System Segment Descriptor It contain the information of

System Segment Descriptor • It contain the information of about OS 1. LDT Their functions are fixed and specified 2. TSS by Intel. 3. Gate • Gate further sub divided into I. Call Gate II. Interrupt Gate III. Task Gate IV. Trap Gate

System Segment descriptor • All system descriptors are present in GDT while some system descriptors are present in LDTs. • The value of S=0 in right access byte. • The type field has 4 bits. • So , no Accessed bit, bit instead the type field (3 bits) is now extended to 4 bits.

Type Field of System descriptors

LDT descriptor (s=0, Type 2)

LDT descriptor (s=0, Type 2) • The LDT are located in the GDT, point to the base of LDT. • LDT contains the segment descriptors that are unique to a each task. • The DPL field of the descriptor is ignored as this descriptor can only be accessed with a privilege level of 0. • Here type field =2 i. e. it specifies a LDT descriptor.

TSS (Task State Segment) Descriptors (S=0, Type 1, 3, 9 and B) • Whenever a computer is performing more than one tasks at a time it may also switch between these tasks. • The task may be a single program or a group of program. • When one task switches to another task , it stores all the necessary information required to restart the task where it was left. This information is called as the “ state of the task” • For storing the state of the task the 80386 processor uses a special segment called the “Task State Segment (TSS)”

TSS (Task State Segment) Descriptors (s=0, Type 1, 3, 9 and B) • The task segment is addressed with the help of TSS descriptor. • It contains information about the location, size and privilege level of a TSS. • A TSS descriptor appears only in GDT and not in IDT or LDT. • The TSS consists the linkage field for the nest task that permits the nesting of the tasks.

TSS (Task State Segment) Descriptors (s=0, Type 1, 3, 9 and B) • B bit indicates whether task is busy or not. B=0 : Task is not busy B=1 : Task is busy

Gate descriptors (S=0, Type 4, 5, 6, 7, C, E, F) • Protection Check. • Control the access to the entry points within the target CS. • All fields are same as specified earlier except the word count, selector and offset. • The word count field specifies the number of parameters that are to be copied from caller’s stack to the called procedure’s stack.

Gate Descriptors • Call gates used to modified privilege levels. • Transfer the program control to a more PL. • Interrupt/Trap gates are used in interrupt and exception handling. (Specify ISR) • Task gates are used in multitasking system