Motorola M68000 User's Guide download pdf (Page 89)

Addressing Modes 75

this memory address at program execution time, we could reference

data using this address value rather than a symbol. This is the essence

of address register indirect addressing.

To use this addressing mode, an address must be placed into one of

the address registers. An address used in this manner is often called a

pointer. If we assume that the address of our data is in register AO, we

can reference the data by specifying the operand as (AO). In other words,

all we do is put parenthesis around the register designator. The general

form of this addressing mode is (An). Let’s assume that a word of data

is at location $10000. If we place the value $10000 into register A0, we

can reference this data. The following instructions will move this data to

MOVEA.L #$10000,A0

MOVE.W (A0),DO

In this case we used the MOVEA.L instruction to place the value $10000

into register A0. Note that we used a MOVEA.L and not a MOVEA.W for

this instruction. Even though we are going to obtain a word of data, an

address is always a 32-bit longword value.

It is rare that we know the actual memory address for a particular

piece of data. How, then, can we find the address of a location repre

sented only by a symbolic name? This is actually quite simple. There

are two methods we can use. The first method simply moves the ad

dress value in as an immediate value. The following program excerpt

will move the address of COUNT into A0 and then move the data value

at COUNT into register DO.

MOVEA.L #COUNT,AO

MOVE.W (A0),DO

•

♦

COUNT: DC.W 1234

The second method is to use a special instruction specifically designed

for this purpose, the load effective address instruction, LEA. Its general

form is:

LEA <ea>,An

It has the advantage that it determines the effective address at execution

time and can therefore be used with a number of different addressing

modes. The above instructions would be rewritten as

LEA COUNT,A0

MOVE.W (A0),DO

1 2 ... 84 85 86 87 88 89 90 91 92 93 94 ... 255 256

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Motorola M68000 User's Guide Page 89