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2.3.2 Addressing Modes
Addressing modes and effective address calculation are described in table 2.8.
Table 2.8 Addressing Modes and Effective Addresses
Addressing
Mode
Mnemonic
Expression Effective Addresses Calculation Equation
Direct
register
addressing
Rn The effective address is register Rn. (The operand
is the contents of register Rn.)
—
Indirect
register
addressing
@Rn The effective address is the contents of register Rn.
Rn Rn
Rn
Post-incre-
ment
indirect
register
addressing
@Rn + The effective address is the contents of register Rn.
A constant is added to the contents of Rn after the
instruction is executed. 1 is added for a byte
operation, 2 for a word operation, and 4 for a
longword operation.
Rn Rn
1/2/4
+
Rn + 1/2/4
Rn
(After the
instruction is
executed)
Byte: Rn + 1
→ Rn
Word: Rn + 2
→ Rn
Longword:
Rn + 4 → Rn
Pre-decre-
ment
indirect
register
addressing
@–Rn The effective address is the value obtained by
subtracting a constant from Rn. 1 is subtracted for
a byte operation, 2 for a word operation, and 4 for a
longword operation.
Rn
1/2/4
Rn – 1/2/4
–
Rn – 1/2/4
Byte: Rn – 1
→ Rn
Word: Rn – 2
→ Rn
Longword:
Rn – 4 → Rn
(Instruction
executed
with Rn after
calculation)
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