Path: utzoo!utgpu!water!watmath!clyde!att!osu-cis!tut.cis.ohio-state.edu!rutgers!bellcore!tness7!tness1!uhnix1!uhnix2!uace0
From: uace0@uhnix2.uh.edu (Michael B. Vederman)
Newsgroups: comp.sys.atari.st
Subject: Re: Questions about ST Video Ram
Message-ID: <657@uhnix2.uh.edu>
Date: 28 Aug 88 00:50:58 GMT
References: <535@skywest.UUCP> <2884@bath63.ux63.bath.ac.uk> <12@usl-pc.usl.edu>
Reply-To: uace0@uhnix2.UUCP
Organization: University of Houston
Lines: 51

In article <12@usl-pc.usl.edu> jpdres13@usl-pc.UUCP (John Joubert) writes:
>In article <535@skywest.UUCP> brenes@skywest.UUCP (Erasmo Brenes) writes:
>>... 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? An initial thought was to use Malloc()
>>to allocate 64Kb and then only use the 32Kb which start in a 32Kb boundary,
>>but this method is too wasteful. There's got to be a better way, isn't there?
>
>--------------------------
[deleted
 lines of
 text]
>
>Here is how I find a 256 byte boundary to put in a screen.  
>
[ deleted
  lines of
  'C' code ]
>                                            /* On a 256 byte boundary. */
>    work_scr=(char *)( (long)(blk=malloc(32768L)) & 0xffffff00 )+0x0100;
>
[ More deleted stuff ]
>Hope this helps,
>----------------------------------------------------------------------------
>John Joubert                         |     /\  |    /\    |     _ 
>jpdres13@usl-pc.USL   or ...         |     \|<>|>|> \|<>|>|><`|`|
>ut-sally!usl!usl-pc!jpdres13         |-----/|-------/|----------------------
>GEnie: J.JOUBERT                     |     \/       \/
>-----------------------------------------------------------------------------

Dear John,


There is really one little problem with the code you have written, that being if
the block you Malloc is just one byte past a 256 byte boundary, then you will
be wasting 255 bytes, therefore you will need to Malloc at least (32K+255) to
insure that you have at least enough memory to draw a full screen in.  Other-
wise you will be overwriting memory which is not yours (a discussion of which

there has been an abundance of these days :-).

- Mike




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