assembly - 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 give an example?

Answer 1

SI = Source Index
DI = Destination Index

As others have indicated, they have special uses with the string instructions. For real mode programming, the ES segment register must be used with DI and DS with SI as in

movsb  es:di, ds:si

SI and DI can also be used as general purpose index registers. For example, the C source code

srcp [srcidx++] = argv [j];

compiles into

8B550C         mov    edx,[ebp+0C]
8B0C9A         mov    ecx,[edx+4*ebx]
894CBDAC       mov    [ebp+4*edi-54],ecx
47             inc    edi

where ebp+12 contains argv, ebx is j, and edi has srcidx. Notice the third instruction uses edi mulitplied by 4 and adds ebp offset by 0x54 (the location of srcp); brackets around the address indicate indirection.

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Though I can't remember where I saw it, but this confirms most of it, and this (slide 17) others:

AX = accumulator
DX = double word accumulator
CX = counter
BX = base register

They look like general purpose registers, but there are a number of instructions which (unexpectedly?) use one of them—but which one?—implicitly.