Editing Orthogonal instruction set (section)

===The PDP-11===
The PDP-11 was substantially orthogonal (primarily excepting its floating point instructions).<ref name=psych>{{cite web |url=http://www.psych.usyd.edu.au/pdp-11/intro.html |title= Introduction to the PDP-11 |website=University of Sydney}}</ref> Most integer instructions could operate on either 1-byte or 2-byte values and could access data stored in registers, stored as part of the instruction, stored in memory, or stored in memory and pointed to by addresses in registers or memory. Even the [[Program counter|PC]] and the [[stack (data structure)|stack]] [[pointer (computer programming)|pointer]] could be affected by the ordinary instructions using all of the ordinary data modes. "Immediate" mode (hardcoded numbers within an instruction, such as ADD #4, R1 (R1 = R1 + 4) was implemented as the mode "register indirect, autoincrement" and specifying the program counter (R7) as the register to use reference for indirection and to autoincrement. (Encoded as ADD (R7)+,R1 .word 4.)<ref name=tor>{{cite web |url=https://www.teach.cs.toronto.edu//~ajr/258/pdp11.pdf |title=PDP-11 instruction reference |website=University of Toronto}}</ref>

The PDP-11 used 3-bit fields for addressing modes (0-7) so there were (electronically) 8 addressing modes. An additional 3-bit field specified the registers (R0–R5, SP, PC). Immediate and absolute address operands applying the two autoincrement modes to the Program Counter (R7), provided a total of 10 conceptual addressing modes.  Most two operand instructions supported all addressing modes for both parameters.<ref name=tor/>