Siemens SIMATIC TIWAY 1 Manual de usuario descargar pdf (Pagina 127)

Floating Point Numbers

B-5

SIMATIC TIWAY I Series 500 NIM User Manual

B.3 IEEE Format

The exponent portion of the IEEE format is similar to the IBM format

except that it is 8 bits in length and is biased by 7F. It is also different in

that it represents a power of 2 instead of 16. The exponent for 2 is

represented in memory by 7F. Negative exponents are represented by

numbers less than 7F. For example, a 2– is represented in the

exponent field by by a value of 7E. The exponents may be any value from

–7E to +7F (2– to 2). The eight exponent bits are stored in

bits 1 – 8 of the first memory word. See Figure B-2.

The fraction portion of the IEEE format is similar to the IBM format. It is a

hexadecimal fraction which is interpreted in one of many ways, depending

on the value of the exponent. Table B-1 describes the different types of

conversions for the IEEE format.

Table B-1 IEEE Conversions























 

































Case A is an illegal value. A Not-a-Number (NaN) will never be returned

from the Series 500 NIM. If this example is written to the NIM, an

exception will be returned.

Case B, +/– infinity, is used to represent numbers too large in magnitude to

be represented with the IEEE format.

Case C represents a normalized IEEE floating point value. (There is a one,

not included in f, to the left of the radix point.)

Case D represents a denormalized IEEE floating point (contains a zero to

the left of the radix point).

Case E represents a value of zero. Numbers too small to be represented in

the IEEE format will be reported as a positive or a negative 0.

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