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Printing To Screen: Difference between revisions
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0x000b8004: 'L', colour_for_L
0x000b8006: 'l', colour_for_l
</pre>
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For instance, using <tt>0x00</tt> as attribute byte means black-on-black (you'll see nothing). <tt>0x07</tt> is lightgrey-on-black (DOS default), <tt>0x1F</tt> is white-on-blue (Win9x's blue-screen-of-death), <tt>0x2a</tt> is for green-monochrome nostalgics.
For colour video cards, you have
When you print to any other page than 0, it will ''not'' appear on screen until that page is ''enabled'' or ''copied'' into the page 0 memory space.
====Color Table====
{| {{wikitable}}
|-
! Color number
! Color name
! RGB value
! Hex value
|-
| 0
| Black
| 0 0 0
| 00 00 00
|-
| 1
| Blue
| 0 0 170
| 00 00 AA
|-
| 2
| Green
| 0 170 0
| 00 AA 00
|-
| 3
| Cyan
| 0 170 170
| 00 AA AA
|-
| 4
| Red
| 170 0 0
| AA 00 00
|-
| 5
| Purple
| 170 0 170
| AA 00 AA
|-
| 6
| Brown
| 170 85 0
| AA 55 00
|-
| 7
| Gray
| 170 170 170
| AA AA AA
|-
| 8
| Dark Gray
| 85 85 85
| 55 55 55
|-
| 9
| Light Blue
| 85 85 255
| 55 55 FF
|-
| 10
| Light Green
| 85 255 85
| 55 FF 55
|-
| 11
| Light Cyan
| 85 255 255
| 55 FF FF
|-
| 12
| Light Red
| 255 85 85
| FF 55 55
|-
| 13
| Light Purple
| 255 85 255
| FF 55 FF
|-
| 14
| Yellow
| 255 255 85
| FF FF 55
|-
| 15
| White
| 255 255 255
| FF FF FF
|-
|}
==Printing Strings==
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If you have a pointer to video memory and want to write a string, here is how you might do it;
<
// note this example will always write to the top
// line of the screen
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}
}
</syntaxhighlight>
This simply cycles through each character in the string, and copies it to the appropriate place in video memory.
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Just like in any environment, you repeatedly divide the value by the base, the remainder of the division giving you the least significant digit of the value.
For example, since 1234 = 4 + 3* 10 + 2 * 100 + 1* 1000, if you repeatedly divide "1234" by ten and use the
<pre>
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Here is an example implementation of the itoa() function (which is not standard, but provided by many libraries):
<
char * itoa( int value, char * str, int base )
{
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return rc;
}
</syntaxhighlight>
[http://www.strudel.org.uk/itoa/ And here is a shorter one]
== Troubleshooting ==
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=== Nothing is Displayed ===
Keep in mind that this way of writing to video memory will
([[GRUB]] does this setup for you.)
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=== Missing Strings ===
Sometimes printing individual characters works, but printing strings fails. This is usually due to the <tt>.rodata</tt> section missing in the linker script.
Previously, GCC had an option <tt>-fwritable-strings</tt> which could be used as a workaround for this, but it was deprecated in version 3.0 and removed in 4.0 and later, which was released in 2005. Even when the option was available, it was a kludge; the real solution was, and still is, to add <tt>.rodata</tt> to the script.
==See Also==
*[[Printing to the screen without a db]]
[[Category:Tutorials]]
[[Category:Video]]
[[Category:Text UI]]
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