It uses the IPU for rendering games now. Normally on the GCW0, it's Power+A.
However, there is no such thing on the RS-90 and i'm not even sure that it's implemented anywhere... (i don't know the button shortcut myself)
You will probably have to wait for an update if it's not implemented.
It looks great for SMS but some GG games look terrible at full screen. What is the latest version that has scaling options?
GPD Win, GPD XD 64gb, PlayGo, RS-90, 3DS XL, DSi XL, GBA SP, GBBC Clone, Gameboy Pocket c/w screen mod, PSP Go
Medion Erazer, Toshiba Z20t, Dell Mini 9, Psion 5MX
Tons of other old laptops and tablets.....