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62 lines
2.1 KiB
62 lines
2.1 KiB
10 years ago
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*Concepts you may want to Google beforehand: VGA character cells, screen offset*
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**Goal: Write strings on the screen**
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Finally, we are going to be able to output text on the screen. This lesson contains
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a bit more code than usual, so let's go step by step.
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Open `drivers/screen.h` and you'll see that we have defined some constants for the VGA
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card driver and three public functions, one to clear the screen and another couple
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to write strings, the famously named `kprint` for "kernel print"
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Now open `drivers/screen.c`. It starts with the declaration of private helper functions
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that we will use to aid our `kprint` kernel API.
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There are the two I/O port access routines that we learned in the previous lesson,
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`get` and `set_cursor_offset()`.
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Then there is the routine that directly manipulates the video memory, `print_char()`
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Finally, there are three small helper functions to transform rows and columns into offsets
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and vice versa.
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kprint_at
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---------
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`kprint_at` may be called with a `-1` value for `col` and `row`, which indicates that
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we will print the string at the current cursor position.
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It first sets three variables for the col/row and the offset. Then it iterates through
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the `char*` and calls `print_char()` with the current coordinates.
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Note that `print_char` itself returns the offset of the next cursor position, and we reuse
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it for the next loop.
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`kprint` is basically a wrapper for `kprint_at`
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print_char
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----------
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Like `kprint_at`, `print_char` allows cols/rows to be `-1`. In that case it retrieves
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the cursor position from the hardware, using the `ports.c` routines.
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`print_char` also handles newlines. In that case, we will position the cursor offset
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to column 0 of the next row.
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Remember that the VGA cells take two bytes, one for the character itself and another one
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for the attribute.
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kernel.c
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--------
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Our new kernel is finally able to print strings.
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It tests correct character positioning, spanning through multiple lines, line breaks,
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and finally it tries to write outside of the screen bounds. What happens then?
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In the next lesson we will learn how to scroll the screen.
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