the ipl is exactly what determines the kernal mode. it is (i think) the first code that is run when the psp is booted so it has to determine its valid before anything else can run. but its more complex than this because how does the psp recognise different ipl's. well, to be able to load an ipl they must have a common piece of code so the psp hardware recognises that it is valid code and then continues to load it (confirm?). its probably like the boot sector on a hard disk - the pc checks for this after the hardware is initialised (bios) and if it doesn't find it it simply doesn't continue. so you have a boot sector for xp and a boot sector for win98 - very different but both are loaded from the same point - its how they load that determines what can and can't run. look at win98 as 1.5 psp firmware and look at xp as 3.71. hope this is clear - its a simple explanation but this is how i understand it. i might add it could be wrong but it fits.