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EGA video corruption?

el_VR said:
Ah hey there Scali, fancy meeting you here :)
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Oh hi Vile btw! :)
Missed that post earlier.

el_VR said:
From my experience, most EGA cards out there were clones, and nearly all clones had 256K on board (our family XT had one of those at some point).
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Well, interesting.
I figured that EGA was very rare as a stock-option in clone PCs (it seems that quite a few of them used an ATi Graphics Solution/Small Wonder), so most EGA cards out there would have been original IBM cards in original IBM PCs.
There was only a small window of opportunity for upgrading a CGA/MDA/Hercules machine to EGA, because VGA arrived on the scene only shortly after third-party EGA cards did, so in my experience, EGA aftermarket was never big.
Then again, as I say, it could be a geographical thing. In my part of the world, PC gaming didn't take off until the early 90s anyway (386SX-16 with Trident VGA seemed to be an early 'standard', around the time that Wolfenstein 3D became popular). So there weren't many people who had a PC at home, let alone that they were interested in upgrading the graphics capabilities.
 
Most game developers probably were not using the original IBM card, they probably used third-party clones that either came with 256KB or were much easier to upgrade to 256KB. Most 16-color games used 320x200x16, and EGA supported eight pages of those graphics for such things like page flipping, double buffering and to assist with fine scrolling. I know that certain 16-color games, like Commander Keen 4, 5, 6 and Keen Dreams, have very serious difficulties with scrolling graphics with EGA cards outside a narrow speed "window" (roughly a 286 or 386SX 16Mhz) but work fine with VGA cards regardless of CPU speed.
 
Great Hierophant said:
Most game developers probably were not using the original IBM card, they probably used third-party clones that either came with 256KB or were much easier to upgrade to 256KB.
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Either that, or they just relied on the EGA-compatibility of their VGA cards? :)

Great Hierophant said:
I know that certain 16-color games, like Commander Keen 4, 5, 6 and Keen Dreams, have very serious difficulties with scrolling graphics with EGA cards outside a narrow speed "window" (roughly a 286 or 386SX 16Mhz) but work fine with VGA cards regardless of CPU speed.
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Well, my experience is that VGA cards tend to have WAY faster memory access than EGA cards.
Scrolling on PC will always be a clumsy affair, even if you abuse the EGA ALU latches for moving data in all bitplanes at a time... It still requires a lot of CPU-work, and lots of waiting on the slow videomemory.
Perhaps VGA cards made the difference with more advanced memory interfaces (something like an ET4000 is incredibly fast even over an ISA bus).
 
Scali said:
Well, my experience is that VGA cards tend to have WAY faster memory access than EGA cards.
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That's exactly what Great Hierophant implied. Except the fact 256k EGA cards did exist, a fare few folk had them, systems even shipped from OEMs with EGA build in. The fact they weren't that common around your location is here nor there.
 
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The IBM EGA simply doesn't support 16 colors with only 64k RAM. Attached is an excerpt of the manual. It appears only two bitplanes are populated without the expansion card.
 

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H-A-L-9000 said:
The IBM EGA simply doesn't support 16 colors with only 64k RAM. Attached is an excerpt of the manual. It appears only two bitplanes are populated without the expansion card.
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That only applies to the 640x350 16/64 color mode. EGA supports 16 color 320x200 and 640x200 graphics modes with the minimum of 64KB.
 
Great Hierophant said:
That only applies to the 640x350 16/64 color mode. EGA supports 16 color 320x200 and 640x200 graphics modes with the minimum of 64KB.
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Yup, that excerpt of the manual is from a section that discusses mode 10h specifically. See the full manual here: http://www.minuszerodegrees.net/oa/OA - IBM Enhanced Graphics Adapter.pdf
That is from page 11. Page 10 is important here :)
Elsewhere in that manual you will indeed find that all the other 16-colour modes will work with 64K (on page 4 it says that you always have 4 bitplanes, 16K each, with 64K, hence enabling 16 colours. More memory will just give you extra pages in these modes).

The tweakmode I discussed by the way is to make use of the 64-colour palette. Namely, EGA will use the hardwired 16-colour CGA palette in 200-line modes to remain compatible with CGA monitors (what a crazy idea). So you need to use 350-line mode for the full palette.
However, it should be possible to switch the card into a 320x350 graphics mode, where you get 4 bitplanes, and the full 64-colour palette. If you then reduce the visible area to something like 320x204 or less, you can even do double-buffering on a 64K card.
 
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I filled up all the banks of memory on the expansion card, but there must still be an issue. The graphics on Commander Keen are almost correct, except for some odd garbage on the intro Screen. The levels don't show any immediate corruption like before.

When I ran the advanced diagnostics disk, it only detects 192KB memory installed. When I say that's not correct, and run the test, it bombs out with an error "failing graphics memory 030101."


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I may have not inserted a RAM chip correctly. I think I may have accidentally installed the chip with a pin or two not in the socket...
I'll have to take it out and check.:confused:
 
mikey99 said:
error "failing graphics memory 030101

This error is pointing to the fourth chip from the left in the third bank of ram

Take a look at the diagram on page 37 of the HW maintenance EGA section from modem7's site :
http://www.minuszerodegrees.net/hms/HMS%20-%206280087%20-%20Maps%20-%202400.pdf
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Yup, that was the exact chip I didn't insert properly. I was in a rush, I should have taken my time. Anyway, now all the RAM is detected by the Diag Disk.

Also, Commander Keen now works fine. Along with SimAnt and the PGA Golf game. That corruption was not due to faulty memory... Just not enough memory.
 
offensive_Jerk said:
That corruption was not due to faulty memory... Just not enough memory.
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Yes, apparently Keen needs the full 256K for its scrolling trickery (apparently it sets up a much larger playfield than the viewing window)... Not really sure what a golf game would need 256K for though.
 
Timo W. said:
To store animation frames, I'd guess.
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Yup, things that make you go hmmm.
If they had used some more main memory, they could easily make the game work on cards with 128K or even 64K.
Looks almost as if they load the graphics of the whole game in videomemory at startup, and then do everything from there. Quite bruteforce.
 

I have another problem with EGA stuttering on my 286/10/EGA. I don't know why this happens.
(Only the starship in the intro, the rest is OK in my opinion)

Doc
 
You'll notice that the stars stay in the same place while the entire ship is moving; this is due to the hardware address being changed while the stars are redrawn to make them look like they're stationary.

It's possible that a 286/10 + slow EGA card + bad programming is the cause. I don't think it means anything is broken, it's just the way it is.
 
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