Can someone please explain to me (in very simple terms) what the difference between the esp, ebp, and esi register is?

Question

I've got to learn assembly and I'm very confused as to what the different registers do/point to.

Answer 1

On some architectures, like MIPS, all registers are created equal, and there is really no difference beyond the name of the register (and software conventions). On x86 you can mostly use any registers for general-purpose computing, but some registers are implicitly bound to the instruction set.

Lots of information about special purposes for registers can be found here.

Examples:

• eax, accumulator: many arithmetic instructions implicitly operate on eax. There are also special shorter EAX-specific encodings for many instructions: add eax, 123456 is 1 byte shorter than add ecx, 123456, for example. (add eax, imm32 vs. add r/m32, imm32)
• ebx, base: few implicit uses, but xlat is one that matches the "Base" naming. Still relevant: cmpxchg8b. Because it's rarely required for anything specific, some 32-bit calling-conventions / ABIs use it as a pointer to the "global offset table" in Position Independent Code (PIC).
• edx, data: some arithmetic operations implicitly operate on the 64-bit value in edx:eax
• ecx, counter used for shift counts, and for rep movs. Also, the mostly-obsolete loop instruction implicitly decrements ecx
• esi, source index: some string operations read a string from the memory pointed to by esi
• edi, destination index: some string operations write a string to the memory pointed to by edi. e.g. rep movsb copies ECX bytes from [esi] to [edi].
• ebp, base pointer: normally used to point to local variables. Used implicitly by leave.
• esp, stack pointer: points to the top of the stack, used implicitly by push, pop, call and ret

The x86 instruction set is a complex beast, really. Many instructions have shorter forms that implicitly use one register or another. Some registers can be used to do certain addressing while others cannot.

The Intel 80386 Programmer's Reference Manual is a irreplaceable resource, it basically tells you everything there is to know about x86 assembly, except for newer extensions and performance on modern hardware.

The PC Assembly (e)book is a great resource for learning assembly.