Purpose of ESI & EDI registers?

Question

What is the actual purpose and use of the EDI & ESI registers in assembler?

I know they are used for string operations for one thing.

Can someone also gi

Answer 1

In addition to the string operations (MOVS/INS/STOS/CMPS/SCASB/W/D/Q etc.) mentioned in the other answers, I wanted to add that there are also more "modern" x86 assembly instructions that implicitly use at least EDI/RDI:

The SSE2 MASKMOVDQU (and the upcoming AVX VMASKMOVDQU) instruction selectively write bytes from an XMM register to memory pointed to by EDI/RDI.