memory,memory-management,operating-system,cpu-architecture , memory segments and physical RAM [closed]


memory segments and physical RAM [closed]

Question:

Tag: memory,memory-management,operating-system,cpu-architecture

The memory map of a process appears to be fragmented into segments (stack, heap, bss, data, and text),


Answer:

In a modern OS supporting virtual memory, it is the address space of the process that is divided into these segments. And in general case that address space of the process is projected onto the physical RAM in a completely random fashion (with some fixed granularity, 4K typically). Address space pages located next to each other do not have to be projected into the neighboring physical pages of RAM. Physical pages of RAM do not have to maintain the same relative order as the process's address space pages. This all means that there is no such separation into segments in RAM and there can't possibly be.

In order to optimize memory access an OS might (and typically will) try to map sequential pages of the process address space to sequential pages in RAM, but that's just an optimization. In general case, the mapping is unpredictable. On top of that the RAM is shared by all processes in the system, with RAM pages belonging to different processes being arbitrarily interleaved in RAM, which eliminates any possibility of having such "segments" in RAM. There's no process-specific ordering or segmentation in RAM. RAM is just a cache for virtual memory mechanism.

Again, every process works with its own virtual address space. This is where these segments can exist. The process has no direct access to RAM. The process doesn't even need to know that RAM exists.


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