If you try and start a 64-bit executable under Windows command.com, it tells you: Code: c:\testlab4>pfddfc.exe This version of C:\testlab4\PFDDFC.exe is not compatible with the version of Windows you're running. Check your computer's system information to see whether you need a x86 (32-bit) or x64 (64-bit) version of the program, and then contact the software publisher. If you do the same under TCC, it merely says: Code: [c:\testlab4]pfddfc.exe 4DOS/NT: "C:\TESTLAB4\PFDDFC.EXE" It would be helpful if TCC gave more complete diagnostics, like command.com, because it may not be immediately obvious that it is not running because of that issue. Suppose you are only running TCC and you try and run the executable. All TCC does is echo the name of the executable. The average user goes "So? Yes, thats the exe, I know that, but why dont you execute it like I asked?" I had to switch to command.exe to find out why it wasnt running.