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Local Descriptor Table

Local Descriptor Table. LDTs contain descriptors which are Task Specific. LDT may contain only code, data, stack, task gate and call gate descriptors. LDTs isolates a given task's code and data segments from the rest of the OS.

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Local Descriptor Table

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  1. Local Descriptor Table • LDTs contain descriptors which are Task Specific. • LDT may contain only code, data, stack, task gate and call gate descriptors. • LDTs isolates a given task's code and data segments from the rest of the OS. • Unlike the 6 byte GDT or IDT registers, the visible portion of LDT register contains only a 16-bit selector. • This selector refers to a Local Descriptor Table Descriptor in the GDT.

  2. Local Descriptor Table Register

  3. Interrupt Descriptor Table • IDT contains descriptors to point to the location of up to 256 interrupt service routines. • IDT should be atleast256 bytes to hold 32 Intel Reserved Interrupts(32*8) • Every interrupt used must have an entry in IDT. • IDT entries are referenced via INT instructions, external interrupt vectors and exceptions. • Interrupts handle asynchronous external events(eg.pressing a key) and exceptions handle instruction faults (e.g. page permission fault)

  4. Interrupt Descriptor Table

  5. Descriptors • The object to which the segment selector points to is called a descriptor. • Descriptors are 8 byte quantities which contain 32 bit base address, 20-bit limit and attributes (12 bits) about a given segment.

  6. General Format of a Descriptor UQ: Write short note on structure of segment descriptor field

  7. General Format of a Descriptor • All segments have 3 attribute fields in common: the P-bit, the DPL bit and the S-bit. • P-bit : P=1  segment is loaded in physical memory. If P=0, then exception 11

  8. General Format of a Descriptor • DPL-bit: 2-bit field which specifies the protection level 0-3 associated with a segment. • There are two main categories of segments: system segments and non-system segments( for code and data) • S-bit: S=1  non-system segment. S=0  system segment

  9. Non-system segment descriptor • G-bit (Granularity Bit): It specifies if a segment length is byte granular or page granular. • G = 0  byte granular (max 1MB) segment size may be 1, 2, ..., 220 bytes G = 1  page granular (max 4GB) segment size may be 1 × 212,2 × 212,...,220 ×212 bytes

  10. Non-system segment descriptor • D-bit : It indicates default length for operands and effective addresses. • D = 1  32-bit operands and 32-bit addressing modes D = 0  16-bit operands and 16-bit addressing modes

  11. Non-system segment descriptor • B-bit : It indicates size of stack pointer • B = 1  32-bit ESP register B = 0  16-bit SP register

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