What is the practical difference between the SI and DI registers?

Question

I don't get what is the difference.

Answer 1

When you are using instructions like movsb, si is considered as the source register, while di is considered as the destination register. But they are both normal x86 registers.

Answer 2

My assembly is a bit rusty, but one's the Source Index, the other the Destination Index. An instruction like movsb will copy a byte from the memory location pointed at by SI, and move it to the memory location pointed at by DI, and then increment both, so if you want to copy the byte stored at SI+1 to DI+1, it only takes a further movsb instruction.