What are index register and segment register?

A segment register is a register that contains the base address, or something related to the base address, of a region of memory. In the 8086/8088, the four segment registers are multiplied by 16 and added to the effective address to form the physical address.

An index register, on the other hand, is a register that contains an address that is added to another address to form the effective address.

In the 8086/8088, four address components are involved; 1.) the displacement contained within the instruction, often called the offset, 2.) a base address specified by the r/m field, often the BP or BX register, 3.) an index address specified by the r/m field, often the SI or DI register, and 4.) the segment address specified by context or by a segment override prefix, often the CS, DS, SS, or ES register.