Okay, I've had a look and finally figured out the middle argument:
Argument 2: Fractional value, so 7F equates to 0.5.
Argument 3: Decimal value for speed.
This is quite cool, since you can set walk speeds that are inbetween, say, 3 and 4. Unfortunately this leaves argument 1... hmm. As far as I can tell, it's not a bank pair for the two arguments, as it is with most opcodes. Placing values into memory and then attempting to use the first argument as such just doesn't seem to work. I'll have another look later (in the middle of an essay...
).
[Edit: 500th post. Yay
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