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Difference between Jmp and Call?

Posted on 2006-11-12
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Last Modified: 2012-08-14
P:=@Button2.OnClick;
 Asm
  Call P;
 End;

Works like a charm

 P:=@Button2.OnClick;
 Asm
 Jmp P;
 End;


What's the difference betwen Jmp and Call? Are they not the same thing?

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Question by:tobjectpascal
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Russell Libby earned 35 total points
ID: 17927966

Call expects a ret (return), to return to the calling location. Jmp is a like a "goto". If you jmp to code that performs a return, then an exception will be raised, as there won't be a call return on the stack.

Russell



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by:tobjectpascal
ID: 17928045
So Call jumps to that segment of memory and starts executing the instructions/opcodes, it then hits the Ret and then it jumps back to the code that made the jmp execution?

Would that not mean Jmp requires a Ret; not Call? or could you not Push a Ret onto the Stack?

To me it makes more Sense if Jmp expected a Ret ...
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by:tobjectpascal
ID: 17928080
procedure TForm1.Button1Click(Sender: TObject);
Var
 P: pointer;
 P2: POinter;
begin
 P:=@Button2.OnClick;
 P2:=@Button1.OnClick;
 Asm
  Push P2;
  Jmp P;
 End;
end;


Interesting.... That actually brings it back to Button1.Click and then crashes lol
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by:tobjectpascal
ID: 17928103
ok So any idea which Register contains the current code execution memory address? i Tried pushing EDI assuming that's where it's currently being exectued..

es:di right? do i need to push both? if you're not sure, that's ok I figured out from making the program crash that Pushing some address before jmping makes it return to that locaiton, just trying to work out how to make it get back again lol...

But thanks :)
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Expert Comment

by:Russell Libby
ID: 17930005

> Would that not mean Jmp requires a Ret; not Call? or could you not Push a Ret onto the Stack?
> To me it makes more Sense if Jmp expected a Ret

In a word: NO.

The CALL saves procedure linking information on the stack and branches to the procedure specified with the destination (target) operand. The target operand specifies the address of the first instruction in the called procedure. This operand can be an immediate value, a general-purpose register, or a memory location. And when performing a near call, the processor pushes the value of the EIP register (which contains the offset of the instruction following the CALL instruction) onto the stack for use later as a return-instruction pointer.

The JMP is most commonly seen in two situations.

1. Branching logic within the same block of procedure code.
2. Hooking code that has been injected, where the calling code has already performed a CALL, and the hook code JMP's back to the original source.

JMP can be used in place of a call, but would require handling of the stack frame before making the JMP. Not usually a recommended idea.

Russell
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