Difference in registers

[Joker2u]

Hi all I am new to ASM and I am using Kip Irvine's Assembly Language for Intel Based Computers 5th edition.  I was wondering if anyone could explain registers other than EAX, EBX, ECX, and EDX, as I am confident in my use of these registers.  I mainly need a brief explanation of ESI, EIP, EDI, EBP, and ESP, any explanations would be greatly appreciated whether they are short or long.  Thank you.

[jj2007]

Joker,
If you have already imnstalled Masm32, this is the right page for you.

[BlackVortex]

ESI and EDI are usually used as "source" and "destination" when copying memory or equivalent operations. There are some opcodes that work with esi/edi specifically.

ESP is the stack pointer, it changes automatically by pushing,popping,ret,call etc. Usually you don't want to change it manually.

EBP is usually used at the start of procedures to save the stack pointer, so that local variables can easily be accessed and then the original pointer is restored before returning.

EDIT: Yes, like JJ says, you need to understand which registers get thrashed when using APIs and other external calls. And macros.

[Joker2u]

Anyone of EIP or EDI?

[MichaelW]

EIP holds the 32-bit instruction pointer. It is not one of the general-purpose registers and is not directly accessible to the programmer. It is used as an offset into the code segment when the processor fetches instructions.

[hutch--]

It is actually spelt out in the Asm Intro Help file in masm32 that you get to from the help menu. Select the helptopic "Register Preservation Convention".

[dedndave]

the letters actually do mean something - lol
it gives you a guideline to go by, but most of them are called "general registers" - you can use them as needed

Code Select Expand


EAX - accumulator
EBX - base
ECX - count
EDX - data
ESI - source index
EDI - destination index
EBP - base pointer
ESP - stack pointer


[jj2007]

EIP holds the 32-bit instruction pointer. It is not one of the general-purpose registers and is not directly accessible to the programmer. It is used as an offset into the code segment when the processor fetches instructions.

"not so directly" accessible...

Code Select Expand

include \masm32\include\masm32rt.inc

.data?
zebuffer db 100 dup(?)

.code
start: mov ebx, $
call @F
@@: pop esi
sub esi, ebx
MsgBox 0, cat$(offset zebuffer, "EIP=", hex$(ebx), ", and the mov ebx, $ plus call @F costs 2*5=", str$(esi), " bytes"), "Hellooo...!", MB_OK
exit
end start


[MichaelW]

It's not directly accessible in that you cannot use it as an instruction operand.

[Joker2u]

what would be a valid instruction for changing the EIP from where it points to 100 spots of memory before that point??

[dedndave]

to change EIP, use JMP or CALL (or one of the conditional branches)
RET also changes it, of course
that is sometimes done in special cases - PUSH the address, then RET

[BlackVortex]

jmp $-100

100 bytes before that (decimal)

but better use a label  :bg