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Addressing Modes 73

least one word of memory. If we have an immediate operand, this

immediate constant data is placed in one or two successive words of

memory, right along with the instruction—a byte or word constant takes up

a word of memory, while a longword constant takes up the next two words.

The data may be a numeric constant, a symbol, or an expression. Here

are some examples of valid immediate addresses:

MOVE.B #$FF,BDATA

ADDI.L I1000/D5

MAX: EQU 200

SUBI.W #MAX*3,COUNT

Note that ADDI and SUBI were used instead of ADD and SUB.

Absolute Addressing

This is the third addressing mode that you have already used. In order

to understand the operation of this addressing mode, you will first have

to recall the structure of memory. Remember that each byte of memory

has an associated address or location. These addresses start at zero and

continue up to the maximum size of allowable memory. A byte, word, or

longword can be stored in memory. Words or longwords must be located

at even-numbered memory addresses. So, we can specify a particular

piece of data by its address in memory. There are actually two different

absolute addressing modes: absolute short and absolute long. The normal

one is absolute long. This allows a full 32-bit address to be specified.

When we want to directly access the contents of a location in memory,

we can use absolute addressing. The instruction

MOVE.B 5000rDO

moves the byte of data at location 500010 in memory into register DO. We

normally don’t use a numeric address, but a symbolic one.

MOVE.B DATA,DO

would be more common. Don’t get confused by the first example. This

not the same thing as immediate mode; without the # character it is taken

as absolute addressing. In the second example, the assembler substitutes

the correct value for the memory address of symbol DATA. For absolute

long addressing, the address is held in two words that follow the opcode

1 2 ... 82 83 84 85 86 87 88 89 90 91 92 ... 255 256

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Motorola M68000 Guida Utente Pagina 87