Assembly Calling Convention: __fastcall (Patreon)

Calling Convention: __fastcall

• Calling convention may be specified by the developer or added by the compiler as an optimization
• The Borland compiler (Delphi) uses a different version of __fastcall as the default calling convention
• Microsoft: first two DWORD arguments are passed in
  ECX and EDX (left-to-right)
• Borland: first three DWORD arguments are passed in
  EAX, EDX and ECX (left-to-right)
• Callee is responsible for stack cleanup

MSDN __fastcall documentation 

Practice Examples

Attached to this post is a copy of the HelloWorld PE file from our tutorial. You are encouraged to open this sample in IDA and identify the function calling conventions, pay close attention to how the variadic function is handled.