From:	Matt Craighead <craighea AT citilink DOT com>
Newsgroups:	comp.os.msdos.djgpp
Subject:	Virtual Screens with DJGPP
Date:	Sat, 16 Nov 1996 12:58:09 -0800
Organization:	(missing)
Lines:	32
Message-ID:	<328E2AE1.2EA@citilink.com>
Reply-To:	craighea AT citilink DOT com
NNTP-Posting-Host:	ppp31.citilink.com
Mime-Version:	1.0
To:	djgpp AT delorie DOT com
DJ-Gateway:	from newsgroup comp.os.msdos.djgpp

I'm writing a program that uses a virtual screen to eliminate screen
flicker when something changes.  So what's the best way to update the
screen?  (640x480x256, supporting VESA 1.x)  This is what I'm doing,
essentially:

#define set_bank(bank) // I forget the two interrupts... put them in
here
#define FULL_BANKS 4
#define REMAINING_BYTES (640*480 - FULL_BANKS*65536)

void screen_update(void)
{
    int bank;
    __dpmi_regs regs;

    for (i = 0; i < FULL_BANKS; i++) {
        set_bank(bank);
        dosmemput(&virtual_screen[0] + 65536*bank, 65536, 0xA0000);
    }
    set_bank(FULL_BANKS);
    dosmemput(&virtual_screen[0] + 65536*4, REMAINING_BYTES, 0xA0000);
}

This seems to run in about 1/75 of a second on my Pentium 133 with an
STB Vision PCI video card, which is going to be fine for that, but for
slower computers, it starts to get pretty bad, 1/9 of a second on my
486-33 with an ISA video card, and I want to support down to that
level.  Is there some fast way to do this without requiring VBE 2.0 or
such?

-- 
Matt Craighead

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