Motorola Axiom CMD11E1 Podręcznik Użytkownika Pobierz pdf (Strona 17)

2.3. BRANCHING 17

Exercise:

1. Using the MM command (memory modify) enter the following values in

memory locations D000, D001, · · ·: 30 31 32 33 41 42 43 44. Verify the

values using the memory dump, MD, command.

2. Using RM, the register modify command, change the value in the X register

to D000

3. Execute the command CALL FFBB and write down what the output was and

also the value in the X register after the command is executed.

4. Repeat the the command CALL FFBB and write down the output and the

new value in the X register.

5. Repeat the last part until you have performed 7 calls to $FFBB.

2.3 Branching

Conditional branching in HC11 is controlled by the state of one or more hardware

ﬂags. The state of a ﬂag depends on the most recently executed instruction that

aﬀects the ﬂag. Thus, if your branching depends on the result of some instruction,

then it is your responsibility to make sure that none of the instructions between

the instruction you are interested in and the branching instruction aﬀects the

ﬂags that control the branching instruction. Thus, it is a good idea to follow the

instruction that sets the ﬂag by the branching instruction. In this lab, we will use

the following conditional branch instructions:

BEQ Branch if the Z ﬂag is set

BNE Branch if the Z ﬂag is not set

Now, the Z ﬂag is set after most instructions if the result of the instruction is a

zero; or else it is cleared, i.e. not set. The two most important instructions that

are often used to set/clear the ﬂag are

CMP Perform a subtraction and discard the answer. However, set the ﬂags.

TST Same as CMP except subtract the number zero

Thus, after the CMP command, the Z ﬂag would be set if the two values that are

compared are equal. Similarly, the TST command will compare a value (memory

or register) with zero and set the Z ﬂag if the value is zero.

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Motorola Axiom CMD11E1 Podręcznik Użytkownika Strona 17