Date   : Thu, 29 Feb 1996 17:40:33 +0000 (GMT)
From   : James Turner <turnerjf@...>
Subject: Re: Planetoid/Snapper problems

On Thu, 29 Feb 1996, lamcw wrote:

> Tom Seddon wrote:
> 
> ;I believe this has been discussed before on the previous mailing list, 
> ;but I couldn't find a solution to the problem: Snapper doesn't work! 
> ;Apparently this is due to &FE60 not producing values it should--what 
> ;should it do? Should there be random values on PB (as I have now) or 
> s;teady values?

PB0-PB7 (&FE60) are the data bits for the User Port. I think that the 
value you get from reading &FE60 depends on the value of &FE62 (data 
direction register B). DDRB allows each bit to be considered as either an 
input or an output. With a bit in DDRB set to 0, the corresponding bit in 
&FE60 is considered an input. On a real BBC, the inputs would drift high 
with no connection to them (they were, I think, partially TTL compatible). 
Setting a bit in DDRB to 1 sets the corresponding bit in &FE60 to 
an output. In this condition, reading the port shows that bit set to 0, 
as far as I remember.

> ;And also, Planetoid doesn't work. It works for about 2 seconds and then
> ;stops. And, finally, Killer Gorilla doesn't draw any platforms or
> ;levels--most odd.
> 
> Yes, &FE60 affects Snapper and Planetoids. I poke vaguely random values
> into this location and it seems to work okay. The speed of the ghosts
> and their behaviour are determined by this value. Bizarre.

I have absolutely no idea why either Snapper or Planetoids should be 
looking at this location! On most machines, I would imagine that nothing 
is connected to the user port, giving the behaviour described above. With 
something connected, the result of reading and writing could be pretty 
much anything, and would be a bad idea, as for all the program knows, it 
could be messing up some piece of hardware.

> I've tried Snapper with the new graphics (i.e. not the Pacman characters
> but silly little men) and that seems to work better.

I think Acornsoft were forced to change from Pacman's graphics to "silly 
little men" as you call them due to some sort of copyright problem...


-- 
_____James Turner - turnerjf@...                   
       "I need a vacation" - The Terminator
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