Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 66)

52 Assembly Language Programming for the 68000 Family

initializing Dn to —1 results in repeating the loop the maximum number

of times—65,536, to be exact. Only the low-order 16 bits of the register

are used as a counter. As a simple example, let’s say you want to output

20 blank lines. You could call the newline subroutine 20 times by writing

20 lines of assembler source, or you could write the following three lines:

MOVE.W #19#D2 INITIAL VALUE IS COUNT-1

NEXT: JSR NEWLINE

DBRA D2,NEXT

You must be careful not to modify the contents of the Dn register within

the program loop, since it then would no longer represent the loop count

and would not yield the result desired. If you must use the Dn register

within the loop, you must save and restore it. You could move the contents

to another register or to a memory location. Here’s how to do it with a

variable:

MOVE.W

#100,D1

NEXT: MOVE.W

•

D1,SAVED1

•

. <Use Dl>

MOVE.W

SAVED1,D1

DBRA D1 ,NEXT

SAVED1: DS.W 1

Recall from Chapter 3 that DS.W is not an instruction, but rather a

directive to reserve one or more words in memory. In this case one word

of uninitialized memory has been reserved at location SAVED1.

There is one restriction with the DBRA instruction that is not found

with the JMP instruction. With the JMP instruction the programmer can

transfer control to any distant label. In other words, there can be loops of

arbitrary size. Unfortunately, the DBRA instruction, and many others you

will discover, only allow transfer of control over a limited distance. This

distance is approximately plus-or-minus 32,768 bytes from the position of

the DBRA instruction itself. This distance can’t be represented as a fixed

number of instructions, because the number of bytes per instruction varies

with the particular instruction. It is a very rare occasion when a loop must

contain a greater number of instructions than this maximum. Fortunately,

the assembler tells the user if all the instructions do not fit into the loop.

In this case there are several methods to get around the problem. You

will have to read on to find out how.

1 2 ... 61 62 63 64 65 66 67 68 69 70 71 ... 255 256

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 66