s.

The layout of the instruction word is shown in Fig. 2.

The octal digits 0 and 1 define the instruction code, thus, there are 64
possible instruction codes, not all of which are used. The first bit of
octal digit 2 is the indirect address bit. If this bit is a ONE,
indirect addressing occurs.

The index address, X, is in octal digits 3, 4, and 5. These digits
address an index register for memory-type instructions. If these digits
are all ZERO, indexing will not take place. In main memory, 511 of the
registers can be used as automatic index registers.

The instruction base address, Y, is in octal digits 7 through 11. These
digits are sufficient to address 32,768 words of memory. Octal digit 6
is reserved for further memory expansion. Space is available in the
equipment frame for this expansion, should it prove desirable.

In those instructions which do not refer to memory, the memory address
digits, Y, and in some cases the index address digits also, are used to
specify the variations in any group of instructions. An example of this
is in the shift and rotate instructions in which the memory address
digits determine the number of shifts.

NUMBER SYSTEM

The PDP-3 is a "fixed" point machine using binary arithmetic. Negative
numbers are represented as the 1's complement of the positive numbers.
Bit 0 is the sign bit which is ZERO for positive numbers. Bits 1 to 35
are magnitude bits with bit 1 being the most significant and bit 35
being the least significant.

The actual position of the binary point may be arbitrarily assigned to
best suit the problem in hand. Two common conventions in the placement
of the binary point are:

1. The binary point is to the right of the least significant digit,
thus, numbers represent integers.

2. The binary point is to the right of the sign digit, thus the numbers
represent a fraction which lies between +-1.

The conversion of decimal numbers into the binary system for use by the
machine may be performed automatically by subroutines. Similarly the
output conversion of binary numbers into decimals is done by subroutine.
Operations for floating point numbers are handled by programming. The
utility program system provides for automatic insertion of the routines
required to perform floating point operations and number base conversion
(see Utility Programs).

INDEXING

In PDP-3, 511 registers of the main magnetic core memory are available
for use as automatic index regis