Calling Conventions

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TMT Pascal Language Description

Implementation Issues

Calling Conventions

Calling conventions match those in Borland Pascal with the following differences:

all parameters use 4 bytes on the stack, or a multiple of 4 (BP:2)
all procedures must preserve the contents of registers ebx, ecx, edx, ds, and es!
the direction bit should be cleared after the exit from a procedure, if it has been modified by the procedure.

To call external procedures written for different languages, TMT Pascal provides the conv operator. The conv operator should be used in the function (procedure) declaration to define a calling convention, which in turn will be used to call a declared function (procedure).

Syntax:

  [function] conv conv_method FunctionName [Arguments] : ReturnType;

Where conv_method is a constant, which defines the calling conversion to be used. The System units contains the following constants to define conventional method:

const
// Base calling conventions to construct any possible convention
  arg_reverse    = [0];
  arg_proc_16    = [2];
  arg_noregsave  = [3];
  arg_no_drop_1  = [4];
  arg_no_drop_2  = [5];
  arg_no_drop_3  = arg_no_drop_1 + arg_no_drop_2;
  arg_no_drop_4  = [6];
  arg_no_drop_5  = arg_no_drop_1 + arg_no_drop_4;
  arg_no_drop_6  = arg_no_drop_2 + arg_no_drop_4;
  arg_no_drop_all= [4..6];
  arg_IO_test    = [8];
  arg_save_edi   = [9];
  arg_save_esi   = [10];
// Composite calling conventions
  arg_pascal     = arg_noregsave;
  arg_stdcall    = arg_reverse + arg_noregsave + arg_save_edi +
                   arg_save_esi;
  arg_cdecl      = arg_reverse + arg_no_drop_all;
  arg_os2        = arg_cdecl + arg_noregsave;
  arg_os2_16     = arg_proc_16 + arg_no_drop_all + arg_noregsave;

The arg_pascal convention passes 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 arg_cdecl, arg_stdcall, arg_os2 and arg_os2_16 conventions pass parameters from right to left. For all conventions except arg_cdecl, the procedure or function removes parameters from the stack upon returning.

With the arg_cdecl convention, the caller must remove parameters from the stack when the call returns. The register convention uses up to three CPU registers to pass parameters, whereas the other conventions always pass all parameters on the stack.

The calling conventions are summarized in the following table.

Directive	Order	Cleanup	Registers
arg_pascal	Left-to-right	Function	No
arg_cdecl	Right-to-left	Caller	No
arg_stdcall	Right-to-left	Function	No

The arg_pascal and arg_cdecl conventions are mostly useful for calling routines in dynamic-link libraries written in C, C++, or other languages. The arg_stdcall convention is used for calling Windows API routines.


Memory Organization	Table of Content	Limitations

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