Branches and Loops

Branches and Loops #

Now that you can write conditional code, you need to know how to move around the code in a non-linear manner. Assembly only has one such concept and that is branch. Branch is just like GOTO in languages that support it.

    mov     r0, #2      @ Move 2 into register 0
    add     r0, #1      @ Add 1 to r0 
    b       loop        @ return to loop label 

As you can see above, when we branch we will branch to labels. This is another time where the compiler and linker will turn the label into a relative address while it’s being compiled. (You could write an offset to branch to, but it’s a lot less practical.) This first loop would be beneficial if you were writing something that never stopped, but this isn’t practical for much of what you’ll program.

IF statements #

The most fundamental conditional concept you may come across is the if statement. Because there is no if keyword, the code will look similar to this:

    mov         r0, #10     @ Move 10 into register 0 
    mov         r1 ,#7      @ Move 7 into register 1
    cmp         r0, r1      @ Compare r0 and r1 
    bne         lblne       @ If r0 != r1, branch to lblne 
    add         r1, #3      @ else add 3 to r1 
    b           lblexit     @ Go to end of code 
    add         r0, #3      @ Add 3 to r0 
                            @ don't need to branch to lblexit here as it's the 
                            @ next command 
    @ Continue code.... 

With this you can then start making loops.

WHILE loop #

@ While r0 < 5 

    cmp     r0, #5      @ Compare r0 and 5 
    bge     exit        @ If greater than or equal, exit loop 
    @ Do loop operations here 
    b       loop 
    @ continue with code 

FOR loop #

@ For each r0 = 0 to 5 
    mov     r0, #0      @ Mov 0 into r0 as first number 
    cmp     r0, #5      @ Compare r0 to 5 
    bgt     exit        @ Leave loop if r0 > 5 
    @   Do loop operations here 
    add     r0, #1      @ Increment counter 
    b       loop 
    @ continue with code