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Calling Conventions: Difference between revisions
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| System V i386 || eax, edx || none || stack (right to left)<sup>[[#Note1|1]]</sup> || || eax, ecx, edx || ebx, esi, edi, ebp, esp || ebp
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| System V X86_64<sup>[[#Note2|2]]</sup> || rax, rdx || rdi, rsi, rdx, rcx, r8, r9 || stack (right to left)<sup>[[#Note1|1]]</sup> || 16-byte at call<sup>[[#
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| Microsoft x64 || rax || rcx, rdx, r8, r9 || stack (right to left)<sup>[[#Note1|1]]</sup> || 16-byte at call<sup>[[#Note3|3]]</sup> || rax, rcx, rdx, r8, r9, r10, r11 || rbx, rdi, rsi, rsp, rbp, r12, r13, r14, r15 || rbp
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| ARM || r0, r1 || r0, r1, r2, r3 || stack || 8 byte<sup>[[#Note4|4]]</sup> || r0, r1, r2, r3, r12 || r4, r5, r6, r7, r8, r9, r10, r11, r13, r14 ||
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==Passing Function Arguments==
GCC/x86 passes function arguments on the stack. These arguments are pushed in reverse order from their order in the argument list. Furthermore, since the x86 protected-mode stack operations operate on 32-bit values, the values are always pushed as a 32-bit value, even if the actual value is less than a full 32-
To pass arguments to a C function, the calling function must push the argument values as described above. Thus, to call foo() from a [[NASM]] assembly program, you would do something like this
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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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