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Exchange Exelvision French Computer build by two french TI engineer


humeur

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Here are some more infos on that beauty:

 

http://www.old-computers.com/museum/computer.asp?c=123&st=1

You can find more information on my website:

www.exelvision.fr

Yes, it is French :) But Google Translator may help: https://translate.google.fr/translate?hl=fr&tab=wT&sl=fr&tl=en&u=http%3A%2F%2Fwww.ti99.com%2Fexelvision%2Fwebsite%2F

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Fascinating. http://www.old-computers.com/museum/computer.asp?c=123&st=1says its engineers were from TI. This has a new to me (old) chip, the TMS3556.


The TMS3556 video display processor (VDP) provides a 40x25 color text mode, 320x250 bitmapped graphics, or mixed mode. The pattern name table is 16 bits per screen position: 3 bits for forgeground color, 2 bits for pattern bank, 1 inversion bit, 2 bits for double width / double height, 1 bit flashing, and 7 bits for character code. This gives an effective 9 bits in the character codes, for up 512 characters. Character definitions are 8x10.




Space characters' attribute bits have a completely different meaning. They set the background color for the following characters! Also bits for M)asking, I)ncrustation, and U)nderlining. Masking hides the following characters. Incrustation raises an external sync signal. Underlining does what it says.


Full bitmap mode consumes 3 bytes of BGR data per 8 pixels, each pixel using 1 bit of each byte.


Mixed mode allows the video mode to switch from text to bitmap after any row. The first row must be text, (80 bytes) but then the 81st and 82nd bytes are loaded into command register CM4, which can effect a switch to bitmap mode. A bitmap line takes 120 bytes per line, plus the 2 command bytes which go into CM4. (Additionally, the top row can be hidden.)


An external circuit is shown which adds 8 levels of grayscale at the cost of the I bit. The I bit can be switched on and off by the attributes on the Space character.


I found this to be a fascinating chip, something I dreamed about having in 1984, but never heard of. There are many more nice features in there than I cover in this brief summary.
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