[Open-graphics] bootstrapping the video mode (outputting VGA text)

[James Richard Tyrer]

Hamish Marson wrote:
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> James Richard Tyrer wrote:
>> Timothy Miller wrote:
>>> On 8/8/06, James Richard Tyrer <tyrerj at acm.org> wrote:
>>>
>>>> Note that mode 1 only has 350 or 400 vertical lines in the VGA
>>>> emulation
>>>> modes and the default vertical is 70 Hz with 900 total lines.  How
>>>> would
>>>> our TV out handle this?  OTOH, real CGA character mode (which is
>>>> in the
>>>> IBM VGA spec) will work on a TV since it is non-interlaced with 200
>>>> visible lines and 262 total lines at vertical 60 Hz  .
>>> Keep in mind that we have a disconnect between what the host thinks
>>> VGA is doing and what we're actually scanning out of the video
>>> controller.  The VGA emulator program converts continuously from one
>>> to the other.  Thus, we can have any physical resolution that we want.
>>> For the TV, we configure the video controller appropriately and set
>>> up the converstion program to handle the appropriate size of the
>>> output framebuffer.
>>>
>>> Also, if you're wondering how to tell if there's a TV on there, you
>>> just don't need to.  Head 0 has the analog DACs, useful for a
>>> multisync monitor.  We'll configure that via DDC (typically).  Head 1,
>>> IIRC, is connected to the TV chip.  We'll just configure the TV
>>> according to the region (different BIOS images?) on head 2.  Then we
>>> set up the two (independent) video controllers to scan out the same
>>> framebuffer at two different resolutions (one or both will use the INC
>>> instruction in the vc to skip the extra space), and viola!
>> Problem!  Which you missed.
>>
>> In video mode 1, (with a real VGA) the VGA output needs to be either
>> a 350 or 400 lines image while the TV output needs to be 200 lines.
>> How do we handle this?  Do we have the VGA output 480 lines and the
>> TV output 240 lines?
>>
> 
> On PAL that would be (IIRC) 256 lines.

Well that is another issue isn't it.  VGA is based on NTSC.  How is it 
displayed on PAL?

-- 
JRT