[A83] Re: What does port 4 do?

To: assembly-83@lists.ticalc.org
Subject: [A83] Re: What does port 4 do?
From: Tijl Coosemans <tijl@ulyssis.org>
Date: Wed, 18 Dec 2002 16:10:19 +0100
In-Reply-To: <13EECC26-0EE9-11D7-B4BC-0030657AAC4A@charter.net>
References: <13EECC26-0EE9-11D7-B4BC-0030657AAC4A@charter.net>
Reply-To: assembly-83@lists.ticalc.org

On Fri, 13 Dec 2002 16:20:25 -0600
Grant Miller <iluvmimac@charter.net> wrote:

> More specifically, what does outputting a value of 77h to it do?
> 
> If you look at the beginning of the calculator's rom on the 83+,
> you'll notice that first it reads from port 2, and checks to see if
> its running on an SE.  If it isn't, it maps rom page 1f (the boot
> page) to 4000-7FFF, and then outputs 77h to port 4.  I'm guessing this
> does something to the memory mapping on the calculator, because
> immediately after that, it jumps 80d5.  Does anybody know what this
> really does? When I try it, it crashes my calculator.
> 
> Does anybody here know anything about this?  Thanks for any help
> anyone can provide.

Setting bit 0 of this port locks every calculator from the 82 to the 86,
but nobody knows what it actually does. Bit 1 and 2 set the frequency of
the timer. 11 is slowest, 00 is fastest. I've no idea about the other
bits.

References:

[A83] What does port 4 do?

From: Grant Miller <iluvmimac@charter.net>