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Posted: **Sun Apr 17, 2016 12:05 pm**

In VGA text mode, 8th bit of attribute byte has two meanings. In real mode, it is possible to toggle between them via Int 0x10/AX=1003h.

I try to do it in protected mode, so no BIOS is available. I think it should be possible to do it via Mode Control register (0x3C0/0x10). I tried to do it, but it resulted in black screen. Here's my code:

Code: Select all

inb(0x3DA); // to index state
outb(0x3C0, 0x10); // register index
// if I stop at this point or beyond then black screen
uint8_t value = inb(0x3C1);
inb(0x3DA);
outb(0x3C0, 0x10);
outb(0x3C0, value & 0xF7); // disable 4th bit (numbering from 1) to go to 16 background colors

Did someone have this problem before? What am I really doing with this code? Am I misinterpreting the documentation? Can someone help me?

Posted: **Sun Apr 17, 2016 1:46 pm**

Hi bokjezd,

When you select an index via register 0x3C0, you'll want to set bit 5 (counting from 0). This means you'll be operating the attribute controller in "normal" mode instead of messing with the internal palette.

For further reference, I recommend FreeVGA.

Posted: **Mon Apr 18, 2016 12:42 am**

Hi!

I just made it work. One thing I didn't know is Palette Address Source bit in Attribute Address Register. I also found some simple bugs after video began working again, but blinking wasn't disabled (spurious return in the middle of my code or extra tilde in my copy to /boot command)

Thanks for pointing that bit out

PS. QEMU doesn't support blinking at all. I had to reboot my physical computer to test this code.

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