Date   : Mon, 12 Sep 1994 11:33:38 BST
From   : David Alan Gilbert <gilbertd@...>
Subject: Re: port &FE60 output

> thanks to all for your help about &FE60. in snapper there's the code
> 
> loop:
>   LDA &FE60
>   CMP &FE60
>   BEQ loop
> 
> now either the output must change (which it can't do unless something is
> attached to the user port OR programmed in some weird way) OR it shouldn't
> reach this bit of code. i've got snapper to work by poking random values into
> this location but it's hit and miss. also the game seems very fast as Alan
> Hart mentioned from his experiences. maybe a value of &FF will bypass
> the above code. does the original snapper run on a Master?

Hmm..There are two things which strike me about this:
  1) One of the timers in the VIA can be made to toggle PB7 (or is it PB6) every
     time the timer reaches 0 - is it possible that what the programmer has done
     is to configure the timer into this mode, and then wait for PB7 to flip?
But this
     does seem an odd way of doing it.

  2) Has the port been configured as an output somewhere - in which case
reading FE60
     would be reading the last written value - which I suppose could be done
by something
     running off an interrupt somewhere. Also is there a second place that
can be used for
     writing Port B (I know there is for Port A - I think its FE6F - but
I didn't think
     there was for Port B).

Dave (gilbertd@...)