78IV datasheet(43/523 Pages) NEC

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CHAPTER 2 CONSTRUCTS OF C LANGUAGE

User’s Manual U15556EJ1V0UM

(i)

Floating-point number (float type) specifications

•Format

The floating-point number format is shown below.

(Higher address)

(Lower address)

31 30

23 22

The numerical values in this format are as follows.

(Value of sign)

(Value of exponent)

(–1)

* (Value of mantissa) *2

s: Sign (1 bit)

0 for a positive number and 1 for a negative number.

e: Exponent (8 bits)

An exponent with a base of 2 is expressed as a 1-byte integer (expressed by two’s complement

in the case of a negative), and used after having a further bias of 7FH added.

These

relationships are shown in Table 2-4 below.

Table 2-4. Exponent Relationships

Exponent (Hexadecimal)

Value of Exponent

127

•

–1

•

–126

m: Mantissa (23 bits)

The mantissa is expressed as an absolute value, with bit positions 22 to 0 equivalent to the 1st to

23rd places of a binary number. Except for when the value of the floating point is 0, the value of

the exponent is always adjusted so that the mantissa is within the range of 1 to 2 (normalization).

The result is that the position of 1 (i.e. the value of 1) is always 1, and is thus represented by

omission in this format.

• Zero expression

When exponent = 0 and mantissa = 0, ±0 is expressed as follows.

(Value of sign)

(–1)

* 0

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