Subtraction

Subtraction in RISC-V assembly is done using one of two instructions:

• sub for subtracting the values of two registers
• addi with a negative immediate value for subtracting an immediate value from a register

Subtracting using registers

Suppose we have two registers, x1 and x2, which contain the values 10 and 3 respectively, and we want to subtract the value in x2 from the value in x1, and store the result in x3.

In other words:

x1 = 10
x2 = 3
x3 = x1 - x2

We can do that using the sub instruction like this:

sub x3, x1, x2

Or, the full code:

.text
main:
    li x1, 10
    li x2, 3
    sub x3, x1, x2

stop:
    li a7, 10
    ecall

View in Creatorsim!

Once we run this code, the value in register x3 will be 7, or 07 in hex!

Subtracting an immediate value

Suppose we have a register, x1, which contains the value 10, and we want to subtract 3 from it and store the result in x2.

In other words:

x1 = 10
x2 = x1 - 3

But we can also subtract the number 3 by adding -3 to x1!

In Python, that would look like this:

x1 = 10
x2 = x1 + (-3) # (brackets optional)

We can do that in RISC-V assembly using the addi instruction with a negative immediate value:

addi x2, x1, -3

The full code being:

.text
main:
    li x1, 10
    addi x2, x1, -3

stop:
    li a7, 10
    ecall

View in Creatorsim!

As this code effectively does the same as the previous example, the value in register x2 will also be 7 when running the code!