Ok, a first try, please tell if this exist... Also, should we use raster or maybe other terms ? Concepts : Raster, split raster, Raster lines or Scanlines ?
In programation, the raster is often referred as the act to modify the setting of inks between 2 scanlines.
This is a way to display more than the usual Video modes number of colours.
The time between the display of 2 Scanlines by the CRTC allow the CPU to order the Gate array to modify the setting of inks.
The screen display at 50Hz...The CPU (Z80) runs at about 4 MgHz.
So It has enough cycles between 2 scanlines to order this.
You can also use Split rasterto "actually" display 2 different screens, sometoimes with different Video Modes.
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