Understanding the Linux Kernel, 3rd Edition

Segmentation in Linux

Segmentation has been included in 80 × 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.

The 2.6 version of Linux uses segmentation only when required by the 80 × 86 architecture.

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 ...