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For unsigned integer types other than
If there are
595 this shall be known as the value representation.
596 The values of any padding bits are unspecified.44)
597 For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit.
598 There need not be any padding bits;
599 there shall be exactly one sign bit.
600 Each bit that is a value bit shall have the same value as the same bit in the object representation of the corresponding unsigned type (if there are M value bits in the signed type and N in the unsigned type, then M ≤ N).
42)Thus, for example, structure assignment
43) It is possible for objects
In particular, if
other operations on values of type
606 44) Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit.
607 Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow, and this cannot occur with unsigned types.
608 All other combinations of padding bits are alternative object representations of the value specified by the value bits.
609 If the sign bit is zero, it shall not affect the resulting value.
610 If the sign bit is one, the value shall be modified in one of the following ways:
611 the corresponding value with sign bit 0 is negated (sign and magnitude);
612 the sign bit has the value -(2N) (two's complement);
the sign bit has the value
614 Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for one's complement), is a trap representation or a normal value.
615 In the case of sign and magnitude and one's complement, if this representation is a normal value it is called a negative zero.
616 If the implementation supports negative zeros, they shall be generated only by:
619 compound assignment operators based on the above cases.
620 It is unspecified whether these cases actually generate a negative zero or a normal zero, and whether a negative zero becomes a normal zero when stored in an object.
If the implementation does not support negative zeros, the behavior
622 The values of any padding bits are unspecified.45)
623 A valid (non-trap) object representation of a signed integer type where the sign bit is zero is a valid object representation of the corresponding unsigned type, and shall represent the same value.
625 The precision of an integer type is the number of bits it uses to represent values, excluding any sign and padding bits.
626 The width of an integer type is the same but including any sign bit;
627 thus for unsigned integer types the two values are the same, while for signed integer types the width is one greater than the precision.
628 45) Some combinations of padding bits might generate trap representations, for example, if one padding bit is a parity bit.
629 Regardless, no arithmetic operation on valid values can generate a trap representation other than as part of an exceptional condition such as an overflow.
630 All other combinations of padding bits are alternative object representations of the value specified by the value bits.
Created at: 2008-01-30 02:39:40
The text from WG14/N1256 is copyright © ISO
Created at: 2008-01-30 02:39:40 The text from WG14/N1256 is copyright © ISO