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Calling Conventions: Difference between revisions
→Accessing Function Arguments
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In the GCC/x86 C calling convention, the first thing any function that accepts formal arguments should do is push the value of <tt>EBP</tt> (the frame base pointer of the calling function), then copy the value of <tt>ESP</tt> to <tt>EBP</tt>. This sets the function's own frame pointer, which is used to track both the arguments and (in C, or in any properly reentrant assembly code) the local variables.
To access arguments passed by a C function, you need to use the <tt>EBP</tt> an offset equal to 4 * (n + 2), where n is the number of the parameter in the argument list (not the number in the order it was pushed by), zero-indexed. The + 2 is an added offset for the calling function's saved frame pointer and return pointer (pushed automatically by <tt>CALL</tt>, and popped by <tt>RET</tt>).
Thus, in function <tt>fee</tt>, to move <tt>fie</tt> into <tt>EAX</tt>, <tt>foe</tt> into <tt>BL</tt>, and <tt>fum</tt> into <tt>EAX</tt> and <tt>EDX</tt>, you would write (in NASM):
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