In a compiled procedural programming language, when calling a procedure, the calling convention determines who is supposed to put the parameters on and off the stack, when and in what order.

This is not important for the programmer to know inside the programming language, but is important to know when calling a dynamically or statically linked function that was compiled in a different language.

For the details of which calling convention does what, here's the dope stolen from the only documentation that I have on this, the Delphi 5 Help file:

When you declare a procedure or function, you can specify a calling convention using one of the directives register, pascal, cdecl, stdcall, and safecall. For example,

function MyFunction(X, Y: Real): Real; cdecl;

Calling conventions determine the order in which parameters are passed to the routine. They also affect the removal of parameters from the stack, the use of registers for passing parameters, and error and exception handling. The default calling convention is register.

The register and pascal conventions pass parameters from left to right; that is, the leftmost parameter is evaluated and passed first and the rightmost parameter is evaluated and passed last. The cdecl, stdcall, and safecall conventions pass parameters from right to left.

For all conventions except cdecl, the procedure or function removes parameters from the stack upon returning. With the cdecl convention, the caller removes parameters from the stack when the call returns.

The register convention uses up to three CPU registers to pass parameters, while the other conventions pass all parameters on the stack.

The safecall convention implements COM error and exception handling.

The default register convention is the most efficient, since it usually avoids creation of a stack frame. (Access methods for published properties must use register.) The cdecl convention is useful when you call functions from DLLs written in C or C++, while stdcall and safecall are used for Windows API calls. The safecall convention must be used for declaring dual-interface methods. The pascal convention is maintained for backward compatibility.

The directives near, far, and export refer to calling conventions in 16-bit Windows programming. They have no effect in 32-bit applications and are maintained for backward compatibility only.

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