Motorola M68000 User Manual download pdf (Page 8)

MOTOROLA

M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL

2-1

SECTION 2

ADDRESSING CAPABILITIES

Most operations take asource operand and destination operand, compute them, and store

the result in the destination location. Single-operand operations take a destination operand,

compute it, and store the result in the destination location. External microprocessor

references to memory are either program references that refer to program space or data

references that refer to data space. They access either instruction words or operands (data

items) for an instruction. Program space is the section of memory that contains the program

instructions and any immediate data operands residing in the instruction stream. Data space

is the section of memory that contains the program data. Data items in the instruction stream

can be accessed with the program counter relative addressing modes; these accesses

classify as program references.

2.1 INSTRUCTION FORMAT

M68000 family instructions consist of at least one word; some have as many as 11 words.

Figure 2-1 illustrates the general composition of an instruction. The ﬁrst word of the

instruction, called the simple effective address operation word, speciﬁes the length of the

instruction, the effective addressing mode, and the operation to be performed. The

remaining words, called brief and full extension words, further specify the instruction and

operands. These words can be ﬂoating-point command words, conditional predicates,

immediate operands, extensions to the effective addressing mode speciﬁed in the simple

effective address operation word, branch displacements, bit number or bit ﬁeld

speciﬁcations, special register speciﬁcations, trap operands, pack/unpack constants, or

argument counts.

Figure 2-1. Instruction Word General Format

SINGLE EFFECTIVE ADDRESS OPERATION WORD

(ONE WORD, SPECIFIES OPERATION AND MODES)

SPECIAL OPERAND SPECIFIERS

(IF ANY, ONE OR TWO WORDS)

IMMEDIATE OPERAND OR SOURCE EFFECTIVE ADDRESS EXTENSION

(IF ANY, ONE TO SIX WORDS)

DESTINATION EFFECTIVE ADDRESS EXTENSION

(IF ANY, ONE TO SIX WORDS)

15 0

1 2 3 4 5 6 7 8 9 10 11 12 13 ... 91 92

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Motorola M68000 User Manual Page 8