Segmentation in Linux : Segmentation & Paging are redundant?

Question

Hello,

I'm reading "Understanding Linux Kernel". This is the snippet that explains how Linux uses Segmentation which I didn't understand.

Segmentation has been included in 80 x 86 microprocessors to encourage programmers to split their applications into logically related entities, such as subroutines or global and local data areas. However, Linux uses segmentation in a very limited way. In fact, segmentation and paging are somewhat redundant, because both can be used to separate the physical address spaces of processes: segmentation can assign a different linear address space to each process, while paging can map the same linear address space into different physical address spaces. Linux prefers paging to segmentation for the following reasons:

Memory management is simpler when all processes use the same segment register values that is, when they share the same set of linear addresses.

One of the design objectives of Linux is portability to a wide range of architectures; RISC architectures in particular have limited support for segmentation.

All Linux processes running in User Mode use the same pair of segments to address instructions and data. These segments are called user code segment and user data segment , respectively. Similarly, all Linux processes running in Kernel Mode use the same pair of segments to address instructions and data: they are called kernel code segment and kernel data segment , respectively. Table 2-3 shows the values of the Segment Descriptor fields for these four crucial segments.

I'm unable to understand 1st and last paragraph.

Answer 1

Modern operating systems (i.e. Linux, other Unixen, Windows NT, etc.) do not use the segmentation facility provided by the x86 processor. Instead, they use a flat 32 bit memory model. Each user mode process has it's own 32 bit virtual address space.

(Naturally the widths are expanded to 64 bits on x86_64 systems)

Answer 2

The 80x86 family of CPUs generate a real address by adding the contents of a CPU register called a segment register to that of the program counter. Thus by changing the segment register contents you can change the physical addresses that the program accesses. Paging does something similar by mapping the same virtual address to different real addresses. Linux using uses the latter - the segment registers for Linux processes will always have the same unchanging contents.

Answer 3

Windows uses the fs segment for local thread storage. Therefore, wine has to use it, and the linux kernel needs to support it.

Answer 4

Intel first added segmentation on the 80286, and then paging on the 80386. Unix-like OSes typically use paging for virtual memory.

Anyway, since paging on x86 didn't support execute permissions until recently, OpenWall Linux used segmentation to provide non-executable stack regions, i.e. it set the code segment limit to a lower value than the other segment's limits, and did some emulation to support trampolines on the stack.