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
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
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
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
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
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
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-
in long precision 16-
or approximately 2.4 .