operations. For adding numbers, we have:

• add: for adding the values of two registers together
• addi: for adding an immediate value (a constant) to a register

We’ll be using the second one for now - addi.

The syntax for addi is:

addi <destination register>, <source register>, <immediate value>

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

So, if we wanted to add 5 to the value in register x5, and store the result in x4, we would write:

addi x4, x5, 5

Now, the value in x4 is 47 (because 42 + 5 = 47)!

Our full code

Going back to our example before, we can now write:

.text
main:
    li x5, 42        # Load the value 42 into register x5
    addi x4, x5, 5   # Add 5 to the value in x5, store result in x4

stop:
    li a7, 10        # Load the exit system call number into register a7
    ecall            # Make the system call to exit the program

Run on creatorsim

Now, when we run this code, the value in register x4 will be 47, or 2F in hex!

An expanded li

The li instruction isn’t really an instruction at all - it’s generally just expanded to a wrapper around an addi instruction that adds the immediate value we want to set it to, to the zero register (x0), which always contains 0.

So, to set the register r1 to 127, we can also write:

addi r1, x0, 127

This does exactly the same thing as:

li r1, 127