The LCDGameModes RISC OS Module written by Steve Harrison (of QTM fame) is a small module that is targeted at VIDC1a based Acorn RISC computers. Its job is to patch the screen modes that games use on the fly, in order that they work correctly with a VGA or SVGA monitor.
As you can see below, Gribbly's Day Out does not work correctly when attempting to play it on a VGA monitor and it produces a garbled screen display.
Once LCDGameModes has been loaded, it intercepts all mode changes and determines if they require patching in order that they display correctly on a VGA style monitor. In the photo below, you can see Gribbly's Day Out running but this time, the screen is not garbled as it was previously thanks to LCDGameModes patching the VIDC registers.
LCDGameModes not only makes games work correctly with a (S)VGA monitor, but it also attempts to fix the aspect ratio of the game that is being played. This is particularly true for games that already display on (S)VGA monitors but don't necessarily have (S)VGA awareness built in. Zarch as pictured below is one of these games. The screen mode Zarch uses is a standard RISC OS mode and as such, RISC OS 3 is able to re-map it to one that works with a VGA monitor.
Although the RISC OS based re-mapping allows the game to work, the aspect ratio is nowhere near what it should be. This phenomenon is well known as the "letterbox effect" on RISC OS 3. LCDGameModes overrides the RISC OS re-mapping process with settings which more closely match the original aspect ratio of the game as it would have appeared on a 15kHz TV/monitor. The result is a physical screen display which is the same height as the original RISC OS letterbox mode, but a slightly smaller width - to achieve the 4:3 ratio. It is normal that the LCDGameModes screen doesn't completely fill your (S)VGA monitor, so for the best result you may have to adjust your monitor's V-size and H-size settings to maximise the screensize.
As you can see in the comparison below, the LCDGameModes display is much closer to the intended 4:3 aspect ratio that the game should be played in. When using an Archimedes that supports different Sync. Polarity settings such as the A5000 and A30x0 machines, the aspect ratio is exact.