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blog/content/edition-2/posts/08-paging-introduction/index.md
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One main task of an operating system is to isolate programs from each other. Your web browser shouldn't be able to interfere with your text editor, for example. To achieve this goal, operating systems utilize hardware functionality to ensure that memory areas of one process are not accessible by other processes. There are different approaches depending on the hardware and the OS implementation.
As an example, some ARM Cortex-M processors (used for embedded systems) have a [_Memory Protection Unit_] (MPU), which allows you to define a small number (i.e., 8) of memory regions with different access permissions (i.e., no access, read-only, read-write). On each memory access, the MPU ensures that the address is in a region with correct access permissions and throws an exception otherwise. By changing the regions and access permissions on each process switch, the operating system can ensure that each process only accesses its own memory and thus isolates processes from each other.
As an example, some ARM Cortex-M processors (used for embedded systems) have a [_Memory Protection Unit_] (MPU), which allows you to define a small number (e.g., 8) of memory regions with different access permissions (e.g., no access, read-only, read-write). On each memory access, the MPU ensures that the address is in a region with correct access permissions and throws an exception otherwise. By changing the regions and access permissions on each process switch, the operating system can ensure that each process only accesses its own memory and thus isolates processes from each other.
[_Memory Protection Unit_]: https://developer.arm.com/docs/ddi0337/e/memory-protection-unit/about-the-mpu
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### Virtual Memory
The idea behind virtual memory is to abstract away the memory addresses from the underlying physical storage device. Instead of directly accessing the storage device, a translation step is performed first. For segmentation, the translation step is to add the offset address of the active segment. Imagine a program accessing memory address `0x1234000` in a segment with an offset `0x1111000`: The address that is really accessed is `0x2345000`.
The idea behind virtual memory is to abstract away the memory addresses from the underlying physical storage device. Instead of directly accessing the storage device, a translation step is performed first. For segmentation, the translation step is to add the offset address of the active segment. Imagine a program accessing memory address `0x1234000` in a segment with an offset of `0x1111000`: The address that is really accessed is `0x2345000`.
To differentiate the two address types, addresses before the translation are called _virtual_, and addresses after the translation are called _physical_. One important difference between these two kinds of addresses is that physical addresses are unique and always refer to the same distinct memory location. Virtual addresses, on the other hand, depend on the translation function. It is entirely possible that two different virtual addresses refer to the same physical address. Also, identical virtual addresses can refer to different physical addresses when they use different translation functions.