These registers can also be called as Special Purpose registers.

Stack Pointer (SP) is a 16-bit register pointing to program stack, ie it is used to hold the address of the top of stack. The stack is maintained as a LIFO with its bottom at the start of the stack segment (specified by the SS segment register).Unlike the SP register, the BP can be used to specify the offset of other program segments.

Base Pointer (BP) is a 16-bit register pointing to data in stack segment. It is usually used by subroutines to locate variables that were passed on the stack by a calling program. BP register is usually used for based, based indexed or register indirect addressing.

Source Index (SI) is a 16-bit register. SI is used for indexed, based indexed and register indirect addressing, as well as a source data address in string manipulation instructions. Used in conjunction with the DS register to point to data locations in the data segment.

Destination Index (DI) is a 16-bit register. Used in conjunction with the ES register in string operations.  DI is used for indexed, based indexed and register indirect addressing, as well as a destination data address in string manipulation instructions. In short, Destination Index and SI Source Index registers are used to hold address.