1.3.1 Integer Data Formats
The three integer data formats (byte, word, and long word) are the standard twos com-
plement data formats defined in the M68000 Family
architecture.
Whenever an integer is
used in a floating-point operation, the integer is automatically converted by the FPCP to
an extended precision floating-point number before being used. For example, to add an
integer constant of five to the number in floating-point data register 3 (FP3), the following
instruction can be used:
FADD.W #5,FP3
(The Freescale assembler syntax uses "#" to denote immediate addressing.)
The ability to effectively use integers in floating-point operations saves user memory since
an integer representation of a number, if representable, is usually smaller than the equiv-
alent floating-point representation.
1.3.2 Floating-Point Data Formats
The floating-point data formats, single precision (32-bits) and double precision (64-bits),
are implemented in the FPCP as defined by the IEEE standard. These data formats are the
main floating-point formats and should be used for most calculations involving real num-
bers. Table 1-1 lists the exponent and mantissa sizes for single, double, and extended
precision. The exponent is biased, and the mantissa is in sign and magnitude form. Since
single and double precision require normalized numbers, the most significant bit of the
mantissa is implied as a one and is not included, thus giving one extra bit of precision.
Table 1-1. Exponent and Mantissa Sizes
Data Exponent Mantissa
Format
Bits Bits
Single 8 23(+ 1)
Double 11 52( + 1 )
Extended 15 64
The extended precision data format is also in conformance with the IEEE standard, but the
standard does not specify this format to the bit level as it does for single and double
precision. The memory format for the FPCP consists of 96 bits (three long words). Only 80
bits are actually used; the other 16 bits are for future expandability and for long-word
alignment of floating-point data structures in memory. Extended format has a 15-bit ex-
ponent, a 64-bit mantissa, and a 1-bit mantissa sign.
Extended precision numbers are intended for use as temporary variables, intermediate
values, or where extra precision is needed. For example, a compiler might select extended
precision arithmetic for evaluation of the right side of an equation with mixed sized data
and then convert the answer to the data type on the left side of the equation. It is anticipated
that extended precision numbers will not be stored in large arrays due to the amount of
memory required by each value.
1.3.3 Packed Decimal String Real Data Format
The packed decimal data format allows packed BCD strings to be transferred to and from
the FPCP. The strings consist of a 3-digit base 10 exponent and a 17-digit base 10 mantissa.
MC68881/MC68882 USER'S MANUAL FREESCALE
1-11
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