VGA Scanline Effects

[Mills32]

I want to know (or calculate) the number of CPU cycles a VGA scanline takes to be drawn. I'll do it for 8088 and 8086 only, because 286 is fast enough to wait for scanlines (reading the vga ports). I don't think there is another way to synchronize code with the crt display.

Any ideas on how to do it?.

I could write pixels to vram with changing colours, and just count myself, but maybe this is not very accurate .

Thanks!

 

[reenigne]

The VGA pixel clock in standard 320 pixel and 640 pixel graphics modes is 25.2/1.001 MHz, and there are 800 of these pixels per scanline total. The CPU clock speed at 4.77MHz is 4.7775/1.001 MHz, so there are 800*4.7775/25.2 = 455/3, so 455 CPU cycles for every 3 scanlines. These calculations are exact using the derivations of the NTSC and VGA clock frequencies. However, the VGA uses a separate crystal than the one on the PC/XT motherboard, so (depending on details of the hardware and even environmental conditions) these clocks may drift from each other. It might be sufficient to resynchronize every frame - I haven't tried it myself.

 

[Mills32]

So, it is possible to make scanline effects on slow PC's without modifying the problematic hblank crt registers (which won't work on modern lcds and most emulators). This is a test on pcem (8088 4.77), the function can be tweaked to produce a stable image on the emulator, so I think any real PC can cope with it.



Here I changed VGA register 0x03D4 port 13, so that it jumps to specific locations ahead (you can only jump 256 bytes in 8 byte steps). The image was loaded by interlacing 4 copies of every scanline, with 2, 4, 6 pixel offsets, so VGA can jump to any of the next lines, and the image and vram logic width, is 256 pixels max.

You see we can only move in 2 pixel steps to the right or left, but it looks ok . The big problem is the array needed can't be animated by changing offset .

I'll try to update the fine X scroll register (3C0h/13h) each scanline, just in case it might also help to get another approach.