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DATA ORGANIZATION AND ADDRESSING CAPABILITIES
The addressing mode of an instruction can specify the value of an operand (an imme-
diate operand), a register that contains the operand (register direct addressing mode),
or how the effective address of an operand in memory is derived. An assembler syntax
has been defined for each addressing mode.
Figure 3-1 shows the general format of the single-effective-address instruction oper-
ation word. The effective address field specifies the addressing mode for an operand
that can use one of the numerous defined modes. The designation is composed of two
3-bit fields, the mode field and the register field. The value in the mode field selects a
mode or a set of modes. The register field specifies a register for the mode or a sub-
mode for modes that do not use registers.
15
14
13
X
X
X
Figure 3-1 Single-Effective-Address Instruction Operation Word
Many instructions imply the addressing mode for only one of the operands. The for-
mats of these instructions include appropriate fields for operands that use only a single
addressing mode.
Additional information may be needed to specify an operand address. This information
is contained in an additional word or words called the effective address extension, and
is considered part of an instruction. Address extension formats are discussed in 3.4.4
Effective Address Encoding Summary.
When an addressing mode uses a register, the register is specified by the register field
of the operation word. Other fields within the instruction specify whether the selected
register is an address or data register and how the register is to be used.
3.1 Program and Data References
An M68000 Family processor makes two classes of memory references, each of which
has a complete, separate logical address space.
References to opcodes and extension words are program space references.
Operand reads and writes are primarily data space references. Operand reads are
from data space in all but two cases — immediate operands embedded in the instruc-
tion stream and operands addressed relative to the current program counter are pro-
gram space references. All operand writes are to data space.
CPU32
DATA ORGANIZATION AND ADDRESSING CAPABILITIES
REFERENCE MANUAL
SECTION 3
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11
10
9
X
X
X
X
8
7
6
5
X
X
X
4
3
2
1
EFFECTIVE ADDRESS
MODE
REGISTER
MOTOROLA
0
3-1
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