(HP50g) HpPascal and contrast of the screen

[DavidM]

The document you referenced seems to describe a different protocol than the examples provided. Whereas the documentation talks about data register D0 being used to designate storage locations that are set for function arguments and return values, the examples appear to expect a pointer to the parameter being passed in R1a. Is it possible the document provided is for a different version of HP Pascal? Or perhaps they are both right, but simply used in different contexts (possibly when only one argument/return value is needed?). In that case, it might make sense to mimic the methods used in the examples you provided (R1a holding a pointer to the passed argument/return value).

I believe it should be possible to do what you're wanting, probably with a fairly simple asm routine if the protocol can be verified.

Also, unless I'm misunderstanding something (highly possible!), it appears that the "Contraste" function writes the low-order 4 bits of the supplied byte to address location 101 and then simply clears the hi bit (for contrast anyway) at location 102. This would imply that the function only succeeds at writing (param MOD 16) to the contrast I/O location. Is this something that you have noticed, or am I simply misunderstanding what it's doing?