ters. Their addresses are specified by
octal digits 3 to 5 of the instruction word. These registers are memory
locations 001-777 (octal). Address 000 specifies that no index register
is to be used with the instructions. The contents of octal digits 7
through 11 of the selected index register are added to the unmodified
address (octal digits 7 through 11 of the instruction).

This sum is used to locate the operand. The addition is done in the
Index Adder which is a 15 bit 1's complement adder. The contents of the
Accumulator and the In-Out Register are unaffected by the indexing
process. An instruction which has used indexing is retained in memory
with its original address unmodified. Memory registers 1-777 (octal) are
available for use as normal memory registers if they are not being used
as index registers. The left half of these registers is available for
the storage of constants, tables, etc., when octal digits 7 through 11
act as index registers.

Three special instructions snx, spx and lir, are available to facilitate
resetting, advancing, and sampling of the index registers. Since the
index registers are normal memory registers, their contents can also be
manipulated by the standard computer instructions.

INDIRECT ADDRESSING

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.

INSTRUCTION LIST

This list includes the title of the instruction,