592 Part 6 Computer families Section 3 The IBM System/360-a series of planned machines which span a wide performance range
the fractions, as indicated in Fig. 3. The fraction of a floating-point number is expressed in 4-bit hexadecimal (base 16) digits. In the short format, the fraction has six hexadecimal digits; in the long format, the fraction has 14 hexadecimal digits. The short length is equivalent to seven decimal places of precision. The long length gives up to 17 decimal places of precision, thus eliminating most requirements for double-precision arithmetic.
The radix point of the fraction is assumed to be immediately to the left of the high-order fraction digit. To provide the proper magnitude for the floating-point number, the fraction is considered to be multiplied by a power of 16. The characteristic portion, bits 1 through 7 of both formats, is used to indicate this power. The characteristic is treated as an excess 64 number with a range from -64 through + 63, and permits representation of decimal numbers with magnitudes in the range of 10-78 to 1075.
Bit position 0 in either format is the fraction sign, S. The fraction of negative numbers is carried in true form.
Floating-point operations are performed with one operand from a register and another from either a register or storage. The result, placed in a register, is generally of the same length as the operands.
Operations for comparison, translation, editing, bit testing, and bit setting are provided for processing logical fields of fixed and variable lengths. Fixed-length logical operands, which consist of one, four, or eight bytes, are processed from the general registers.
Fig. 4. Extended binary-coded-decimal interchange code.