In computing, signedness is a property of data types representing numbers in computer programs. A numeric variable is signed if it can represent both positive and negative numbers, and unsigned if it can only represent non-negative numbers (zero or positive numbers).
As signed numbers can represent negative numbers, they lose a range of positive numbers that can only be represented with unsigned numbers of the same size (in bits) because roughly half the possible values are non-positive values, whereas the respective unsigned type can dedicate all the possible values to the positive number range.
For example, a two's complement signed 16-bit integer can hold the values −32768 to 32767 inclusively, while an unsigned 16 bit integer can hold the values 0 to 65535. For this sign representation method, the leftmost bit (most significant bit) denotes whether the value is negative (0 for positive or zero, 1 for negative).
In programming languages
For most architectures, there is no signed–unsigned type distinction in the machine language. Nevertheless, arithmetic instructions usually set different CPU flags such as the carry flag for unsigned arithmetic and the overflow flag for signed. Those values can be taken into account by subsequent branch or arithmetic commands.
The C programming language, along with its derivatives, implements a signedness for all integer data types, as well as for "character". For Integers, the unsigned modifier defines the type to be unsigned. The default integer signedness outside bit-fields is signed, but can be set explicitly with signed modifier. By contrast, the C standard declares signed char, unsigned char, and char, to be three distinct types, but specifies that all three must have the same size and alignment. Further, char must have the same numeric range as either signed char or unsigned char, but the choice of which depends on the platform. Integer literals can be made unsigned with U suffix.
Compilers often issue a warning when comparisons are made between signed and unsigned numbers or when one is cast to the other. These are potentially dangerous operations as the ranges of the signed and unsigned types are different.
| Bits | Min | Max |
|---|---|---|
| 8 (signed) | −128 | 127 |
| 16 (signed) | −32768 | 32767 |
| 32 (signed) | −2147483648 | 2147483647 |
| 64 (signed) | −9223372036854775808 | 9223372036854775807 |
| 128 (signed) | −170141183460469231731687303715884105728 | 170141183460469231731687303715884105727 |
| 8 (unsigned) | 0 | 255 |
| 16 (unsigned) | 0 | 65535 |
| 32 (unsigned) | 0 | 4294967295 |
| 64 (unsigned) | 0 | 18446744073709551615 |
| 128 (unsigned) | 0 | 340282366920938463463374607431768211455 |
See also
- Sign bit
- Signed number representations
- Sign (mathematics)
- Binary Angular Measurement System, an example of semantics where signedness does not matter
External links
- "Numeric Type Overview". MySQL 5.0 Reference Manual. mysql.com. 2011. Retrieved 6 January 2012.
- "Understand integer conversion rules", CERT C Coding Standard, Computer emergency response team, retrieved December 31, 2015
In computing signedness is a property of data types representing numbers in computer programs A numeric variable is signed if it can represent both positive and negative numbers and unsigned if it can only represent non negative numbers zero or positive numbers As signed numbers can represent negative numbers they lose a range of positive numbers that can only be represented with unsigned numbers of the same size in bits because roughly half the possible values are non positive values whereas the respective unsigned type can dedicate all the possible values to the positive number range For example a two s complement signed 16 bit integer can hold the values 32768 to 32767 inclusively while an unsigned 16 bit integer can hold the values 0 to 65535 For this sign representation method the leftmost bit most significant bit denotes whether the value is negative 0 for positive or zero 1 for negative In programming languagesFor most architectures there is no signed unsigned type distinction in the machine language Nevertheless arithmetic instructions usually set different CPU flags such as the carry flag for unsigned arithmetic and the overflow flag for signed Those values can be taken into account by subsequent branch or arithmetic commands The C programming language along with its derivatives implements a signedness for all integer data types as well as for character For Integers the unsigned modifier defines the type to be unsigned The default integer signedness outside bit fields is signed but can be set explicitly with signed modifier By contrast the C standard declares signed char unsigned char and char to be three distinct types but specifies that all three must have the same size and alignment Further char must have the same numeric range as either signed char or unsigned char but the choice of which depends on the platform Integer literals can be made unsigned with U suffix Compilers often issue a warning when comparisons are made between signed and unsigned numbers or when one is cast to the other These are potentially dangerous operations as the ranges of the signed and unsigned types are different Data types Bits Min Max8 signed 128 12716 signed 32768 3276732 signed 2147483648 214748364764 signed 9223372036854775808 9223372036854775807128 signed 170141183460469231731687303715884105728 1701411834604692317316873037158841057278 unsigned 0 25516 unsigned 0 6553532 unsigned 0 429496729564 unsigned 0 18446744073709551615128 unsigned 0 340282366920938463463374607431768211455See alsoSign bit Signed number representations Sign mathematics Binary Angular Measurement System an example of semantics where signedness does not matterExternal links Numeric Type Overview MySQL 5 0 Reference Manual mysql com 2011 Retrieved 6 January 2012 Understand integer conversion rules CERT C Coding Standard Computer emergency response team retrieved December 31 2015
