Hercules character clock incorrect for graphics mode

[dbalsom]

Hey, I've noticed something odd about emulation of Hercules graphics mode where V-Sync rate appears to be ~100 Hz rather than expected ~50 Hz. This ~100 Hz shown in the GUI matches a small tool I wrote to programmatically measure V-Sync interval as well.

For context, I'm considering making a game using Hercules and evaluating state of emulation and it's a bit of a mess:

• DOSBox derivatives run at expected 50 Hz
• 86Box runs at 90 Hz (?)
• PCem runs at 100 Hz (?)

Unfortunately without real hardware I'm not able to work out who's right and who's wrong. Perhaps someone here with access to real hardware would able to run the tool and confirm whether it should run at 50 Hz...?

My tester tool w/ source code:
herctest.zip

Originally posted by @mausimus in #226

[dbalsom]

Fix: change HGC_CHAR_CLOCK from 8 to 16.

[MrMadguy64]

Yes, HGC's manual mentions, that it uses following CRTC settings: 35h, 2Dh, 2Eh, 07h, 5Bh, 02h, 57h, 57h, 02h, 03h, 00h, 00h.

I.e. yeah, these settings suggest, that HGC uses 16 pixels per character clock in graphic mode. I'm not sure, why it was done, but HGC also uses slightly different horizontal and vertical frequences. 18816Hz horizontal one instead of 18432Hz. And 51.41Hz vertical one instead of 50Hz.

P.S. I recalculated my numbers and got even weirder numbers. 19171Hz and 52.38Hz. I don't even know, why in some cases instead of trying to achieve exact match between text and graphic modes, video card manufacturers were choosing different timings instead. May be they tried to achieve more stable picture? For example why all 320x200 CGA modes have exactly the same timings, but 640x200 color differs from 640x200 B/W?

[dbalsom]

P.S. I recalculated my numbers and got even weirder numbers. 19171Hz and 52.38Hz. I don't even know, why in some cases instead of trying to achieve exact match between text and graphic modes, video card manufacturers were choosing different timings instead. May be they tried to achieve more stable picture? For example why all 320x200 CGA modes have exactly the same timings, but 640x200 color differs from 640x200 B/W?

can you show your math?

[MrMadguy64]

can you show your math?

Ahh. Sorry. I've forgotten, that HTotal is one less. Initial calculation was correct. I had it in my old Excel file, decided to recalculate it yesterday, but was too sleepy to make it correctly.

Math is standard for any mode:
HFreq = PixelClock / (HTotal * PixelsPerCharClock)
VFreq = HFreq / VTotal

In this case:
HFreq = 16257000 / (54 * 16) = 18815
VFreq = 18815 / (92 * 4 + 2) = 18815 / 370 = 50.851

So, for some reason higher than usual frequences are used. Using HTotal = 55 (36h instead of 35h) would give correct values. 18474Hz and 49.92Hz. I really doubt, that HGC uses EGA-like CRTC, that reserves extra character clock for VRAM prefetching. But who knows.

P.S. I guess, in case of CGA different settings for color and B/W modes are connected to color burst.

P.P.S. Sorry, VTotal is also one less.

[dbalsom]

It's almost exactly 50Hz since the Hercules uses a straight 16Mhz clock compared to the MDA's 16.257.

[MrMadguy64]

It's almost exactly 50Hz since the Hercules uses a straight 16Mhz clock compared to the MDA's 16.257.

Are you sure it's 16Mhz? Because oscillator marked as 16Mhz doesn't mean it's exactly 16Mhz.

Problem is - text mode timings wouldn't match in this case.

16000000 / (98 * 9) = 18140Hz
18140 / (26 * 14 + 6) = 49.027Hz

Remember? HGC doesn't have it's own BIOS. It's controlled by system BIOS. And therefore should be MDA-compatible.

I suspect, that there can be hidden reasons for it. For example HGC can require even number of character clocks due to some VRAM access cycle considerations. Like in case of EGA.

[dbalsom]

Are you sure it's 16Mhz?

yes. this is fairly well known and has been widely discussed before, see: https://forum.vcfed.org/index.php?threads/mda-hercules-and-other-clones-whats-the-frequency-kenneth.52966/

even clone MDA cards have used a 16.000MHz crystal. It is "close enough" and for Hercules it provides 50Hz in graphics mode which was more important than .. whatever in text mode.

Problem is - text mode timings wouldn't match in this case.

does it need to? despite the notorious nature of the IBM 5151 there still a safe range of horizontal and vertical sync where your monitor does not actually explode.

[MrMadguy64]

Ok. I thought, that compatibility was the highest priority. But it seems like video card manufacturers had different reasoning.

[dbalsom]

Ok. I thought, that compatibility was the highest priority. But it seems like video card manufacturers had different reasoning.

Compatibility back then was measured by being able to run the most popular software titles, not strict conformance to a hardware spec. Of course, the latter generally implies the former.

I can't think of any text-mode application that would care if your vertical or horizontal refresh was slightly off from the IBM MDA.