Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 64)

50 Assembly Language Programming for the 68000 Family

tion, STOP, but if this instruction is used to terminate your program, the

microprocessor will literally stop and you will have to reboot (start from

scratch) your operating system. It is much better to return control such

that you can continue to issue system commands.

So that you can actually start to write programs using the input/output

procedures presented above, these must be included along with your

program and the shell. If you require one or more of these subroutines

in your program, just copy the appropriate source statements. You will

have to consult your system documentation to adapt these procedures for

other operating systems and/or assemblers.

Looping

With the instructions you have learned so far, it is possible to write a

few simple programs that perform addition and subtraction of a limited

number of values. To form the sum of 20 numbers entered from your

keyboard would take 20 lines of assembler source code just to obtain

these values. An additional 20 ADD instructions would be required as

well. If you desire to add up a larger number of values, you would soon

tire of all the typing needed to produce the source program. Consider

also that each assembly language instruction will take one or more words

of memory space when it is translated into machine language. Often it is

important to write a small program as well as an efficient one.

The solution to this problem is the use of a program loop. There

are many ways to write a program loop for the 68000 microprocessor.

The simplest type of program loop is the infinite program loop. While it

may seem of no value to write an infinite loop, if there is a method for

escaping the loop, it is often useful. You might like a program that repeats

itself over and over again for an arbitrary number of input data values.

This would eliminate the need to reinvoke the program for each new

value. The methods of escaping from an infinite loop will be discussed

in Chapter 5. For now, let us see how we can write one.

Execution normally passes from one instruction to the next. This is

known as sequential execution. The 68000 provides a special instruction

to alter the normal sequential program flow. The jump instruction, JMP,

provides the ability to transfer control to any instruction that has a label.

The following is a simple infinite loop:

OVER:

JMP

OVER

1 2 ... 59 60 61 62 63 64 65 66 67 68 69 ... 255 256

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Motorola M68000 Betriebsanweisung Seite 64