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).