@Piru
Thanks, installing the update has sorted the problem - screenmodes are now appearing.
Forgot to mention the configuration - OS3.9 BB2 + Cybergraphics 4.
I installed cybergraphics 4 on a fresh OS3.9 installation, and had no problems choosing a screenmode then.
I guess this is the "exec.library version == v40" check, and that oine of the boing bags updates exec.library beyond v40?
@Keropi
I couldn't even test the screenmodes - just got an error message stating "could not open test screen". Workbench managed to boot up on a 16-bit cgx screen though, and both cgxmode and showcgxconfig returned expected values.
Oh well, problem solved. Thanks all.
Rich