Motorola M68000 Uživatelská příručka stáhnout pdf (Strana 85)

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ADDRESSING MODES

The majority of 68000 instructions have one or more operands. An

operand is used either as a source operand or a destination operand. A

source operand is always a data value that is only read by the instruction

and never modified. A destination operand may be read as well as

written. In other words, its value can be modified by the action of the

instruction.

The programmer normally explicitly specifies the operand or

operands of an instruction. A small number of instructions have implicit

operands. This means that a register is used as an operand without be

ing so specified. The JSR instruction is an example of this. As you will

see in the next two chapters, address register A7 is implicitly used by this

instruction.

Operands always ultimately specify a register or location in memory

where the data for the instruction is found. It is quite clear in

MOVE.L D0,D1

that the source operand is to be found in data register DO and the

destination operand is in data register Dl. If we write

MOVE.L (A0)+,D0

what does (AO) +mean? In order to answer this and many other questions

concerning operands, it is necessary to introduce the concept of an

addressing mode. The addressing mode is the method we specify for

the instruction to find a particular operand. The effective address of an

instruction is the actual location of the data. When a simple data register

is specified, the effective address is the data register itself. When an

instruction such as

ADD.L COUNT,DO

is specified, the effective address of the source operand is the location in

memory that contains the variable COUNT.

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Motorola M68000 Uživatelská příručka Strana 85