Re: [RFC] Multiboot ammendment: non-VBE video

["Vladimir 'φ-coder/phcoder' Serbinenko" <phcoder@gmail.com>]

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Robert Millan wrote:
>
>> Bootloader will use a mode it
>> chooses. Perhaps we should remove recommended mode fields from the spec
>> altogether or make them somehow optional
>>     
>
> Is that important?  I'm hesitant to do that untill we have better understanding
> on what lead to this decision.
>   
>>>   
>>>       
>> It is useful. It allows payload to say: "I'm ok with video mode but
>> prefer text one"
>>     
>
> Oh, I think I missunderstood what "EGA-standard text mode" means.  Is this
> the same text mode that survived to this day, and is known in GRUB codebase
> as "vga_text"?
>
>   
Yes
>>> Perhaps it would be simpler for us to arrange it so that flags 11 and 12
>>> can't be used at the same time.  We could just say that flag 11 is reserved
>>> and unused, and then put these declarations in the offset that used to be for
>>> VBE.  IIRC there were no possible sections after VBE, so the Framebuffer
>>> section size wouldn't be constrained by that.
>>>
>>> But if you prefer to keep flag 11 operational, I don't object.  I would
>>> probably object to GRUB implementing it though.
>>>
>>>   
>>>       
>> If payload wants to use VBE it will. I think that providing this
>> informtion saves payload a trip to real mode. So this way we encourage
>> saner real-mode-free OS design.
>>     
>
> I'd rather encourage them to use modern flag 12 instead.  In any case, it
> doesn't to keep flag 11 as part of the standard as long as it's optional.
> I'm still not convinced GRUB should implement it, but that's a separate
> discussion.
>
>   
Flag 12 doesn't allow to e.g. change the resolution or do the double
buffering.
>> Do you think it would be useful to make flag 12 structure describe EGA
>> text too? I attach new diff for multiboot.texi. If it's approved it
>> should be simple to implement in grub2 and example kernel
>>     
>
> You forgot to attach it.
>
>   
Fixed


-- 
Regards
Vladimir 'φ-coder/phcoder' Serbinenko


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=== modified file 'doc/multiboot.texi'
--- doc/multiboot.texi	2010-01-01 20:02:24 +0000
+++ doc/multiboot.texi	2010-01-04 19:32:13 +0000
@@ -477,9 +477,7 @@
 
 All of the graphics fields are enabled by flag bit 2. They specify the
 preferred graphics mode. Note that that is only a @emph{recommended}
-mode by the OS image. If the mode exists, the boot loader should set
-it, when the user doesn't specify a mode explicitly. Otherwise, the
-boot loader should fall back to a similar mode, if available.
+mode by the OS image. Boot loader may also choose another mode if it sees fit.
 
 The meaning of each is as follows:
 
@@ -488,7 +486,9 @@
 Contains @samp{0} for linear graphics mode or @samp{1} for
 EGA-standard text mode. Everything else is reserved for future
 expansion. Note that the boot loader may set a text mode, even if this
-field contains @samp{0}.
+field contains @samp{0}. If video adapter doesn't support EGA text mode or
+bootloader doesn't know how to initialise this mode it may set video
+mode even if field contains @samp{1}
 
 @item width
 Contains the number of the columns. This is specified in pixels in a
@@ -635,6 +635,15 @@
 84      | vbe_interface_off |
 86      | vbe_interface_len |
         +-------------------+
+88      | framebuffer_addr  |    (present if flags[12] is set)
+96      | framebuffer_pitch |
+100     | framebuffer_width |
+104     | framebuffer_height|
+108     | framebuffer_bpp   |
+109     | framebuffer_type  |
+110-115 | color_info        |
+        +-------------------+
+
 @end group
 @end example
 
@@ -911,9 +920,7 @@
 @uref{http://www.microsoft.com/hwdev/busbios/amp_12.htm, Advanced Power
 Management (APM) BIOS Interface Specification}, for more information.
 
-If bit 11 in the @samp{flags} is set, the graphics table is available.
-This must only be done if the kernel has indicated in the
-@samp{Multiboot Header} that it accepts a graphics mode.
+If bit 11 in the @samp{flags} is set, the @sc{vbe} table is available.
 
 The fields @samp{vbe_control_info} and @samp{vbe_mode_info} contain
 the physical addresses of @sc{vbe} control information returned by the
@@ -935,6 +942,44 @@
 Multiboot boot loaders may simulate @sc{vbe} on non-@sc{vbe} modes, as
 if they were @sc{vbe} modes.
 
+If bit 12 in the @samp{flags} is set, the @sc{Framebuffer} table is available.
+
+The field @samp{framebuffer_addr} contains framebuffer physical address. This field is 64-bit wide but bootloader @dfn{should} set it under 4GiB if possible for compatibility with payloads which aren't aware of PAE or amd64. The field @samp{framebuffer_pitch} contains pitch in bytes. The fields @samp{framebuffer_width}, @samp{framebuffer_height} contain framebuffer dimensions in pixels. The field @samp{framebuffer_bpp} contains number of bits per pixel. If @samp{framebuffer_type} is set to 0 it means indexed color. In this case color_info is defined as follows:
+@example
+@group
+        +----------------------------------+
+110     | framebuffer_palette_addr         |
+114     | framebuffer_palette_num_colors   |
+        +----------------------------------+
+@end group
+@end example
+@samp{framebuffer_palette_addr} contains address of palette. Palette is an array of colour descriptors. Each colour descriptor has following structure:
+@example
+@group
+        +-------------+
+0       | red_value   |
+1       | green_value |
+2       | blue_value  |
+        +-------------+
+@end group
+@end example
+If @samp{framebuffer_type} is set to 1 it means direct RGB color. Then color_type is defined as follows:
+
+@example
+@group
+        +----------------------------------+
+110     | framebuffer_red_field_position   |
+111     | framebuffer_red_mask_size        |
+112     | framebuffer_green_field_position |
+113     | framebuffer_green_mask_size      |
+114     | framebuffer_blue_field_position  |
+115     | framebuffer_blue_mask_size       |
+        +----------------------------------+
+@end group
+@end example
+
+If @samp{framebuffer_type} is set to 2 it means EGA text. In this case @samp{framebuffer_width} and @samp{framebuffer_height} are expressed in characters and not in pixels. @samp{framebuffer_bpp} is equal 16 (16 bits per character) and @samp{framebuffer_pitch} is expressed in bytes per text line.
+All further values of @samp{framebuffer_type} are reserved for future expansion
 
 @node Examples
 @chapter Examples


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