Are memory segments defined by the OS or architecture?

Question

The x86 architecture has segment registers for various segments of the address space (ss, ds, etc). If I wanted to add a new memory segment into a process address space, could I do it by just modifying the kernel or would I need hardware support? Not looking to do anything specific just curious and trying to understand how Linux uses segment registers.

Answer 1

from this link https://www.cs.princeton.edu/courses/archive/fall02/cs318/proj2/pc-arch.html

Modern operating system and applications use the (unsegmented) memory model¾ all the segment registers are loaded with the same segment selector so that all memory references a program makes are to a single linear-address space.

When writing application code, you generally create segment selectors with assembler directives and symbols. The assembler and/or linker then creates the actual segment selectors associated with these directives and symbols. If you are writing system code, you may need to create segment selectors directly.

Also you can not add a new segment with out changing a lot of things including hardware support.

Answer 2

Memory is usually managed by a dedicated piece of hardware named Memory Management Unit (MMU). Any x86 CPU has an MMU, but this doesn't mean that memory management must to be done in hardware. Linux itself can run by emulating an MMU in software. Of course without hardware support it would be really difficult (and in some cases even impossible) to implement some features. From a pure theoretical point of view you could be able to emulate in software (kernel space) a segmentation-like behavior with all the segments that you like, but in the real world that would be just a bad idea.

As you said x86_32 has support for memory segmentation, but since i386 there is also support for paging. Nowadays, segmentation is considered deprecated and there is no modern OS (AFAIK) that uses it (except maybe some hackish patch like grsecurity/PaX and their UDEREF feature). It's also important to note that x86_64 completely lacks support for segmentation.