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Writing to a different video page

Posted on 1997-03-17
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Last Modified: 2008-03-10
How do I write to page 1, for instance, in text mode using video mode 3 and directly accessing the video memory?
In this mode video memory starts on 0xB800 I guess. But, if I write from this address I'll be writing to page 0 (as set by INT 10, AH=0x05). Documentation says there are four [0-3] available pages in this mode. I switched to page 1 using INT 10 - AH=0x05, but now how do I write to it? Shall I add an offset of 80X25 = 2000?
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Question by:pneves
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indian earned 5 total points
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INT 10 - function 05h actives the page you want to see in the screen not the page you want to write to, so of course you must add an offset (base adress is always B800h), but this is not 2000 but 4000 because each cell is represented by two bytes (ASCII code and ATTRIBUTE).

This is the formula in 80x25 mode:

offset = P * PageSize + Y * 160 + 2 * X + B

where P is the page (0-3).
      B = 0 if you want to write the ASCII code.
        = 1 if you want to write the attribute.
      PageSize = 80 x 25 * 2 = 4000.

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