Code designed to run in user mode and be interpreted by domains is called “domain code”. There are about 60 such programs in the current release. All of these are written in C but but most of then use a special preprocessor called “kcpp”. kcpp translates commands such as
KC (fen, Node_Fetch + FEN_Parm2) KEYSTO(k3)
{OC(Node_Fetch + FEN_Parm2);XB(0x00C00000);NB(0x0080A000);cjcc(0x00000000,&_jumpbuf); }
This command invokes the key in one of the 16 general key slots. “fen” identifies the key slot and an earlier statement in the program:
   KEY fen    = 12;   /* Receives red front end node key */
provides the concrete value. The order code here is “Node_Fetch + FEN_Parm2” which is passed thru for the standard C compiler to see as an argument. As “fen” names the slot of the invoked key, “k3” names the slot to receive a key that will be returned when control returns to this domain.

main/src/domains/support/cfstart.s is a program that first runs in domains that will later run C code. It sets up the domain for compiled code.