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Memory Addressing Modes
These addressing modes specify that the operand is in memory and is to be
referenced relative to one of the machine's address registers.
The operand is the word in memory whose address equals the value currently
stored in the address register specified in the register sub-field. This mode
is selected when the mode sub-field is 2.
The operand is the word in memory whose address equals the value currently
stored in the address register specified in the register sub-field.
After the operand is used, the value in the address register specified
is incremented by one. This mode is selected when the mode sub-field
is 3.
The value stored in the address register specified by the register
sub-field is first decremented by one. The operand is the word
in memory whose address equals the decremented value stored in the
address register specified. This mode is selected when the mode sub-field
is 4.
This addressing mode requires one extension word. The operand is the
word in memory whose address is the sum of the value in the extension
word and the value in the address register specificed in the register
sub-field of the effective address specification. In computing the address,
the value in the address register is treated as a 16-bit unsigned integer
while the value in the extension word is treated as a signed integer
represented using 2's complement notation. This mode is selected when the
mode sub-field is 5.