Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 93)

Addressing Modes 79

the second string is the same length as the first. If it is not, then the first

string would be a substring of the second and the two are therefore not

identical. If the second string were a substring of the first, the CMPM.B

instruction would fail. I will leave it as an exercise to rewrite this program

to allow matching substrings.

Address Register Indirect With Predecrem ent

This addressing mode is very similar to address register indirect with

postincrement. Its general form is — (An). Notice that the minus sign must

precede the register number. There are two major differences. First, the

value in the specified address register is decremented by the data size

rather than incremented. The other difference is in when this decrementing

takes place. With address register indirect with postincrement, the address

address register indirect with predecrement, the decrementing takes place

before the effective address is computed.

At first, you might be tempted to think of this addressing mode as a

way to access memory in decreasing address order. You might want to

move a chunk of memory starting with the highest memory location. This

can be done with some care. You have to remember that predecrementing

will occur. This means that the initial address value you place in the

address register will not be the address of the first piece of data. It will

be off by data-size bytes. You can certainly take this into consideration by

adjusting the value in the address register. This can be done in two ways.

You can generate a label that is on the next higher memory location:

DS.W 100 WORD ARRAY

BLOCK s EQU *

The asterisk is used to obtain the current value of the assembler’s location

counter. In this case, its value will be equal to the memory location

immediately following the last word of the data. You would then be

able to access this array in reverse order with the following instructions:

LEA BLOCK,AO

MOVE.W -(AO),DO GETS DATA ELEMENT INTO DO

The other method would involve placing a pointer to the last data element

in the address register and then adjusting it before use. Here is how you

would do it:

1 2 ... 88 89 90 91 92 93 94 95 96 97 98 ... 255 256

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Motorola M68000 Uživatelská příručka Strana 93