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.
The 80×86 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.