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Clarifications for VGA text buffer format

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Philipp Oppermann
2019-07-06 11:15:28 +02:00
parent 85b83aed32
commit 67e780691c
1 changed files with 7 additions and 2 deletions
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blog/content/second-edition/posts/03-vga-text-buffer/index.md
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@@ -31,7 +31,12 @@ Bit(s) | Value
12-14 | Background color 12-14 | Background color
15 | Blink 15 | Blink
The following colors are available: The first byte represents the character that should be printed in the [ASCII encoding]. To be exact, it isn't exactly ASCII, but a character set named [_code page 437_] with some additional characters and slight modifications. For simplicity, we proceed to call it an ASCII character in this post.
[ASCII encoding]: https://en.wikipedia.org/wiki/ASCII
[_code page 437_]: https://en.wikipedia.org/wiki/Code_page_437
The second byte defines how the character is displayed. The first four bits define the foreground color, the next three bits the background color, and the last bit whether the character should blink. The following colors are available:
Number | Color | Number + Bright Bit | Bright Color Number | Color | Number + Bright Bit | Bright Color
------ | ---------- | ------------------- | ------------- ------ | ---------- | ------------------- | -------------
@@ -44,7 +49,7 @@ Number | Color | Number + Bright Bit | Bright Color
0x6 | Brown | 0xe | Yellow 0x6 | Brown | 0xe | Yellow
0x7 | Light Gray | 0xf | White 0x7 | Light Gray | 0xf | White
Bit 4 is the _bright bit_, which turns for example blue into light blue. Bit 4 is the _bright bit_, which turns for example blue into light blue. For the background color, this bit is repurposed as the blink bit.
The VGA text buffer is accessible via [memory-mapped I/O] to the address `0xb8000`. This means that reads and writes to that address don't access the RAM, but directly the text buffer on the VGA hardware. This means that we can read and write it through normal memory operations to that address. The VGA text buffer is accessible via [memory-mapped I/O] to the address `0xb8000`. This means that reads and writes to that address don't access the RAM, but directly the text buffer on the VGA hardware. This means that we can read and write it through normal memory operations to that address.