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Function return values
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Hello, i'm developing a simple assembly (AT&T) program as exercise in preparation of an exam and i have a problem:
I need to pass 4 return values of a function using the stack. I know that after the "call", %esp points to the return address that will be used from the "ret". I've tried to pop that value from the stack, push my return values and then push the return address again just before the "ret" but it doesn't work.
Can i have an help please? Where do i need to put those values in the stack? And how do i have to manipulate the %esp or %ebp?
Thanks
I need to pass 4 return values of a function using the stack. I know that after the "call", %esp points to the return address that will be used from the "ret". I've tried to pop that value from the stack, push my return values and then push the return address again just before the "ret" but it doesn't work.
Can i have an help please? Where do i need to put those values in the stack? And how do i have to manipulate the %esp or %ebp?
Thanks
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Join Date: Jun 2009
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Practically a Posting Shark
val1 = func( &apple, &bat, &cat, &dog )
The address of apple, bat, cat, and dog are now on the stack.
here's one method!
The address of apple, bat, cat, and dog are now on the stack.
here's one method!
Assembly Syntax (Toggle Plain Text)
ESP points to the current top of stack,
PUSH & POP implicity decrement and increment ESP.
Right upon entering your function ESP points to a
DWORD containing the return address (address
of the instruction past the CALL).
RET will pop the DWORD off the stack (according
to the value of ESP so make sure ESP points
to the return address) to implicitly
change the contents of IP.
If you reserve space on the stack for a couple
of DWORDs (if you need to return two values)
before you CALL your function, to do so
simply subtract 8 from ESP before calling your function.
[ ] <-------Bottom of stack | Higher Address
[ ] Space 2
[ ] Space 1
[ ] Return Address | Lower Address
Then copy ESP into EBP to index into the stack,
once in your function to place the return values on
the stack.
To copy into Space 1 use movl src,4(%ebp)
To copy into Space 2 use movl src,8(%ebp)
PUSH & POP implicity decrement and increment ESP.
Right upon entering your function ESP points to a
DWORD containing the return address (address
of the instruction past the CALL).
RET will pop the DWORD off the stack (according
to the value of ESP so make sure ESP points
to the return address) to implicitly
change the contents of IP.
If you reserve space on the stack for a couple
of DWORDs (if you need to return two values)
before you CALL your function, to do so
simply subtract 8 from ESP before calling your function.
[ ] <-------Bottom of stack | Higher Address
[ ] Space 2
[ ] Space 1
[ ] Return Address | Lower Address
Then copy ESP into EBP to index into the stack,
once in your function to place the return values on
the stack.
To copy into Space 1 use movl src,4(%ebp)
To copy into Space 2 use movl src,8(%ebp)
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To control a mind violates a man, and all it has been used for is
hurting and afflicting. Nowonder I progam in assembly...
--->Now available http://dotcoding.netai.net/
To control a mind violates a man, and all it has been used for is
hurting and afflicting. Nowonder I progam in assembly...
--->Now available http://dotcoding.netai.net/
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Join Date: Nov 2006
Posts: 134
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Solved Threads: 3
Junior Poster
Assembly Syntax (Toggle Plain Text)
Of course it would be a whole lot easier to use registers to return
a value; or do as Wildgoose says, and push the address of variables onto the stack if you want to return more than one.
Last edited by mathematician; Jul 11th, 2009 at 10:17 pm.
mathematician your right you can just place the address
of the memory variables on the stack, this would be
a good method.
AT&T syntax goes: mnemonic src,dest
16-bit example:
of the memory variables on the stack, this would be
a good method.
AT&T syntax goes: mnemonic src,dest
Assembly Syntax (Toggle Plain Text)
16-bit example:
Assembly Syntax (Toggle Plain Text)
Last edited by NotNull; Jul 11th, 2009 at 11:08 pm.
----------------------------------------------------------
To control a mind violates a man, and all it has been used for is
hurting and afflicting. Nowonder I progam in assembly...
--->Now available http://dotcoding.netai.net/
To control a mind violates a man, and all it has been used for is
hurting and afflicting. Nowonder I progam in assembly...
--->Now available http://dotcoding.netai.net/
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