Well, that's dependant on if your talking about Vertical as opposed to Horizontal Monitor Placement in the Original Cab. Donkey Kong and Galaxians for instance were 'Vertically' placed and there is a Setting in the Config for this. However if your meaning you have a 16:9 Monitor, again, there is a cfg Setting where the Image can be 'Stretched' to fill the screen, however, the Aspect Ration will then be incorrect (most Arcade Cabs were released before 16:9 Monitors). When using Emulators I prefer having the Original A/R Correct (to prevent Image distortion), but its a personal choice. AdvanceMAME has many more Settings for changing these and other features (it was specifically designed for building home Cab's) but its performance is inferior to puddings MAME4ALL which has been specifically optimised for the RPi.
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