According to the help file: ProSystem Emulator Documentation Command Line switches The following list the command line switches allowed in ProSytem. If loading a rom file on the command line, it must be the last parameter in the list. -Fullscreen value Use value = 1 for fullscreen Use value = 0 for windowed -MenuEnabled value Use value = 1 to enable the menu bar Use value = 0 for no menu bar -Palette filename filename is the palette to use. If the name contains spaces, be sure to surround it with quotes -Zoom value Value can be 1 to 4 to increase the window size -Mute value Use value = 1 to mute the sound Use value = 0 to enable sound -Latency value Set to desired Sound Latency -SampleRate value Set to desired Sound Sample Rate -Region value Use value = PAL for PAL Use value = NTSC for NTSC Use value = AUTO for Auto-Detect Example ProSystem -Fullscreen 0 -MenuEnabled 1 C:\centipede.a78 This will start ProSystem in windowed mode, with the menu bar enabled, and with C:\centipede.a78 as the rom to load. So I would personally use the suggested command-line in GameEx. This will set the emulator fullscreen as well as hide the menubar. ProSystem.exe -Fullscreen 1 -MenuEnabled 0 "[ROMPATH]\[ROMFILE]"