usr/sys/reg.h

/*
 * Location of the users' stored
 * registers relative to R0.
 * Usage is u.u_ar0[XX].
 */
#define R0      (0)
#define R1      (-2)
#define R2      (-9)
#define R3      (-8)
#define R4      (-7)
#define R5      (-6)
#define R6      (-3)
#define R7      (1)
#define RPS     (2)

#define TBIT    020             /* PS trace bit */