CLC         clear the carry flag to NC                             (CF 0)
CMC         complement (reverse the value of) the carry flag       (CF ~CF)
STC         set the carry flag to CY                               (CF 1)
CLD         clear direction flag to UP (string ops auto-increment) (DF 0)
STD         set direction flag to DN (string ops auto-decrement)   (DF 1)
CLI         disable maskable hardware interrupts                   (IF 0)
STI         enable maskable hardware interrupts                    (IF 1)

CTS/CLTS    clear task switch flag.                           (TF 1)   286+

HLT         halt processing (perform NOPs until an interrupt occurs)

WAIT/FWAIT  wait for TEST line active (synchronize with coprocessor)
LOCK        (prefix) prevent coprocessor bus access for the next instruction
prevents access only to mem affected by next opcode      386+

SEG segreg  (prefix) override default segreg for next EA calculation

                                                                            

Processor Control Instructions