Excerpt from http://www.rcollins.org/secrets/opcodes/SALC.html:
An undocumented op code that performs an operation common to every Assembly language subroutine to C and many other higher level languages. This instruction is a C programmers 'dream' instruction for interfacing to assembly language. Undocumented: Available to all Intel x86 processors Useful in production source code. SALC Flags: SET Carry flag to AL +-+-+-+-+-+-+-+-+-+ +----------+ |O|D|I|T|S|Z|A|P|C| | 11010110 | +-+-+-+-+-+-+-+-+-+ +----------+ | | | | | | | | | | | D6 | +-+-+-+-+-+-+-+-+-+ +----------+ The name SALC simply stands for SET the Carry flag in AL. This instruction is categorized as an undocumented single-byte proprietary instruction. Intel claims it can be emulated as a NOP. Hardly a NOP, this instruction sets AL=FF if the Carry Flag is set (CF=1), or resets AL=00 if the Carry Flag is clear (CF=0). It can best be emulated as SBB AL,AL. SALC doesn't change any flags, where SBB AL,AL does. This instruction is most useful to high-level language programmers whose programs call assembly language, and expect AL to indicate success or failure. Since it is convenient for assembly language programs to return status in the CF, this instruction will convert that status to a form compatible with high level languages. Over the years, this instruction has been given many names by various discoverers. I originally gave it the name SETCAL, but the most common name I've seen in print is SETALC. The name given above, SALC is an official Intel name. While perusing the P6 opcode map, I always check for known, undocumented opcodes. After weeding through the map for many minutes, my patience and perseverance paid off. I found the opcode, and its name. Intel's name for this opcode is SALC. This would indicate that Intel plans to officially document this instruction, beginning with the P6.