Horiztonal Movement Code Question

[Horiztonal Movement Code Question]

11 minutes ago, NoLand said:

This is in the 6502 Hardware manual, http://www.bitsavers.org/components/mosTechnology/6500-10A_MCS6500hwMan_Jan76.pdf

 

What I do not really know, is how long the TIA takes to latch the signal. I recall somewhat faintly that it may be as much as 2 TIA clocks. But I wouldn't even know where to look this up.

Thanks for the manual link NoLand.  I just recently bought Lance Leventhal's 6502 book on ebay, but I'm not too far in yet. 

Regarding the latch, in the atari course I'm taking, I don't recall that being mentioned yet thus I have no idea of it's importance at this time.

[Horiztonal Movement Code Question]

Well, NoLand, that was fantastic.  I wish I would have explored the clocks, I thought about it, but I was hunting a five and let the opportunity pass.  Much appreciated NoLand. 

[Horiztonal Movement Code Question]

I'm having a challenge understanding the divide portion of this code.

 

        LDA PlayerXPos
        AND #$7F
        
        STA WSYNC
        STA HMCLR
        
        SEC
DivideLoop:
	SBC #15
        BCS DivideLoop
        
        EOR #7
        ASL
        ASL
        ASL
        ASL
        STA HMP0
        STA RESP0
        STA WSYNC
        STA HMOVE

 

The video I watched stated that you wanted the remainder for the "fine" positioning, which I believe is -8 to +7.  If I use the value 20 for PlayerXPos, I expect a remainder of 5, as 15 goes into 20 one time with a remainder of 5.  After doing the exclusive or, and the four left shifts, I end up with the value 0001 for the upper 4 bits, not five.  If I worked from 30, and subtract 10 to arrive at 20, then that's out bounds of the negative range, thus it would have to be 15 + the remainder 5.

 

I don't yet know why the one's complement is taken of the lower three bits, nor where my thinking is flawed, but it's flawed alright.