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Register Direct modes
These addressing modes specify that the operand is one of the machines
16 registers.
The operand is in the data register specified by the register sub-field
of the effective address specification. This mode is selected when the
mode sub-field contains the value 0.
The operand is in the address register specified by the register sub-field
of the effective address specification. This mode is selected when the
mode sub-field contains the value 1.