Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 86)

72 Assembly Language Programming for the 68000 Family

The 68000 has a total of 12 different addressing modes. This may

seem like a lot, but not all of these modes are commonly used, and some

are automatically selected by many assemblers to allow more efficient

execution of instructions. You have actually learned several of these

modes already.

At this point I should mention something about the limitations on the

use of these 12 addressing modes with all instructions. Life would indeed

be nice if we could use any of these 12 addressing modes as the source

and/or destination of each and every instruction. Computer scientists

refer to the orthogonality of a machine architecture as to the degree to

which all addressing modes can be used with all instructions. A fully

orthoganal architecture would allow all combinations. Unfortunately, the

68000 is not fully orthoganal, although it comes remarkably close. We do

have to be careful. Appendix C should be used as a reference as to what

is legal and what is not.

We will now go over the details of each of the addressing modes. I

will show some simple examples of how each mode is used. First, let’s

review the addressing modes that we have already covered in previous

chapters.

These are actually the simplest of the modes. The effective address is

the specified register itself. There are two register direct modes:

1. Dn—Data register direct

2. An—Address register direct

Any of the seven address or seven data registers can be used. Don’t forget

that some instructions will not allow an address register to be specified.

We must use a MOVEA instruction instead of MOVE, and an ADDA or

SUBA instead of ADD or SUB.

Im mediate Data

We have used immediate data addressing to place a constant in a

known at the time of assembly. The general form of this addressing mode

is #<data>, where <data>can be a byte, word, or longword value.

Where is this constant located? It is actually located right along with the

instructor’s opcode word. You may recall that each instruction consists of at

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Motorola M68000 Betriebsanweisung Seite 86