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    <p>I was messing around the other day trying to learn about calling conventions and I wrote some methods to convert between various ones. Here is one for StdCall->FastCall.</p> <pre><code>typedef struct { USHORT ParameterOneOffset; // The offset of the first parameter in dwords starting at one USHORT ParameterTwoOffset; // The offset of the second parmaeter in dwords starting at one } FastCallParameterInfo; __declspec( naked,dllexport ) void __stdcall InvokeFast() { FastCallParameterInfo paramInfo; int functionAddress; int retAddress; int paramOne, paramTwo; __asm { // Pop the return address and parameter info. Store in memory. pop retAddress; pop paramInfo; pop functionAddress; // Check if any parameters should be stored in edx movzx ecx, paramInfo.ParameterOneOffset; cmp ecx,0; je NoRegister; // Calculate the offset for parameter one. movzx ecx, paramInfo.ParameterOneOffset; // Move the parameter one offset to ecx dec ecx; // Decrement by 1 mov eax, 4; // Put 4 in eax mul ecx; // Multiple offset by 4 // Copy the value from the stack on to the register. mov ecx, esp; // Move the stack pointer to ecx add ecx, eax; // Subtract the offset. mov eax, ecx; // Store in eax for later. mov ecx, [ecx]; // Derefernce the value mov paramOne, ecx; // Store the value in memory. // Fix up stack add esp,4; // Decrement the stack pointer movzx edx, paramInfo.ParameterOneOffset; // Move the parameter one offset to edx dec edx; // Decrement by 1 cmp edx,0; // Compare offset with zero je ParamOneNoShift; // If first parameter then no shift. ParamOneShiftLoop: mov ecx, eax; sub ecx, 4; mov ecx, [ecx] mov [eax], ecx; // Copy value over sub eax, 4; // Go to next dec edx; // decrement edx jnz ParamOneShiftLoop; // Loop ParamOneNoShift: // Check if any parameters should be stored in edx movzx ecx, paramInfo.ParameterTwoOffset; cmp ecx,0; je NoRegister; movzx ecx, paramInfo.ParameterTwoOffset; // Move the parameter two offset to ecx sub ecx, 2; // Increment the offset by two. One extra for since we already shifted for ecx mov eax, 4; // Put 4 in eax mul ecx; // Multiple by 4 // Copy the value from the stack on to the register. mov ecx, esp; // Move the stack pointer to ecx add ecx, eax; // Subtract the offset. mov eax, ecx; // Store in eax for later. mov ecx, [ecx]; // Derefernce the value mov paramTwo, ecx; // Store the value in memory. // Fix up stack add esp,4; // Decrement the stack pointer movzx edx, paramInfo.ParameterTwoOffset; // Move the parameter two offset to ecx dec edx; // Decrement by 1 cmp edx,0; // Compare offset with zero je NoRegister; // If first parameter then no shift. ParamTwoShiftLoop: mov ecx, eax; sub ecx, 4; mov ecx, [ecx] mov [eax], ecx; // Copy value over sub eax, 4; // Go to next dec edx; // decrement edx jnz ParamTwoShiftLoop; // Loop NoRegister: mov ecx, paramOne; // Copy value from memory to ecx register mov edx, paramTwo; // push retAddress; jmp functionAddress; } } </code></pre> <p>}</p>
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