Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 62)

48 Assembly Language Programming for the 68000 Family

terminal. Normally, some degree of hardware independence is provided

by the operating system being used. But here again we will not all be

using the same operating system. Some readers may be using an Apple

Macintosh and others may be using an Atari ST or one of many other

operating systems that operate with 68000-based systems.

In order to start to write programs without worrying about the system-

dependent details, we will use a set of input/output subroutines whose

inner workings will be different for different operating systems. These

subroutines will assume a standard ASCII terminal or display. Since the

interface is through your operating system, the details of your particular

hardware are automatically taken care of. It doesn’t matter if you have a

video display or a printing terminal. Appendix B gives the actual source

statements for these subroutines written for the Atari ST, Commodore

Amiga, and Apple Macintosh operating systems. (While this does not

cover every operating system in use, the majority of readers will probably

be accommodated.)

It is possible to write many programs that involve inputting one

or more decimal numbers from the keyboard and outputting one or

more decimal numbers to the screen. It is also necessary to be able to

input and output ASCII characters. We will start by introducing some

useful subroutines to perform these tasks. A procedure, or subroutine,

is a portion of a program that can be referenced, or called, from many

different places within the program without the necessity of repeating

the instructions for this procedure each time it is used. Here are the

procedures; read their descriptions carefully so that you will understand

their use.

1. INDEC—Input an unsigned decimal number from the keyboard.

The number is entered as one or more decimal digits terminated

by a character other than 0-9. This terminating character may be

a carriage return (RETURN key on most keyboards). The number

must be representable by four bytes and must therefore be between

0 and 4,294,967,295. The number is placed in the DO register.

2. OUTDEC—Output an unsigned decimal number to the screen. The

number is taken from the longword in register DO. It is output

without a terminating carriage return and line feed (doesn’t advance

to the next line). The range of the output value is the same as for

INDEC.

3. NEWLINE—Terminate the present output line and output the car

riage return and line feed characters to advance to the start of the *

next line.

4. GETC—Input a single character from the keyboard. The ASCII

value of the character is returned in the lower eight bits of the register

DO. The high-order bits are cleared.

1 2 ... 57 58 59 60 61 62 63 64 65 66 67 ... 255 256

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