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From: meekins@tortoise.cis.ohio-state.edu (Tim Meekins)
Newsgroups: comp.sys.apple2
Subject: Re: FTA Modulae
Keywords: Animation
Message-ID: <110011@tut.cis.ohio-state.edu>
Date: 19 Apr 91 20:23:17 GMT
References: <47417@ut-emx.uucp> <109615@tut.cis.ohio-state.edu> <47484@ut-emx.uucp>
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In article <47484@ut-emx.uucp> daveh@ccwf.cc.utexas.edu (Dave Huang) writes:
>In article <109615@tut.cis.ohio-state.edu> meekins@tortoise.cis.ohio-state.edu (timothy lee meekins) writes:
>>I'm blushing right now. You're right. I really meant to say PEI but caught
>>up in the excitement of answering a question I knew and mistakenly typed
>>PEA. Thanks for noticing this before someone else gets really screwed-up
>>trying to figure out what I was doing.
>
>Okie dokie :) But how do you go about mapping the stack and DP to the
>SHR screen? Don't those have to stay in bank $00 while the SHR screen
>is in bank $E1 and/or $01?
>

And for today's installment of Hacking With Tim:

"Mapping the Stack and Direct Page for Sneaky Tricks"

First off, you're right that the stack and direct page reside in bank $00
and the SHR screen reside in bank $E1 and/or $01. So, how can you possibly
map something on bank 0 to bank 1 or $E1? Well, we use a trick that goes back
to the //e architecture (I think). Bank $01 on the IIgs is treated like the
alternate 64K bank on a 128K Apple //e. Since the 6502 cannot perform 24-bit
addressing like a 65816, you can only use one bank or the other. So, there
are a set of soft swicthes that can be set to determine which bank is being
used. IN ADDITION, a soft switch was added to allow the Zero Page to be used
on either bank. So, on a IIgs, we can access this switch (ALTZP) to move
the DP/S to bank $00 or bank $01. In the code I'm showing below, I'll be
using the STATEREG ($C068) to modify ALTZP. You can also use SETSTDZP ($C008)
and SETZLTZP ($C009).

So, in a typical routine, I'd use the following code "header":

	phd		;Save the direct page
	lda	>$E1C068
	ora	#$0030	;Set ALTZP
	sta	>$E1C068
	tsx		;Save the stack in X register

Now all we need to do is point to the SHR area to "modify"

	lda	#$2000+160-2
	tcs
	lda	#$2000+160
	tcd

now to scroll line 1 to line 0

	pei	(158)
	pei	(156)
	pei	(154)

	...

	pei	(0)

and now we can clean everything up:

	lda	>$E1C068
	and	#$FFCF
	sta	>$E1C068
	txs
	pld

If you need to store X somewhere, there are some great screen holes
available to use from $9E00+200 to $9E00+255.

Also, mapping the DP/S can sometimes cause some funny things to happen
to certain resources, so, if you want to make Todd happy, add a SEI/CLI
pair and turn off interrupts before running this code.

So, is this technique good for anything besides scrolling?

YES!!!!!!

For example, in Z3D I map the DP/S to the same scanline and issue PEI's
to shadow the screen. I also clear the screen by mapping the DP then
issuing STZ dp for the scanlines.

If you a particular sequence of bytes you wish to place on the screen,
you can PEA them directly to the screen.

Happy Hacking! And until next time...

-Tim Meekins

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