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Code Issues Releases Wiki Activity

fixed grammar in linux-bootstrap-3.md, Set up video mode section

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ruthgrace 2015-10-17 21:03:37 -04:00
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Booting/linux-bootstrap-3.md
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@ -120,25 +120,25 @@ which subtracts value of the `HEAP` from the `heap_end` (we calculated it in the
That's all. Now we have simple API for heap and can setup video mode. That's all. Now we have simple API for heap and can setup video mode.
Setup video mode Set up video mode
-------------------------------------------------------------------------------- --------------------------------------------------------------------------------
Now we can move directly to video mode initialization. We stopped at the `RESET_HEAP()` call in the `set_video` function. Next is the call to `store_mode_params` which stores video mode parameters in the `boot_params.screen_info` structure which is defined in the [include/uapi/linux/screen_info.h](https://github.com/0xAX/linux/blob/master/include/uapi/linux/screen_info.h). Now we can move directly to video mode initialization. We stopped at the `RESET_HEAP()` call in the `set_video` function. Next is the call to `store_mode_params` which stores video mode parameters in the `boot_params.screen_info` structure which is defined in [include/uapi/linux/screen_info.h](https://github.com/0xAX/linux/blob/master/include/uapi/linux/screen_info.h).
If we will look at `store_mode_params` function, we can see that it starts with the call to `store_cursor_position` function. As you can understand from the function name, it gets information about cursor and stores it. If we look at the `store_mode_params` function, we can see that it starts with the call to the `store_cursor_position` function. As you can understand from the function name, it gets information about cursor and stores it.
First of all `store_cursor_position` initializes two variables which has type - `biosregs`, with `AH = 0x3` and calls `0x10` BIOS interruption. After interruption successfully executed, it returns row and column in the `DL` and `DH` registers. Row and column will be stored in the `orig_x` and `orig_y` fields from the the `boot_params.screen_info` structure. First of all `store_cursor_position` initializes two variables which have type `biosregs` with `AH = 0x3`, and calls `0x10` BIOS interruption. After the interruption is successfully executed, it returns row and column in the `DL` and `DH` registers. Row and column will be stored in the `orig_x` and `orig_y` fields from the the `boot_params.screen_info` structure.
After `store_cursor_position` executed, `store_video_mode` function will be called. It just gets current video mode and stores it in the `boot_params.screen_info.orig_video_mode`. After `store_cursor_position` is executed, the `store_video_mode` function will be called. It just gets the current video mode and stores it in `boot_params.screen_info.orig_video_mode`.
After this, it checks current video mode and sets the `video_segment`. After the BIOS transfers control to the boot sector, the following addresses are for video memory: After this, it checks the current video mode and sets the `video_segment`. After the BIOS transfers control to the boot sector, the following addresses are for video memory:
``` ```
0xB000:0x0000 32 Kb Monochrome Text Video Memory 0xB000:0x0000 32 Kb Monochrome Text Video Memory
0xB800:0x0000 32 Kb Color Text Video Memory 0xB800:0x0000 32 Kb Color Text Video Memory
``` ```
So we set the `video_segment` variable to `0xB000` if current video mode is MDA, HGC, VGA in monochrome mode or `0xB800` in color mode. After setup of the address of the video segment font size needs to be stored in the `boot_params.screen_info.orig_video_points` with: So we set the `video_segment` variable to `0xB000` if the current video mode is MDA, HGC, or VGA in monochrome mode and to `0xB800` if the current video mode is in color mode. After setting up the address of the video segment, font size needs to be stored in `boot_params.screen_info.orig_video_points` with:
```C ```C
set_fs(0); set_fs(0);
@ -146,16 +146,16 @@ font_size = rdfs16(0x485);
boot_params.screen_info.orig_video_points = font_size; boot_params.screen_info.orig_video_points = font_size;
``` ```
First of all we put 0 to the `FS` register with `set_fs` function. We already saw functions like `set_fs` in the previous part. They are all defined in the [boot.h](https://github.com/0xAX/linux/blob/master/arch/x86/boot/boot.h). Next we read value which is located at address `0x485` (this memory location is used to get the font size) and save font size in the `boot_params.screen_info.orig_video_points`. First of all we put 0 in the `FS` register with the `set_fs` function. We already saw functions like `set_fs` in the previous part. They are all defined in [boot.h](https://github.com/0xAX/linux/blob/master/arch/x86/boot/boot.h). Next we read the value which is located at address `0x485` (this memory location is used to get the font size) and save the font size in `boot_params.screen_info.orig_video_points`.
``` ```
x = rdfs16(0x44a); x = rdfs16(0x44a);
y = (adapter == ADAPTER_CGA) ? 25 : rdfs8(0x484)+1; y = (adapter == ADAPTER_CGA) ? 25 : rdfs8(0x484)+1;
``` ```
Next we get amount of columns by `0x44a` and rows by address `0x484` and store them in the `boot_params.screen_info.orig_video_cols` and `boot_params.screen_info.orig_video_lines`. After this, execution of the `store_mode_params` is finished. Next we get the amount of columns by address `0x44a` and rows by address `0x484` and store them in `boot_params.screen_info.orig_video_cols` and `boot_params.screen_info.orig_video_lines`. After this, execution of `store_mode_params` is finished.
