Operating Systems - Memory Management

compsci ✒ operating-systems ✒ memory-management

Old methods of allocating memory

If your OS can only run one program at a time, all the memory not needed by the OS can be assigned to the program, to do with as it wishes
Of course, only running one program at a time is wasteful - if it is blocked by slow disk reading or having to send something over the network, the CPU will be idle most of the time.
So you might divide the memory into fixed partitions, and assign one partition to each program - programs that need more memory have to use one of the larger positions and will not be started until one becomes free (and is assigned to it).
Each partition has a continuous block of memory, so it can be represented by the start (base) and size of the partition (limit).
But this is still inefficient - programs may not know how much memory they will use in advance, and you're still restricted to having the total memory usage of running programs <= amount of RAM

The first insight is that when a process is paused (e.g. waiting for input), it can be written to disk and the RAM assigned to a different process.