Versions before 6.1 used Borland C/C++, which only supports 16/32-bit Windows, no ppc.

mIRC 2.1a - 6.03 = Borland C/C++
mIRC 6.1 - 6.35 = Visual C++ .NET 7.0
mIRC 7.0 - 7.61(current version) = Visual Studio 2008



https://www.mirc.com/pfaq.html
Quote

Question: Will you be releasing the mIRC source code for public use at some point?
Answer: mIRC is the way I make a living, so it is not currently practical for me to release the source code.


irc.swiftirc.net #msl (mIRC Scripting Language)