For each process its entry in the process manager's process table and also its entry in the kernel's process table contains a data structure to keep track of the segments for that process:

/* Memory map for local text, stack, data segments. */
struct mem_map {
  vir_clicks mem_vir;           /* virtual address */
  phys_clicks mem_phys;         /* physical address */
  vir_clicks mem_len;           /* length */
};

Addresses and segment lengths are both expressed in clicks rather than bytes. Check the definition in the source code to determine the number of bytes in a click. (I found it to be different than stated in the text.) Memory is allocated in integral multiples of this unit. Given access to a process's mem_map structure, a segment, and a virtual address in that segment, it straightforward to to translate the virtual address to a physical address.