Xref: news2.mv.net comp.os.msdos.djgpp:5145 From: matt AT elite DOT powernet DOT co DOT uk (Matt Bernstein) Newsgroups: comp.os.msdos.djgpp Subject: Re: Why is grx20 so slow ? Date: Tue, 18 Jun 1996 17:45:25 GMT Organization: Power Internet Lines: 39 Message-ID: <4q6qap$q8t@power2.powernet.co.uk> References: NNTP-Posting-Host: elite.powernet.co.uk To: djgpp AT delorie DOT com DJ-Gateway: from newsgroup comp.os.msdos.djgpp hector AT leei DOT enseeiht DOT fr (Jean Hector) wrote: >With any driver, the mouse test program MOUSETST is much more slower in >the 16 colors mode than in the 256 colors mode (not possible with stdvga). >Is there a reason ? Is there a solution ? >JH. Not a very good explanation because I've never programmed 16-colour VGA, but whereas 256 colour modes are easily written to with a byte per pixel, VGA memory is organised in a truly hideous way (summat like this) :- PAGE1 BYTE 1 BYTE 2 BYTE 3 ... 11010111 01101110 10110010 PAGE2 BYTE 1 BYTE 2 BYTE 3 ... 01010110 10101001 00011000 PAGE3 BYTE 1 BYTE 2 BYTE 3 ... 00100101 11100110 11011011 PAGE4 BYTE 1 BYTE 2 BYTE 3 ... 00100111 11010101 11101111 The first pixel in this case has the value 0001, the second 0011, the third 1100 and so on... (I hope this makes sense) Because we have to do all this paging nonsense, 16-colour VGA is much slower than 256-c VGA. There may be other factors which slow down the mouse-driver, but this is the obvious one which I can think of. The basic answer is do everything in 640x480x256 minimum... Matt Bernstein (matt AT elite DOT powernet DOT co DOT uk)