On 7/28/06, James Richard Tyrer <[EMAIL PROTECTED]> wrote:
This is how CGA and EGA modes work when you use the Video BIOS to write text in a graphics mode. Don't recall if VGA modes do this but they probably do something similar. But the problem comes up if the user modifies the character maps in RAM. I think that there are Video BIOS calls to do this, so it might still work.
It's not quite the same. For one thing, far too many DOS programs (including command.com) bypass the interrupts and just write to the text screen directly. The VGA BIOS I wrote was meant to try to do 80x25 text mode on a graphics card that didn't have VGA hardware, so I hooked interrupts to try to emulate it. Hooking int 0x10 was worthless, since nothing used it, so I hooked the 18Hz timer interrupt and did a periodic update (updating the graphics for characters that changed or everything if the font changed). That worked okay until an OS would start protected mode, after which we would get no more text display. Anyhow, our VGA emulation will be all on-chip, not relying on the host CPU for anything other than setup, so we'll be able to handle font changes just fine. That's why I like the periodic update idea. This way, when someone writes to the screen memory, it just goes to the memory, and we convert it later. _______________________________________________ Open-graphics mailing list [email protected] http://lists.duskglow.com/mailman/listinfo/open-graphics List service provided by Duskglow Consulting, LLC (www.duskglow.com)
