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168 Assembly Language Programming for the 68000 Family

the main program. Many textbooks on systems programming or operating

system techniques describe these techniques.

There is one special instruction that has been provided just for inter

facing with 8-bit peripheral devices. This is the MOVEP (move peripheral

data) instruction. Its general forms are:

MOVEP[.<size>] Dx,d(Ay)

MOVEP[,<size>] d(Ay),Dx

<size> = W, L

Note that the only forms of the addressing modes it allows are register

and register indirect with displacement. This instruction moves bytes of

data to or from alternate bytes of a memory address. A longword transfer

to/from an even address looks like this:

13 24 23 1G IS 8 7 0

hi-order

mid-upper

mid-lower

low-order

Contents of Memory

8 7

hi-order

mid-upper

mid-lower

low-order

A word transfer to/from an even address looks like the illustration at the

top of the following page.

Miscellaneous Instructions

In this section we will discuss a few miscellaneous instructions not

covered in previous chapters.

The NOP instruction is an instruction that does absolutely nothing. It

stands for No OPeration. It takes a minimum amount of time to execute,

since there are no operands. It occupies one word of memory. It can be

used as a place holder. For example, if you are using a debugger and

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Motorola M68000 Betriebsanweisung Seite 182