Addressing Modes
Most instructions allocate six bits to specify an operand. Three bits select one of eight addressing modes, and three bits select one of the eight general registers. The use of three-bit groups made octal notation natural.
In the following sections, each item includes an example of how the operand would be written in assembly language for a hypothetical single-operand instruction with symbol OPR. Rn means one of the registers, written R0 through R7. (Rn) signifies the contents of that register.
Read more about this topic: PDP-11 Architecture
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