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3.9.2
Floating Point Numbers
Floating point numbers are stored in one of two IEEE 754 standard formats that uses adjacent 16-bit
words: 32-bit single precision or 64-bit double precision.
The REAL data type represents single precision floating point numbers. The LREAL data type
represents double precision floating point numbers. REAL and LREAL variables are typically used to
store data from analog I/O devices, calculated values, and constants.
Types of Floating Point Variables
Data Type
Precision and Range
REAL
Limited to 6 or 7 significant digits, with a range of approximately
±1.401298x10
LREAL
Limited to 17 significant digits, with a range of approximately
±2.2250738585072020x10
Note: The programming software allows 32-bit and 64-bit arguments (DWORD, DINT, REAL, and
LREAL) to be placed in discrete memories such as %I, %M, and %R in the PACSystems target.
This is not allowed on Series 90-70 targets. (Note that any bit reference address that is
passed to a non-bit parameter must be byte-aligned. This is the same as the Series 90-70
CPU.)
Internal Format of REAL Numbers
Bits 17-32
32
Register use by a single floating point number is diagrammed below. For example, if the floating point
number occupies registers R5 and R6, R5 is the least significant register and R6 is the most
significant register.
Most Significant Register
32
Most Significant Bit
Internal Format of LREAL Numbers
Bits 49-64
11-bit exponent
1-bit sign (Bit 64)
GFK-2950C
through ±3.402823x10
-45
-308
17
16
8-bit exponent
1-bit sign (Bit 32)
Bits 17-32
17
Least Significant Bit
Bits 33-48
Bits 17-32
52-bit mantissa
February 2018
.
38
to ±1.7976931348623157x10
Bits 1-16
23-bit mantissa
Least Significant Register
Bits 1-16
16
Most Significant Bit
Bits 1-16
Chapter 3. Program Data
308
.
1
1
Least Significant Bit
1
53
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