Point of memory management algorithms - support sharing of main memory. We will focus on having multiple processes sharing the same physical memory. Key issues:

· Protection. Must allow one process to protect its memory from access by other processes.

· Naming. How do processes identify shared pieces of memory.

· Transparency. How transparent is sharing. Does user program have to manage anything explicitly?

· Efficiency. Any memory management strategy should not impose too much of a performance burden.

Why share memory between processes? Because want to multiprogram the processor. To time share system, to overlap computation and I/O. So, must provide for multiple processes to be resident in physical memory at the same time. Processes must share the physical memory.

Historical Development.

· For first computers, loaded one program onto machine and it executed to completion. No sharing required. OS was just a subroutine library, and there was no protection. What addresses does program generate?

· Desire to increase processor utilization in the face of long I/O delays drove the adaptation of multiprogramming. So, one process runs until it does I/O, then OS lets another process run. How do processes share memory? Alternatives:

o Load both processes into memory, then switch between them under OS control. Must relocate program when load it. Big Problem: Protection. A bug in one process can kill the other process. MS-DOS, MS-Windows use this strategy.

o Copy entire memory of process to disk when it does I/O, then copy back when it restarts. No need to relocate when load. Obvious performance problems. Early version of Unix did this.

o Do access checking on each memory reference. Give each program a piece of memory that it can access, and on every memory reference check that it stays within its address space. Typical mechanism: base and bounds registers. Where is check done? Answer: in hardware for speed. When OS runs process, loads the base and bounds registers for that process. Cray-1 did this. Note: there is now a translation process. Program generates virtual addresses that get translated into physical addresses. But, no longer have a protection problem: one process cannot access another’s memory, because it is outside its address space. If it tries to access it, the hardware will generate an exception