I thought that Endi was asking about Spectrum Basic.... Yes, on the EnterMice wiki there are also Basic examples to download if you don't want to type.
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I've not seen a Kempston mouse system in my life, but if it is the mouse who says where on the screen is the pointer, then it will force total linearity to the movement, without velocity at all.
On EnterMice, the driver multiplies by 2 the relative increment if greater than 1, and I think that "modern" PS/2 mice add velocity internally.
What is mouse velocity? You are in front of a PC now, isn't it? if you move slowly the mouse the pointer is displaced precisely across the screen, and you need a big table to reach from corner to corner.
But if you attempt the same distance with rapid shakes of the mouse you will see that you only need a few centimetres to reach the opposite corner. The faster you move the hand the farther the pointer reaches with lower precision.
Of course, that games can be theoretically emulated to use EnterMice without problems if they manage some type of coordinates. But the gameplay will be better than in the original game, I think.