System Segment Descriptor It contain the information of about OS LDT TSS Gate Gate further sub divided into Call Gate Interrupt Gate Task Gate Trap Gate Their functions are fixed and specified by Intel.

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, 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