Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 90)

76 Assembly Language Programming for the 68000 Family

The LEA instruction takes the address of COUNT, not the value at

location COUNT, and places it in register AO. The LEA instruction always

has a longword size, since it specifies an address value.

You might be wondering, why not just use absolute addressing? The

answer to this is best shown by example, but there are a couple of reasons.

Unlike in absolute addressing, the address can be modified if it is in a

of the address register indirect mode is that it avoids the necessity of

knowing the symbolic name for the location of a data value in order to

reference it. This will start to have a greater meaning when subroutines

are discussed in Chapter 8.

Address register indirect can be of great help in managing data in

arrays. You are no doubt somewhat familiar with the use of arrays from

your experience in a high-level language. An array is a type of data

structure created by grouping together many similar data elements of

the same type in successive locations in memory. For example, we can

create an array of word-sized data items with the DC.W directive.

ARRAY: DC.W 345,862,10000,-26,473

This example is an array composed of arbitrary values. Suppose we wanted

to print out these five values—how could it be done using address register

indirect addressing? First, we observe that the label ARRAY specifies the

first address in the array. If we place this address in register AO, we should

be able to access the first data element using address register indirect

addresssing. How do we then get to the next value? We merely add the

proper constant to AO. In this case it will be a 2, since word data requires two

consecutive memory bytes. The following is a program that does just this:

LEA

CLR.L

MOVE.W

JSR

ADDQ.L

DBRA

ARRAY,A0

#4 ,D1

(A0),D0

OUTDEC

NEWLINE

«2,A0

D1,NEXT

GET ADDRESS OP ARRAY

CLEARS HIGH ORDER WORD

INIT FOR LOOP

GET ARRAY ELEMENT

OUTPUT

GO TO NEXT LINE

BUMP FOR NEXT ELEMENT

BACK FOR MORE

Wouldn’t it be nice if we didn’t have to increment the value of A0

by 2 each time through the loop, if it could be done automatically? The

68000 has granted our wish. The next addressing mode we will discuss,

address register indirect with postincrement, does just that.

1 2 ... 85 86 87 88 89 90 91 92 93 94 95 ... 255 256

Kommentare zu diesen Handbüchern

Keine Kommentare

Motorola M68000 Betriebsanweisung Seite 90

Kommentare zu diesen Handbüchern

Verwandte Produkte und Handbücher für Nein Motorola M68000