Dynamic Loading

• In our discussion so far, the entire program and all data of a process must be in physical memory for the process to execute.
• To obtain better memory-space utilization, we can use dynamic loading.
  • With dynamic loading, a routine is not loaded until it is called.
  • All routines are kept on disk in a relocatable load format.
  • The main program is loaded into memory and is executed. When a routine needs to call another routine, the calling routine first checks to see whether the other routine has been loaded.
  • If not, the relocatable linking loader is called to load the desired routine into memory and to update the program's address tables to reflect this change.
  • Then control is passed to the newly loaded routine.
• The advantage of dynamic loading is that an unused routine is never loaded.
• Dynamic loading does not require special support from the OS. Operating systems may help the programmer, however, by providing library routines to implement dynamic loading.