Motorola Axiom CMD11E1 Podręcznik Użytkownika Pobierz pdf (Strona 18)

18 CHAPTER 2. INTRODUCTION TO LOOPING

Exercise:

1. Using the HC11 reference book, identify 5 instructions that do not aﬀect

the C ﬂag, but aﬀects some other ﬂag.

2. Using the HC11 reference book, can you identify any instruction that does

not aﬀect the V ﬂag, but aﬀects some other ﬂag?

3. Using the HC11 reference book, identify 5 instructions that always clears

the C ﬂag.

4. Using the HC11 reference book, identify an instruction that always sets the

C ﬂag.

2.4 Looping

2.4.1 Counting loops

This is by far the simplest and most used loop structure. In a counting loop, we

perform a speciﬁc operation a given number of times. A counter controls how

many times the loop is executed. The counter could be stored in memory or kept

in a register. If it is kept in a register, it is your responsibility to make sure that

the register is not inadvertently changed either by your code or by some third

party function that you call. If you decide to use a register, your best bet is to

use either the B register or Y register. The structure of the loop is as follows:

1: Initialize the counter to the number of times the loop is to be performed

2: Perform any other initializations

3: Test if the counter is zero. If so quit the loop

4: Perform the desired task

5: Perform any re-initializations

6: Decrement the counter

7: Go back to 3

8: Come here when you quit the loop

Note that there are two places where the code jumps to. One to (3) and the other

to (8). When writing the code in assembly language, we would need two labels.

In the code that follows, I have used the labels FOO and BAR.

1 2 ... 13 14 15 16 17 18 19 20 21 22 23 ... 100 101

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Motorola Axiom CMD11E1 Podręcznik Użytkownika Strona 18