signextended

Signextended refers to the process of enlarging the width of a signed integer by replicating its sign bit into all higher-order bits. In two's complement representation, this preserves the numeric value when promoting from a smaller width to a larger width. If the original value's most significant bit is 0, the new bits are filled with 0s; if it is 1, the new bits are filled with 1s.

Common cases include sign-extending an 8-bit value to 16, 32, or 64 bits, or sign-extending a 16-bit value to 32 or 64 bits. For example, the 8-bit value 0x7F sign-extends to 0x007F in 16 bits, while 0x80 sign-extends to 0xFF80 in 16 bits. A 32-bit value 0x80000000 sign-extends to 0xFFFFFFFF80000000 in 64-bit form.

Sign extension is the counterpart of zero extension, which fills new higher-order bits with zeros regardless of the sign. Sign extension is usually required when performing arithmetic across mixed-width operands, when promoting smaller integers to larger types, or when loading signed immediates from memory.

Hardware and software implementations provide mechanisms for sign extension. Some CPUs offer dedicated instructions for sign-extending smaller operands (for example, sign-extend byte or halfword instructions in certain ISAs; and move-with-sign-extension instructions in x86). Compilers perform sign extension implicitly during type promotion in high-level languages.

See also: sign extension, two's complement, sign bit, type promotion.