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Introduction

Table 1-3. Integer Data Formats

Operand Data Format

Size

Notes

 

 

 

 

Bit

1

Bit

 

 

 

Bit Field

1 – 32 Bits

Field of Consecutive Bit

 

 

 

 

Binary-Coded Decimal

8 Bits

Packed: 2 Digits/Byte; Unpacked: 1 Digit/Byte

(BCD)

 

 

 

 

 

 

Byte Integer

8 Bits

 

 

 

 

Word Integer

16

Bits

 

 

 

 

Long-Word Integer

32

Bits

 

 

 

 

Quad-Word Integer

64

Bits

Any Two Data Registers

 

 

 

16-Byte

128 Bits

Memory Only, Aligned to 16Byte Boundary

 

 

 

 

1.5 FLOATING-POINT DATA FORMATS

The following paragraphs describe the FPU’s operand data formats. The FPU supports seven data formats. There are three signed binary integer formats (byte, word, and long word) that are identical to those supported by the integer unit. The FPU supports the use of the packed decimal real format. The MC68881 and MC68882 support this format in hardware and the processors starting with the MC68040 support it in software. The FPU also supports three binary floatingpoint formats (single, double, and extended precision) that fully comply with the IEEE 754 standard. All references in this manual to extendedprecision format imply the double-extended-precision format defined by the IEEE 754 standard.

1.5.1 Packed Decimal Real Format

Figure 1-11 illustrates the packed decimal real format which is three long words consisting of a 3-digit base 10 exponent and a 17-digit base 10 mantissa. The first two long words, digits 15 – 0, are 64 bits and map directly to bit positions 63 – 0 of the extended-precision real format. There are two separate sign bits, one for the exponent, the other for the mantissa. An extra exponent (EXP3) is defined for overflows that can occur when converting from the extended-precision real format to the packed decimal real format.

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M68000 FAMILY PROGRAMMER’S REFERENCE MANUAL

1-15

Introduction

.

 

 

 

 

 

 

 

 

 

SIGN OF MANTISSA

 

 

IMPLICIT DECIMAL POINT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SIGN OF EXPONENT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

USED ONLY FOR ± INFINITY OR NANS

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

96

 

 

 

 

 

 

 

 

 

 

 

 

 

65

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SM

SE

Y Y

 

 

EXP 0

EXP 1

EXP 0

(EXP 3)

XXXX

XXXX

DIGIT 16

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DIGIT 15

DIGIT 14

DIGIT 13

DIGIT 12

DIGIT 11

DIGIT 10

DIGIT 9

DIGIT 8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DIGIT 7

 

 

DIGIT 6

DIGIT 5

DIGIT 4

DIGIT 3

DIGIT 2

DIGIT 1

DIGIT 0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

32

 

 

 

 

 

 

 

 

 

 

 

 

0

 

 

NOTE: XXXX indicates “don't care", which is zero when written and ignored when read.

Figure 1-11. Packed Decimal Real Format

1.5.2 Binary Floating-Point Formats

Figure 1-12 illustrates the three binary floating-point data formats. The exponent in the three binary floating-point formats is an unsigned binary integer with an implied bias added to it. When subtracting the bias from the exponent’s value, the result represents a signed twos complement power of two. This yields the magnitude of a normalized floating-point number when multiplied by the mantissa. A program can execute a CMP instruction that compares floating-point numbers in memory using biased exponents, despite the absolute magnitude of the exponents.

.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

30

22

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

S

 

8-BIT

 

23-BIT

 

SINGLE REAL

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

EXPONENT

 

FRACTION

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

62

51

 

 

 

 

SIGN OF FRACTION

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

S

 

 

 

11-BIT

 

 

 

 

 

52-BIT

 

 

 

DOUBLE REAL

 

 

 

 

 

 

 

 

 

EXPONENT

 

 

 

 

FRACTION

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

94

 

 

 

 

 

 

 

 

 

SIGN OF FRACTION

 

 

 

 

 

 

 

 

 

 

 

 

80

63

 

 

 

 

 

 

 

 

 

 

 

0

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

S

 

 

15-BIT

ZERO

 

 

 

 

 

 

 

 

 

64-BIT

 

 

 

EXTENDED REAL

 

EXPONENT

 

 

 

 

 

 

 

 

 

MANTISSA

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

SIGN OF MANTISSA

 

 

 

 

 

EXPLICIT INTEGER PART BIT

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 1-12. Binary Floating-Point Data Formats

Data formats for singleand double-precision numbers differ slightly from those for extended-precision numbers in the representation of the mantissa. For all three precisions, a normalized mantissa is always in the range (1.0...2.0). The extended-precision data format represents the entire mantissa, including the explicit integer part bit. Singleand doubleprecision data formats represent only a fractional portion of the mantissa (the fraction) and always imply the integer part as one.

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