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SubjectRe: [OT] Crazy idea: Design open-source graphics chip


Maciej W. Rozycki wrote:
> On Fri, 30 Jan 2004, Timothy Miller wrote:
>
>
>>>Another reason to drop VGA then - money.
>>
>>As soon as PC BIOS's don't require it, we can drop it.
>
>
> No PC BIOS recognizes a VGA. The PC/AT firmware uses int 0x10 to
> communicate with the console and as long as there is a handler there,
> console output works. Most systems will actually run without a handler,
> too, but they'll usually complain to the speaker. The handler is provided
> by the ROM firmware of the primary graphics adapter.
>
> Old PC/AT firmware actually did recognize a few display adapters, namely
> the CGA and the MDA which had no own firmware. These days support for
> these option is often absent, even though the setup program may provide an
> option to select between CGA40/CGA80/MDA/none (the latter being equivalent
> to an option such as an EGA or a VGA, providing its own firmware).
>

You're not entirely correct here. I attempted to write a VGA BIOS for a
card which did not have hardware support for 80x25 text.

I first tried intercepting int 0x10. I quickly discovered that most DOS
programs bypass int 0x10 and write directly to the display memory. As a
result, very little of what should have displayed actually did.

Next, I tried hanging off this timer interrupt. I had two copies of the
text display, "now" and "what it was before". I would compare the
characters and render any differences. This worked quite well for DOS,
but the instant ANY OS switched to protected mode, they took over the
interrupt and all console messages stopped. Actually, the same was true
for int 0x10.

Even just the DOS shell command-line tends to bypass int 0x10 and write
directly to display memory.

Furthermore, 640x480x16 simply won't happen at all without direct
hardware support. Some things rely on that (or mode X or whatever) for
initial splash screens.

In the PC world, too many assumptions are made about the hardware for
any kind of software emulation to work.

The suggestion that a general-purpose CPU on the graphics card could be
used to emulate it is correct, but the logic area of the general-purpose
CPU is greater than that of the dedicated VGA hardware. Furthermore,
you can't just "stick a Z80 onto the board", because multi-chip
solutions up the board cost too much.

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