30 January 2023, 23:39 | #41 | ||||||
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31 January 2023, 00:38 | #42 |
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That's why you need to waste many CPU cycles to do crude "raster bar" and still you don't know where during line your beam is.
To do something useful with this people using timer (8254/8253) but this is workaround. A500 too but this doesn't count as in overall it is workaround (fragile connection). |
31 January 2023, 09:44 | #43 |
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CGA (and by that VGA) includes a register that indicates whether the electron beam is in the active display area, and whether it is in the vertical sync or not. That is as far as it gets, there is no indication of the raster scan position anywhere in the CGA register map. Thus, what the "raster bars" do is that they busy-wait for the vertical sync to be indicated, and then busy-wait through cycle counting until the right spot on the display is reached, then change the color through the attribute controller.
CGA and VGA are quite limited. |
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