OS4.0 and AGA video mode @31.5 kHz...

[Matt_H]

WHDLoad doesn't work on OS4.

The way screen promotion works is that NTSC screens are changed to DblNTSC screens and PAL screens are changed to DblPAL screens. In OS4 I think the Dbl modes can replace the 15KHz modes, but in 3.9 the double modes are added alongside the 15KHz modes, meaning WHDLoad can still force a 15KHz screen.

There are a very (very) small number of WHDLoad slaves that support setting Dbl screenmodes, however. Probably less than 4.

[Matt_H]

Most LCD monitors cannot do 50Hz but just use DBLNTSC which is 60Hz

That's a workaround, but since so much old software (particularly games) demands a (Dbl)PAL screen and simply won't work properly with (Dbl)NTSC (apart from just being cut off on the bottom), I'd say it's not a good long-term solution.

[Matt_H]

@ alexh

I was referring to using the 3.9 bootmenu to force a Dbl mode. I assumed it would work the same way as forcing PAL on an NTSC machine, but, granted, I've never tested its functionality.