The source index (SI) register is required for some string (character) operations. In this context the SI is associated with the DS register. The destination index (DI) register is also required for some string operations. In this context the DI is associated with the ES register.

The various 16-bit registers on the 8085 are BC, DE, HL, SP, PC.

An index register has the function of receiving, storing, and outputting instruction-changing codes in a computer. The index register is in a computer's CPU.

Address Register are designed to make it easy to index into the array of constant register. The address allows you to provide a signed integer offset into the constant register.