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 loop: 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 lblne: add r0, #3 @ Add 3 to r0 @ don't need to branch to lblexit here as it's the @ next command lblexit: @ Continue code....
With this you can then start making loops.
WHILE loop #
@ While r0 < 5 loop: cmp r0, #5 @ Compare r0 and 5 bge exit @ If greater than or equal, exit loop @ Do loop operations here b loop exit: @ continue with code
FOR loop #
@ For each r0 = 0 to 5 mov r0, #0 @ Mov 0 into r0 as first number loop: 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 exit: @ continue with code