Just wondering what's the function of an HT27C512-70 EPROM chip on an old AGP nVidia Vanta VGA board. The board sports an nVidia Riva TNT2 processor. I wonder what's the use of an accessory EPROM chip. Just so, because I'm curious 
robitabu:
Just wondering what's the function of an HT27C512-70 EPROM chip on an old AGP nVidia Vanta VGA board. The board sports an nVidia Riva TNT2 processor. I wonder what's the use of an accessory EPROM chip. Just so, because I'm curious
Firmware? VGA boards have a CPU, too.
I think you will find this chips like this on many boards. Network cards, Hard Drives, sound cards... the list goes on...
The EEPROM contains the "VGA BIOS". This is software called by the main computer's CPU at boot time during the initial POST process (Power On Self Test) and the main system initialization. It also provides standard routines for basic video output, for standard VGA video modes. Without it the computer wouldn't know how to display anything right from bootup, and it wouldn't be able to display those nice text screens that you get when you turn the computer on that tell you all about what hardware you have. There would be nothing at all until the OS had loaded the video card's driver.
I see, and it looks like a VGA BIOS may be modded and burned back into the chip on modern boards ... well, in old ones, like the one I started looking into, it's impossible since it's an EPROM and can't be burned again. Anyway, it's all clear to me now.
I'll keep reading about it, I wonder if that EPROM may be switched with something else, something one can customize ...
First you would need to reverse engineer the assembly language from the binary image on the EEPROM. Then you'd have to understand it 
Not a simple task, but do-able. From that you could glean clues as to how the card actually works. Much of it will be a standard operation as most VGA cards at the most basic use the same style interface. For more complex work, like graphics, you'd need to know just how the card works - for that you may get some detail from things like the Linux drivers (which should be open source) for the card.
Actually you could reuse the EPROM. You would need to find an EPROM programmer. If you are examining the device, see if there is a 'cover' over the window. Take or peal the cover off and take a close look at the intricate design. You would need about a year's worth of direct sunlight, or a high powered Ultraviolet erase to clear out the E(rasable)Programmable Read Only Memory. EPROMS fell out of favor in the early 1990's because of the hassle to program/reprogram.
If it's AGP then it's going to be EEPROM - probably winbond W29C040P, or maybe PMC Flash PM49FL004T, PM39LV512R or similar in a PLCC format. Those are the most common (though you sometimes get an ST or an IDT). I have loads of them rescued from old VGA cards (PCI and newer). UV EPROMs were last used on ISA cards (and maybe Vesa Localbus. Remember those?)
Yep, I remember them, and in some applications they are still in use today.