Path: utzoo!utgpu!attcan!uunet!mcvax!ukc!etive!hwcs!neil
From: neil@cs.hw.ac.uk (Neil Forsyth)
Newsgroups: comp.sys.atari.st
Subject: Re: Questions about ST Video Ram
Message-ID: <1931@brahma.cs.hw.ac.uk>
Date: 4 Aug 88 08:25:55 GMT
References: <535@skywest.UUCP>
Reply-To: neil@cs.hw.ac.uk (Neil Forsyth)
Organization: Computer Science, Heriot-Watt U., Scotland
Lines: 43

In article <535@skywest.UUCP> brenes@skywest.UUCP (Erasmo Brenes) writes:

>The problem that I have is that every time I call the XBIOS routine
>SetScreen(logadr,phyadr,rez), where phyadr = &NextScreen[0], the screen jumps 
>horizontally left to right. The information displayed on the screen is
>correct but is skewed to the right 3/4 of the screen (BTW, the program
>is written using MWC 2.x). What I've noticed is that the video buffer that
>the ST starts with is located in a 32K boundary, 0xf8000, while the starting
>location of NextScreen[] is not. It also seems that if a take &NextScreen[0]
>mod 32K, the remainder is the offset that I see on the screen. So, the
>question is: If the Shifter expects the video ram to start at 32K boundaries,
>then how do I get MWC, or any C program to allocate a 32Kb buffer starting in
>a 32Kb boundary in an efficient manner?

The video RAM address must start on a 256 byte boundary. To be sure that your
buffer is on this boundary try this: (Forgive any lack of style)

	if ( (buffer=malloc(32256)) == NULL)	/* Get memory */
		{
		fprintf(stderr, "Not enough memory\n");	/* Or bust */
		exit(-1);
		}

	screen=(buffer+256) & 0xFFFFFF00L;	/* align on boundary by */
						/* by leaping 256 bytes */
						/* into buffer then     */
						/* clearing lowest bits */

If you want loads of screens you just malloc that many times 32000 and add 256
for allignment.

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