Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 63)

Getting Started 49

5. PUTC—Output a single character to the screen. The character is

taken from the low-order eight bits of register DO.

In order to use these procedures within your program, a special in

struction, JSR (jump to subroutine) is provided in the 68000 instruction

set. The exact operation of this instruction, and of subroutines in gen

eral, will be discussed in Chapter 8. The mnemonic JSR is followed by

the symbol representing the subroutine’s name. For now, assume that

when you use the JSR instruction, the program performs the operations

specified by the subroutine that is called, and then continues on to the

next instruction. The following program excerpt will obtain two num

bers from your keyboard, add them together, and then output the result

to your screen:

JSR

INDEC

JSR

NEWLINE

MOVE.L D0,D1

JSR

INDEC

JSR

NEWLINE

ADD.L

D1,D0

JSR OUTDEC

JSR NEWLINE

Notice that the third instruction is used to save the contents of the DO

the first number to be added. Other than the DO register that is used with

the INDEC procedure, the input/output procedures given above do not

destroy the contents of any of the 68000 registers. The JSR to NEWLINE

ensures the advance to the beginning of a new line after each number

is input and after the result is output. A call to NEWLINE is required

even if you terminate the number you enter with a carriage return. A line

feed must be output to advance to the beginning of the “next” line. The

carriage return only positions you at the beginning of the “current” line.

The Program Shell

For the writing of a complete program, certain assembler directives

and standard code sequences are needed for each different assembler

and operating system. This program shell will enclose each program. So

that we don’t have to depend on one particular assembler or operating

system, we will not include this program code for each program pre

sented. Appendix B shows an appropriate program shell for the Atari ST,

Commodore Amiga, and Apple Macintosh.

Note that the shell is terminated by an appropriate mechanism to re

turn control to the operating system. The 68000 does have a halt instruc

1 2 ... 58 59 60 61 62 63 64 65 66 67 68 ... 255 256

Kommentare zu diesen Handbüchern

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 63

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Nein Motorola M68000