Motorola Axiom CMD11E1 Guide de l'utilisateur télécharger pdf (Page 34)

34 CHAPTER 3. FUNCTIONS AND BIT MANIPULATIONS

the registers the way they were before the functions used them

Here are the basic rules for writing functions

1. Decide on its functionality. Don’t try to create a Swiss army knife that has

multiple functionalities built in. Your function must do only one thing, and

it must do it well. Your documentation for the function must clearly state

the functionality

2. Decide on its name. Pick a meaningful name but keep the name to 8 char-

acters or less

3. Decide on the registers that will be used to pass information to the function.

8-bit values can be sent using A or B registers. 16-bit values can be sent

using X or Y registers

4. Decide what registers will be used to return values back to the caller. 8-bit

values can be returned using A or B registers. 16-bit values can be returned

using X or Y registers.

5. Decide what registers the function will use and which of these will be restored

back at the end. Clearly document which registers will be used and not

restored back as the registers that are modiﬁed by the function.

6. Write the function. Avoid the temptation of writing the function ﬁrst and

then worrying about the other items!

As an example, we will write a simple function called RAND that will return a

random value every time it is called. How does this function work? The function

starts with a seed. We use an 8-bit number as a seed. The seed is used to calculate

a random number using some formula. To make sure we get a diﬀerent number

every time the function is called, the seed value is changed. Typically, the random

number that is generated is used as the seed for the next random number. Thus,

we are actually generating a random sequence starting with some initial seed. The

formula

it uses is simple: It shifts the seed value left, and adds with carry the

value 20. The function needs one byte of storage to keep track of the seed. This

storage will be allocated in the data section using the FCB directive as

SEED FCB 0

The only exception is the register that is used to return a value to the user. Clearly, these

registers should not be restored to their original value!

For example, OUTA expects the data in theA register, while OUT1BYT expects the information

in the X register.

I use this as a quick and dirty 8 bit random number generator. It is not the best, but the

code is only 5 line long!

1 2 ... 29 30 31 32 33 34 35 36 37 38 39 ... 100 101

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Motorola Axiom CMD11E1 Guide de l'utilisateur Page 34