The pm's process table begins with an array of 3 mem_map's:

EXTERN struct mproc {
  struct mem_map mp_seg[NR_LOCAL_SEGS]; /* points to text, data, stack */
  char mp_exitstatus;/* storage for status when process exits */
  char mp_sigstatus;/* storage for signal # for killed procs */
  pid_t mp_pid;/* process id */
  ...
mp_seg[0] is for the text segment,
mp_seg[1] is for the data segment,
mp_seg[2] is for the stack segment.

There are #defines for 0,1, and 2 to help avoid getting the segments mixed up:

#define NR_LOCAL_SEGS      3/* # local segments per process (fixed) */
#define T                  0/* proc[i].mem_map[T] is for text */
#define D                  1/* proc[i].mem_map[D] is for data */
#define S                  2/* proc[i].mem_map[S] is for stack */