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Descriptors and Descriptor Tables

The 386 maintains three different descriptor tables: the Global Descriptor Table (GDT), the Local Descriptor Table (LDT), and the Interrupt Descriptor Table (IDT). The IDT can hold up to 256 interrupt, trap, or task gates; GDT and LDT can hold up to 8191 descriptors each. Most GDT and LDT entries hold segment descriptors, although other descriptors (gates, TSS, etc.) are possible. Among other things, a segment descriptor contains the following information:

• the linear start address of the segment (the Base)
• the size of the segment (the Limit)
• the descriptor privilege level (the DPL)
• execute, read only, or read/write access permission

The main purpose of the IDT is to hold Interrupt or Trap Gates. Basically, these gates simply point to the entrypoint of an interrupt service routine.

RTTarget-32 manages a GDT with a total of 16 descriptors. The following table summarizes the properties of RTTarget-32's GDT descriptors:

RTTarget-32's IDT has 256 entries as follows:

By default, RTTarget-32 does not maintain an LDT (but RTKernel-32 does).

Protected Mode

32-Bit Protected Mode

Privilege Levels