[Coco] Re: Technical: PIA Help

[Brad Grier]

Hi Robert,

I reached my conclusions by stepping through Mocha in debug mode. I also 
used the debugger in Jeff Vavasour's emulator (the game doesn't work 
quite right there either - it does work in David Keil's emulator). I'm 
surprised it doesn't work in MESS.

The strobing appears to be in the interrupt routine (both the write to 
$ff02 and the read of $ff00). The routine starts at $c4fe - you don't 
have to go far to see the *first* write/read of $ff02 and $ff00.

By the way, what is RTSI?

Brad



Robert Gault wrote:

> You correct in your understand of the keyboard strobing. As to the game, 

> my first question would be how do you know what the game is doing? Have 

> you disassembled the code or are you monitoring the emulated registers 

> with a built-in debugging routine.

> 

> There is a Tetris ROM image on RTSI which is 16K and works with a Coco3 

> in high res graphics and a Coco1/2 in low res graphics. A very very 

> quick scan through the code did not find the $FF02 strobing. Does the 

> store $FF02 with $FF occur during the interrupt, or is $FF00 read during 

> the interrupt, or both?

> 

> By the way, this image does not run correctly in MESS. Key presses are 

> detected but the game responds as though several wrong keys were 

> pressed. I'm not sure how or if I can test it on a real Coco without a 

> ROM pack.

> 

> Brad Grier wrote:

> 

>> Hello all,

>>

>> I'm trying to get Tetris to work in Mocha but I'm having trouble 

>> getting the emulator to recognize key presses in the game. This is the 

>> only program I've found that exhibits this behavior.

>>

>> My understanding of the CoCo PIA keyboard routine is as follows: Write 

>> a zero to the bit that corresponds to the keyboard column you're 

>> interested in (with the rest of the bits set to 1) and poke it into 

>> 0xff02. Next read 0xff00 and if a key is pressed you'll get a zero in 

>> the bit corresponding to the keyboard row.

>>

>> Tetris seems to do all of it's keyboard polling from a field sync 

>> interrupt routine. It's constantly poking 0xff into 0xff02 - normally 

>> Mocha would ignore the subsequent read at 0xff00 because no bits are 

>> zero. I've found that if I treat 0xff as a request to see if *any* key 

>> is down regardless of column, I can at least get some keys to kind of 

>> work. Maybe it all boils down to what does a 0xff into 0xff02 really 

>> do? I'm sure I'm overlooking something obvious...

>>

>> Anyway, I've been making a lot of changes to Mocha recently and this 

>> one is last on the list for the time being. That, and I like Tetris.

>>

>> Thanks,

>>

>> Brad

>> http://members.cox.net/javacoco/

>>

>>

>