Next we can see `save_screen` function which just saves screen content to the heap. This function collects all data which we got in the previous functions like rows and columns amount etc. and stores it in the `saved_screen` structure, which is defined as: Next we can see the `save_screen` function which just saves screen content to the heap. This function collects all data which we got in the previous functions like rows and columns amount etc. and stores it in the `saved_screen` structure, which is defined as:
```C ```C
static struct saved_screen { static struct saved_screen {
@ -174,7 +174,7 @@ if (!heap_free(saved.x*saved.y*sizeof(u16)+512))
and allocates space in the heap if it is enough and stores `saved_screen` in it. and allocates space in the heap if it is enough and stores `saved_screen` in it.
The next call is `probe_cards(0)` from the [arch/x86/boot/video-mode.c](https://github.com/0xAX/linux/blob/master/arch/x86/boot/video-mode.c#L33). It goes over all video_cards and collects number of modes provided by the cards. Here is the interesting moment, we can see the loop: The next call is `probe_cards(0)` from [arch/x86/boot/video-mode.c](https://github.com/0xAX/linux/blob/master/arch/x86/boot/video-mode.c#L33). It goes over all video_cards and collects the number of modes provided by the cards. Here is the interesting moment, we can see the loop:
```C ```C
for (card = video_cards; card < video_cards_end; card++) { for (card = video_cards; card < video_cards_end; card++) {
@ -182,7 +182,7 @@ for (card = video_cards; card < video_cards_end; card++) {
} }
``` ```
but `video_cards` not declared anywhere. Answer is simple: Every video mode presented in the x86 kernel setup code has definition like this: but `video_cards` is not declared anywhere. Answer is simple: Every video mode presented in the x86 kernel setup code has definition like this:
```C ```C
static __videocard video_vga = { static __videocard video_vga = {
@ -223,13 +223,13 @@ is in the `.videocards` segment. Let's look in the [arch/x86/boot/setup.ld](http
} }
``` ```
It means that `video_cards` is just memory address and all `card_info` structures are placed in this segment. It means that all `card_info` structures are placed between `video_cards` and `video_cards_end`, so we can use it in a loop to go over all of it. After `probe_cards` executed we have all structures like `static __videocard video_vga` with filled `nmodes` (number of video modes). It means that `video_cards` is just a memory address and all `card_info` structures are placed in this segment. It means that all `card_info` structures are placed between `video_cards` and `video_cards_end`, so we can use it in a loop to go over all of it. After `probe_cards` executes we have all structures like `static __videocard video_vga` with filled `nmodes` (number of video modes).
After `probe_cards` execution is finished, we move to the main loop in the `set_video` function. There is infinite loop which tries to setup video mode with the `set_mode` function or prints a menu if we passed `vid_mode=ask` to the kernel command line or video mode is undefined. After `probe_cards` execution is finished, we move to the main loop in the `set_video` function. There is an infinite loop which tries to set up video mode with the `set_mode` function or prints a menu if we passed `vid_mode=ask` to the kernel command line or video mode is undefined.
The `set_mode` function is defined in the [video-mode.c](https://github.com/0xAX/linux/blob/master/arch/x86/boot/video-mode.c#L147) and gets only one parameter, `mode` which is the number of video mode (we got it or from the menu or in the start of the `setup_video`, from kernel setup header). The `set_mode` function is defined in [video-mode.c](https://github.com/0xAX/linux/blob/master/arch/x86/boot/video-mode.c#L147) and gets only one parameter, `mode`, which is the number of video modes (we got it from the menu or in the start of `setup_video`, from the kernel setup header).
`set_mode` function checks the `mode` and calls `raw_set_mode` function. The `raw_set_mode` calls `set_mode` function for selected card i.e. `card->set_mode(struct mode_info*)`. We can get access to this function from the `card_info` structure, every video mode defines this structure with values filled depending upon the video mode (for example for `vga` it is `video_vga.set_mode` function, see above example of `card_info` structure for `vga`). `video_vga.set_mode` is `vga_set_mode`, which checks the vga mode and calls the respective function: The `set_mode` function checks the `mode` and calls the `raw_set_mode` function. The `raw_set_mode` calls the `set_mode` function for the selected card i.e. `card->set_mode(struct mode_info*)`. We can get access to this function from the `card_info` structure. Every video mode defines this structure with values filled depending upon the video mode (for example for `vga` it is the `video_vga.set_mode` function. See above example of `card_info` structure for `vga`). `video_vga.set_mode` is `vga_set_mode`, which checks the vga mode and calls the respective function:
```C ```C
static int vga_set_mode(struct mode_info *mode) static int vga_set_mode(struct mode_info *mode)
@ -265,11 +265,11 @@ static int vga_set_mode(struct mode_info *mode)
} }
``` ```
Every function which setups video mode, just calls `0x10` BIOS interrupt with certain value in the `AH` register. Every function which sets up video mode just calls the `0x10` BIOS interrupt with a certain value in the `AH` register.
After we have set video mode, we pass it to the `boot_params.hdr.vid_mode`. After we have set video mode, we pass it to `boot_params.hdr.vid_mode`.
Next `vesa_store_edid` is called. This function simply stores the [EDID](https://en.wikipedia.org/wiki/Extended_Display_Identification_Data) (**E**xtended **D**isplay **I**dentification **D**ata) information for kernel use. After this `store_mode_params` is called again. Lastly, if `do_restore` is set, screen is restored to an earlier state. Next `vesa_store_edid` is called. This function simply stores the [EDID](https://en.wikipedia.org/wiki/Extended_Display_Identification_Data) (**E**xtended **D**isplay **I**dentification **D**ata) information for kernel use. After this `store_mode_params` is called again. Lastly, if `do_restore` is set, the screen is restored to an earlier state.
After this we have set video mode and now we can switch to the protected mode. After this we have set video mode and now we can switch to the protected mode.