how to kludge a 486 PC into thinking it has a video card?

From: Steve Thatcher <melamy_at_earthlink.net>
Date: Mon Feb 21 10:26:33 2005

okay, time to chime in...

in order for you to fool the bios you will need some hardware that will
"respond" to key areas that the bios is interested in.

Picking a mono card as a target, you will need hardware that allows for
writes and reads of memory from b000:0 to b000:7fff (the end address may be
a lower location). I don't think the bios does a real memory test, so you
may be able to use one register that lets the cpu write a value to it that
can be read back (the register should respond to all addresses in the range.

Once bios has determined that a mono card is present by the memory check,
then it will attempt to setup the crt controller on the board. You will
need to allow for writes to these I/O locations (but I do not believe you
will have to have them read/writable with the actual data). You will need
to allow them to be read and return specific values in order to allow the
bios to preceed further.

Once here, operation with the bios may need some status values from the
"crt controller" and this may be the same information you return during setup.

best regards, Steve Thatcher

At 11:05 AM 02/21/2005, Jules Richardson wrote:
>I wondered if there was some standard for the BIOS detecting card types
>(write to this port, expect this value back kind of thing) that meant I
>might be able to bodge something with a few LS logic chips, but maybe
>not. Hopefully I can find enough boards with a BIOS that's sensible
>enough not to check for video :)
>
>cheers
>
>Jules
Received on Mon Feb 21 2005 - 10:26:33 GMT