Noldor from CPC Zone has posted some clever source to automatically decrypt Appleby Associates Amstrad CPC tape loader (a.k.a. the “GO! Loader” as it first appeared on U.S. Gold “GO!” branded games). Outwardly these loaders are characterised by a three digit counter smoothly counting down to zero, usually in the bottom left corner of the screen. Examples of games that use this system are A.T.F., Rygar, Strider, Rygar, Forgotten Worlds, but there are many more.
Internally there are a long series of encryption loops that need to be decrypted one by one in order to gain access to the loading code itself. While not a particularly tricky exercise, it can be a very time consuming one as there are perhaps as many as 50 loops that need to be executed in order to fully decrypt the loader. I know this from personal experience as I had to do this painful stepwise decryption process for a number of my Amstrad CPC game cheats !
Fortunately, while there may be many loops, there are only a few distinct types of loop so it would be possible to create an automatic system to detect the type of loop, run it, detect the type of the next loop, run that and so on until decryption is complete.
Noldor’s source code does just this and so automates the decryption process for this type of loader. Hurrah! Now, where were you in 1989 when this would have been very useful !?