HCS12 Assembly Programming - The HCS12/9S12: An Introduction to Software and Hardware Interfacing - page 67

bra loop ; go back to the loop body

next …
The implementation of that statement “for i 5 n2 to n1 do S” can be modified from the
previous instruction sequence as follows:
n1 equ xx ; start index (a nonnegative number)

n2 equ yy ; end index (a positive number)

…

i ds.b 1

…

movb #n2,i ; initialize loop index i

loop ldaa i ; check loop index

cmpa #n1 ; “

blt next ; if all iterations have been performed, then exit

… ; perform the loop operation

… ; “

dec i ; decrement loop index

bra loop ; go back to the loop body

next …
Like the for loop, the while-loop construct also checks the condition at the start of the
loop, and its implementation is similar with the following exceptions:
• The condition to be checked may be an external event instead of a variable.
• Updating the condition may be done by an external event such as an interrupt or
the change of an input signal.
Assume that the CPU will keep performing a certain operation as long as the variable icount
is not zero and icount is decremented by the interrupt service routine (discussed in Chapter 6).
Then the following instruction sequence implements a loop using the while-loop construct:
N equ xx

…

icount ds.b 1

…

movb #N, icount

wloop ldaa #0

cmpa icount

beq next

… ; performed loop operation

…

bra wloop

next …
The “repeat S until C” looping construction is used more often to perform some operation a
certain number of times. The following instruction sequence will perform a certain operation
N times:
N equ xx ; define the constant N

…

…

ldy #N ; use Y to hold loop count

loop … ; perform operations

…

dbeq Y,loop ; is loop count decremented to 0 yet?