Memory and addresses

In this section, we will write a program that subtracts the value of one register from another and stores the result in register 0.

In order to do this, we will create a number of variables with integers that are then assigned to the register. This will introduce you to the concepts of memory and addresses. During this process you will also understand a few new terms that are important to the Assembly language.

We will use the following program to explore these items:

.data
/* Variable definition */
.balign 4
wordvar1:
    .word 7
.balign 4
wordvar2:
    .word 3

.text
/* code definition */
.balign 4
.global main
main:
    LDR R1, wordvar1addr
    LDR R1, [R1]
    LDR R2, wordvar2addr
    LDR R2, [R2]
    SUB R0, R1, R2 @ Subtract 3 from 7
    BX LR

wordvar1addr : .word wordvar1
wordvar2addr : .word wordvar2

Add this program to a new file called third_assem_prog.s and save it.

This should be stored with the other Assembly programs you created in the assem_programs directory...