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Special Addressing Modes
When one of the special addressing modes described below is used, the
mode sub-field of the effective address specification is set to 7 and
the register sub-field is used to specify exactly which of the special
addressing modes is to be used. Each of the special addressing modes
requires one extension word.
The operand is the word in memory whose address equals the value
stored in this extension word. The value in the extension word is
interpreted as a 16-bit unsigned integer. This mode is selected when
the value in the mode sub-field is 7 and the value in the register
sub-field is 0.
The operand is the word in memory whose address equals the sum
of the value in the extension word and the value currently in the
program counter register.(1) In computing
the address, the value in the program counter is treated as a 16-bit
unsigned integer, while the value in the extension word is treated as
a 16-bit signed integer represented using 2's complement notation.
This mode is selected when the mode sub-field is 7 and the value in the
register sub-field is 2.
The operand is the extension word. This mode is selected when the
mode sub-field is 7 and the value in the register sub-field is 4.