CPL DPL

2.2.1. Segment Selectors and Segmentation Registers

To make it easy to retrieve segment selectors quickly, the processor providessegmentation registers whose only purpose is to hold Segment Selectors; these registers are called cs, ss, ds, es, fs, and gs. Although there are only six of them, a program can reuse the same segmentation register for different purposes by saving its content in memory and then restoring it later.

Three of the six segmentation registers have specific purposes:
cs
The code segment register, which points to a segment containing program instructions
ss
The stack segment register, which points to a segment containing the current program stack
ds
The data segment register, which points to a segment containing global and static data

The remaining three segmentation registers are general purpose and may refer to arbitrary data segments.
The cs register has another important function:it includes a 2-bit field that specifies the Current Privilege Level (CPL) of the CPU.The value 0 denotes the highest privilege level, while the value 3 denotes the lowest one. Linux uses only levels 0 and 3, which are respectively called Kernel Mode and User Mode.

2.2.2. Segment Descriptors

Each segment is represented by an 8-byteSegment Descriptor that describes the segment characteristics. Segment Descriptors are stored either in theGlobal Descriptor Table (GDT ) or in theLocal Descriptor Table(LDT).

Usually only one GDT is defined, while each process is permitted to have its own LDT if it needs to create additional segments besides those stored in the GDT. The address and size of the GDT in main memory are contained in the gdtr control register, while the address and size of the currently used LDT are contained in the ldtr control register.

Figure 2-3 illustrates the format of a Segment Descriptor; the meaning of the various fields is explained inTable 2-1.

Table 2-1. Segment Descriptor fields

Field name	Description
Base	Contains the linear address of the first byte of the segment.
G	Granularity flag: if it is cleared (equal to 0), the segment size is expressed in bytes; otherwise, it is expressed in multiples of 4096 bytes.
Limit	Holds the offset of the last memory cell in the segment, thus binding the segment length. When G is set to 0, the size of a segment may vary between 1 byte and 1 MB; otherwise, it may vary between 4 KB and 4 GB.
S	System flag: if it is cleared, the segment is asystem segment that stores critical data structures such as the Local Descriptor Table; otherwise, it is a normal code or data segment.
Type	Characterizes the segment type and its access rights (see the text that follows this table).
DPL	Descriptor Privilege Level: used to restrict accesses to the segment. It represents the minimal CPU privilege level requested for accessing the segment. Therefore, a segment with its DPL set to 0 is accessible only when the CPL is 0 that is, in Kernel Mode while a segment with its DPL set to 3 is accessible with every CPL value.
P	Segment-Present flag : is equal to 0 if the segment is not stored currently in main memory. Linux always sets this flag (bit 47) to 1, because it never swaps out whole segments to disk.
D or B	Called D or B depending on whether the segment contains code or data. Its meaning is slightly different in the two cases, but it is basically set (equal to 1) if the addresses used as segment offsets are 32 bits long, and it is cleared if they are 16 bits long (see the Intel manual for further details).
AVL	May be used by the operating system, but it is ignored by Linux.

There are several types of segments, and thus several types of Segment Descriptors. The following list shows the types that are widely used in Linux.

Code Segment Descriptor

Indicates that the Segment Descriptor refers to a code segment; it may be included either in the GDT or in the LDT. The descriptor has the S flag set (non-system segment).

Data Segment Descriptor

Indicates that the Segment Descriptor refers to a data segment; it may be included either in the GDT or in the LDT. The descriptor has the S flag set. Stack segments are implemented by means of generic data segments.

Task State Segment Descriptor (TSSD)

Indicates that the Segment Descriptor refers to a Task State Segment (TSS) that is, a segment used to save the contents of the processor registers (see the section "Task State Segment" in Chapter 3); it can appear only in the GDT. The corresponding Type field has the value 11 or 9, depending on whether the corresponding process is currently executing on a CPU. The S flag of such descriptors is set to 0.

Local Descriptor Table Descriptor (LDTD)

Indicates that the Segment Descriptor refers to a segment containing an LDT; it can appear only in the GDT. The corresponding Type field has the value 2. The S flag of such descriptors is set to 0. The next section shows how 80 x 86 processors are able to decide whether a segment descriptor is stored in the GDT or in the LDT of the process.