IBM System/7 Functional Characteristics - All about the IBM 1130 ...

Numeric, alphabetic, special character, or logical information can be represented bythe bit values in a word, as determined by the combination of the bits in a word and theintention of the programmer who organizes the program and data in storage.Numeric Data Formats for Arithmetic OperationsNumeric computations in the System/7 are performed in binary arithmetic by the arithmeticinstructions. The value of bit 0 (the high-order bit) of a word specifies whether theword represents a positive or negative number as shown in Figure 2-4. This bit is calledthe sign bit. If the value of bit 0 is 0, the number represented in the word is positive;if the value of bit 0 is 1, the number represented in the word is negative.A value of0 herespecifiesa positivenumber here0I 1 1 1 1 1. 1 1 1 1 1 1 1 1Bit 0 1 15A value of1 here/specifiesa negativenumber here1 1 1 1 1 1 1 1 1 I 1 I 1 IBit 0 1 15Figure 2-4. Numeric data formatThe ranges of numeric values that can be represented in a 16-bit word are shown asfollows:16-bit wordBinaryDecimalBit 0 (sign)Bits 1-15 (numeric value)000000000000000000000+toto011111111111111132,7671000000000000000–32,768–toto1111111111111111–00001Positive numbers are always represented in true binary notation with a 0 sign bit.Negative numbers are represented in twos-complement form: the sign bit contains a 1,signifying the negative number. For example, the number 1111 1111 1111 1111 is shownabove as having an equivalent decimal value of -00001. (The left 0's are shown since16 binary bits can represent a five-digit decimal number.) This binary value is reallythe twos-complement of negative 1.