Motorola M68000 User's Guide download pdf (Page 57)

Getting Started 43

of subtle errors that are hard to debug. If you must perform a move

between two data elements of different lengths, there are techniques that

can be used. We will discuss them as we move along. The following is an

example of a MOVE instruction used to move the contents of the byte

at memory location ALPHA to the byte at memory location BETA.

HOVE.B ALPHA,BETA

A constant value can be moved into a register or into a memory

location. A constant as a source operand is specified by preceding it

with the special character #. This is known as an immediate operand.

The following instruction will move the constant 100 into register DO:

MOVE.L #100,D0

Although it makes no sense, it is possible to write a MOVE instruction

indicating that the contents of a register or a memory location are to be

stored into a constant. This is not permitted, but if you forget the order

of the source and destination operands it may come out this way.

MOVE.L DO ,#100

is not legal. Fortunately, your assembler will detect this error and let you

know.

If the byte or word form of an instruction is used with a data register

as the destination, only the lower byte or word of the register is changed.

All the high-order bits remain unchanged. This is important to remember,

since we may move a byte into a register and then subsequently use

the register as a longword. All those high-order bytes will most likely

be meaningless garbage and cause an erroneous value to be used. For

example, if register DO contains the value $12345678,

MOVE *B #$00,DO

would result in DO containing $12345600, and not $00000000. Further

along in the book I will discuss ways to handle this problem.

Quite often a programmer desires to swap the contents of two regis

ters. The 68000 provides a special instruction to perform this operation.

Before looking at this instruction, let’s see how to program a swap opera

tion using only the MOVE instruction. To swap the contents of registers

DO and Dl, a programmer may at first be tempted to write:

MOVE.L DO,D1

MOVE.L D1,D0

1 2 ... 52 53 54 55 56 57 58 59 60 61 62 ... 255 256

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Motorola M68000 User's Guide Page 57