Normally, the machine would require 36 bits or 12 octal digits to be written for each instruction used in the machine.
Memory registers 1-777 (octal) are available for use as normal memory registers if they are not being used as index registers.
This instruction will rotate the bits of the Accumulator right N positions, where N is octal digits 7-11 of the instructions word.
The contents of octal digits 0-5 of the index register location are unaffected by this instruction.
Base 8. A number representation using the digits 0-7 only, with the right-most digit counting ones, the next counting multiples of 8, then 8^2 = 64, etc. For example, octal 177 is digital 127:
digit weight value 1 8^2 = 64 1* 64 = 64 7 8^1 = 8 7* 8 = 56 7 8^0 = 1 7* 1 = 7 --- 127
Octal system used to be widespread back when many computers used 6-bit bytes, as a 6-bit byte can be conveniently written as a two-digit octal number. Since nowadays a byte is almost always 8-bit long the octal system lost most of its appeal to the hexadecimal system.
For a brief discussion on the word `octal' see hexadecimal.