An instruction which is to use an indirect address will have a ONE in
bit six of the instruction word. The original address, Y, of the
instruction will not be used to locate the operand of the instruction,
as is the normal case. Instead, it is used to locate a memory register
whose contents in octal digits 7 through 11 will be used as the address
of the original instruction. This new address is known as the indirect
address for the instruction and will be used to locate the operand. If
the memory register containing the indirect address also has a 1 in bit
six, the indirect addressing procedure is repeated again and a third
address is located. There is no limit to the number of times this
process can be repeated.
Index registers may be used in conjunction with indirect addressing. In
this case, the address after being modified by the selected index
register is used to locate the indirect address.
The indirect address can be acted on by an index register and deferred
again if desired. Each use of an index register or an indirect address
extends the operating time of the original instruction by 5
microseconds.