Editing Sign extender

[[File:Sign extender.svg|right|200px]]
[[File:Sign extender square.svg|right|150px]]
[[Sign extension]] is an operation in which the number of bits representing a specific value is increased while preserving the original sign and value. The exact method used to perform this operation depends on underlining signed number representation used on that machine. A '''Sign Extender''' or a '''Sign Extension Unit''' is a black box representation of such operation. The symbols on the right are usually used to represent such unit.

== Two's complement sign extension ==
In today's systems, the most common signed number representation is [[two's complement|2's complement]]. In two's complement, signed extension is done by simply repeating the most significant bit, the sign bit, into all the new bits we introduce. For example:
[[File:Sign extender 2s comp.svg|left]]

{| class="wikitable" style="float: right;"
|-
! colspan="4" | 8 to 32 bit extension
|-
! colspan="2" | Input || colspan="2" | Output
|-
! Base 10 || Base 2 || Base 10 || Base 2
|-
| 4 || 00000100 || 4 || 00000000000000000000000000000100
|-
| -4 || 11111100 || -4 || 11111111111111111111111111111100
|-
| 42 || 00101010 || 42 || 00000000000000000000000000101010
|-
| -42 || 11010110 || -42 || 11111111111111111111111111010110
|-
| -126 || 10000010 || -126 || 11111111111111111111111110000010
|-
| X || MNNNNNNN || X || MMMMMMMMMMMMMMMMMMMMMMMMMNNNNNNN
|}
{{clear}}
== Sign-magnitude sign extension ==
Similar to two's complement, in a [[sign-magnitude representation]], the most-significant bit is also used to indicate the sign. Sign extension on sign-magnitude numbers works the same as two's complement. The MSB is repeated across all the new bits that were added.

== C programming ==
In the sample [[C| code]] below. Assuming on this implementation <code>CHAR_BIT = 8</code> and <code>sizeof (int) = 4</code>. The assignment of c to i will 
undergo a [[sign extension]] - 24 extra bits will be added. In a [[two's complement]] sign representation those 24 bits will be identical to the 8th bit of c.
<source lang="C">
char c;
int i;

c = -42;
i = c;
</source>

[[Category:Digital circuits]]