virtual address space in a variable partition scheme

I know this question is more on OS than on programming,sorry for that.

In a multiprogrammig scheme with variable 开发者_Go百科partitions who has to decide the size of a process virtual address space??

what if during execution we find out a process needs more memory??

With variable partition scheme I mean a scheme that does not involve segmentation or paging but countigous allocation.

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Do you mean something without Virtual Memory? Systems without MMU give a fixed space to each process, even the maximum stack size is encoded in the binary file format, see the bFLT format for an example.