The purpose of the floating-point instruction set is to

perform calculations using operands with a wide range

of magnitude and yielding results scaled to preserve

precision.

A floating-point number consists of a signcd

by this number is the product of the fraction and the

number 16 raised to the power of the exponent. The

exponent is expressed in excess 64 binary notation; the

fraction

a radix point to the left of the high-order digit.

To avoid unnecessary storing and loading operations

for results and operands, four floating-point registers

are provided. The floating-point instruction set pro

vides for loading, adding, subtracting, comparing,

multiplying, dividing, and storing, as well as the sign

control of short or long operands.

erally provide faster processing and require less

operands provide greater accuracy of computation.

Operations may be either register to register or storage

to register. All floating-point instructions and registers

are part of the floating-point feature.

To preserve maximum precision, addition, subtrac

tion, multiplication, and division are performed with

normalized results. Addition and subtraction may also

be performed with unnormalized results. Normalized

and unnormalized operands may be used in any

The condition code is set as a result of all sign

Data Format

Floating-point data occupy a fixed-length format,

which may be either a fullword short format or a

double-word long format. Both formats may be used

in main storage and in the floating-point registers. The

four floating-point registers are numbered

and 6.

o I 78 31

The first bit in either format is the sign bit

subsequent seven bit positions are occupied by the

characteristic. The fraction field may have either six

or 14 hexadecimal digits.

The entire set of floating-point instructions is avail

able for both short and long operands. When short

precision is specified, all operands and results are 32-

bit floating-point words, and the rightmost 32 bits of

the floating-point registers do not participate in the

operations and remain unchanged. An exception is the

product in

all operands and results are 64-bit floating-point words.

Although final results in short-precision have six

fraction digits, intermediate results in addition,

digits. Thc low-order digit of a seven-digit fraction is

called the guard digit and serves to increase the

Number Representation

The fraction of a floating-point number is expressed in

hexadecimal digits. The radix point of the fraction is

assumed to be immediately to the left of the

for the floating-point number, the fraction is consid

ered to be multiplied by a power of 16. The character

istic portion, bits 1-7 of both floating-point formats,

excess 64 number with a range from -64 through +63,

coresponding to the binary values 0-127.

Both positive and negative quantities have a true

fraction, the difference in sign being indicated by the

sign bit. The number is positive or negative according

ly as the sign bit is zero or one.

The range covered by the magnitude (M) of a

normalized floating-point number is

in short preciSion 16-

65

6

)

63

, and

in long precision 16-

65

14

)

63

,

or approximately 2.4 .

78

75