PCjs Machines

Home of the original IBM PC emulator for browsers.


x86 Opcodes: SALC (0xD6)


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.
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.