superfury wrote:
Anyone figured out the exact 8/16 clocks of those? Are the Dosbox-X clocks already accurate? Or do they need adjusting to be using the clocks in the photo's?
Clocks for 8MHz and 16MHz formats in ET3000 and ET4000?
That depends wildly, but let's assume ideal case first.
According to pictures and datasheets, there would be a nominal 32.514 MHz oscillator that internally can be divided by two to get 16.257 MHz for EGA formats.
There is also mentions of using 28.322MHz divided by two to get CGA formats - most likely 14.161 MHz is close enough for CGA, and the line width may be fine tuned if necessary.
superfury wrote:
I see 6 onboard clocks, but 16 documented? Anyone?
In most ET3000 pictures, there really are separate oscillators for each frequency, so that's easy, just look at the pictures. However, the BIOS must know which oscillators are present and which frequencies they have to select correct oscillator for each mode, so which frequencies to emulate depends only on which video BIOS you are targeting.
In most ET4000 pictures, there are many different PLL chips used, so the actual frequency is whatever the PLL can do, so if you want to know the exact frequencies of a specific card you have in mind, you must look at what PLL chip it has and then read the PLL chip datasheet, and possibly also how the video BIOS sets up the PLL chip parameters. Although, many PLL chips had preprogrammed frequencies and ET4000 chip has three (four?) clock select wires, so it's possible that only the presets in the PLL are ever used.
As I explained in one of my previous posts, on a PLL-clocked ET4000 card, the clocks will not be exactly 25.175 and 28.322 for example, and can be far off.
So, There is no single answer to what frequencies are used and what are their exact values. Therefore, any approximation should be OK, or then just stick to emulating a single known card/bios combination.
superfury wrote:
(S)VGA is odd: 640x480@60Hz is actually 59.94Hz?
It really is and should not be a surprise as it is a standardized format. 25175000/(800*525) is indeed very close to 59.94Hz, but not exact.
Most of the other video resolutions of interest can be found in standards as well.