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28 lines
1.2 KiB
Markdown
28 lines
1.2 KiB
Markdown
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*Concepts you may want to Google beforehand: I/O ports*
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**Goal: Learn how to use the VGA card data ports**
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We will use C to communicate with devices via I/O registers and ports.
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Open `drivers/ports.c` and examine the inline C assembler syntax. It has
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some differences, like the order of the source and destination operands,
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and the funny syntax to assign variables to operands.
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When you understand the concepts, open `kernel/kernel.c` for an example
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of use.
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In this example we will examine the I/O ports which map the screen cursor
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position. Specifically, we will query port `0x3d4` with value `14` to request
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the cursor position high byte, and the same port with `15` for the low byte.
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When this port is queried, it saves the result in port `0x3d5`
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Don't miss the opportunity to use `gdb` to inspect the value of C variables,
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since we still can't print them on the screen. To do so, set a breakpoint
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for a specific line, `breakpoint kernel.c:21` and use the `print` command
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to examine variables. Aren't you glad now that we invested some time in
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compiling the cross-compiled gdb? ;)
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Finally, we will use the queried cursor position to write a character
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at that location.
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