Rasters

When programming, the raster is often referred to as the act of modifying the setting of inks between 2 scanlines, in interrupt mode.

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 is updated with a frequency of 50Hz...The CPU (Z80) runs at about 4 MHz.

So It has enough cycles between 2 scanlines to order this.

You can also use Split raster to "actually" display 2 different screens, sometimes with different Video Modes. This also allows a Graphic heavy HUD with no interference with Scrolling.

Links

• Wikipedia link to Raster concepts

CPCWiki Links

• Gate Array
• CRTC
• ASIC

• Synchronising with the CRTC and display

• Programming:Simple Raster Example 1
• Programming:Simple Raster Example 2 (uses CPC plus features)
• Programming:Simple Raster Example 3 (uses CPC plus features)
• Programming:Simple Split Raster Example 1