The next step is MFT which divide the real storage into partitions predefined with fixed size. Program are fixed to particular partition based on their size. Use of storage is not optimal as small program wasted storage in the partition. Program loaded will have its base address be relocated by the loader to the partition real address for run.
MVT is similar to MFT but the partition size is set dynamically to reduce storage wastage for small program. The downside is the possibility of memory fragmentation. If the freed up memory (when a program ends) is not large enough for the next program to run, the new program will have to wait.
PCP, MFT and MVT addressed real memory directly. Program is compiled with origin 0. Loader will load the starting real memory address in the base register. This is called static relocation.
MVT supports roll out and roll in operation to dynamically swap the whole program out the external store during run. Roll in will load the program in its original address before rollout so it is not very effective memory management.
The next step is to develop virtual memory. Firstly, a program address space is divided into segments of block. Secondly, the address for each block is translate during run time which means the block load address can change while it runs.
OS/VS1 and OS/VS2 release 1 (SVS) implemented virtual memory. However, there is only 1 virtual address space in the whole system. So they are corresponding to PCP and MVT using a larger virtual memory instead of real memory.
MVS gave each user its own virtual memory space.
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