CS

CS returns a FIXED BINARY(31) value that indicates if the old and current values in a compare and swap were equal.

p

Address of the old FIXED BINARY(31) value.

q

Address of the current FIXED BINARY(31) value.

x

The new FIXED BINARY(31) value.

CS compares the "current" and "old" values. If they are equal, the "new" value is copied over the "current", and a value of 0 is returned. If they are unequal, the "current" value is copied over the "old", and a value of 1 is returned.

  cs: proc( old_Addr, current_Addr, new )
   returns( fixed bin(31) byvalue )
   options( byvalue );

   dcl old_Addr     pointer;
   dcl current_Addr pointer;
   dcl new          fixed bin(31);

   dcl old          fixed bin(31) based(old_addr);
   dcl current      fixed bin(31) based(current_addr);

   if current = old then
     do;
       current = new;
       return( 0 );
     end;
   else
     do;
       old = current;
       return( 1 );
     end;
 end;

On z/OS, the CS built-in function implements the CS instruction. For a detailed description of this function, read the appendices in the Principles of Operations manual.

On Intel, the CDS built-in function uses the Intel cmpxchg4 instruction. The cmpxchg4 instruction takes the address of a "current" value, a "new" value and an "old" value. It returns the original "current" value and updates it with the "new" value only if it equaled the "old" value.

  cs: proc( old_Addr, current_Addr, new )
   returns( fixed bin(31) byvalue )
   options( byvalue );

   dcl old_Addr     pointer;
   dcl current_Addr pointer;
   dcl new          fixed bin(31);

   dcl old          fixed bin(31) based(old_addr);
   dcl current      fixed bin(31) based(current_addr);

   if cmpxchg4( current_Addr, new, old ) = old then
     do;
       return( 0 );
     end;
   else
     do;
       old = current;
       return( 1 );
     end;
 end;