Is there a way to blank out a portion of a screen on Commdore C64?

Question

I would like to blank out a couple of lines between two given rasterlines. Above that I'm planning to have a scrolling background and sprites, below that is a textual info area.

I'm thinking of a rasterline interrupt that turns on the blanking and a second one turning it off a couple of rasterlines later. Half-drawn characters and sprites should be covered by the bar. For this reason I cannot realize it with sprites, because the sprites are in use.

For scrolling, the VIC chip provides the feature of increasing the borders, which covers scrolling artifacts, but this feature only works at the border. Bit 4 of register $D011 provides a blank out feature, but this one only starts at rasterline 0 and blanking cannot be controlled to turn on/off during one frame.

Any good idea how to realize this?

Answer 1

I found a very compact solution that requires only two memory writes to enable the bar:

It uses a combination of Extended Color Mode (ECM) with Bitmap Mode (BMM), which the VIC cannot display - it is going into some sort of blanking mode instead. This provides a good separation line for the scrolled characters. After a couple of rasterlines (I used 12) the normal text mode is turned back on. To avoid jumping of the lines, it is necessary to force a badline. Luckily this is done via the same register as for the ECM and bitmap mode, so the following code does the trick (raster1 is the rasterline where the blanking area starts):

lda #$78 + ((raster1+1) & 7) ;turn on ECM and BMM and cause a badline at line raster1+1
sta $D011

As a welcome side effect, this also sets the 9th bit of the next raster line IRQ.

Unfortunately, the ECM+Bitmap blank does not cover the sprites, to cut off a sprite in mid-drawing I temporarily switch the screen pointer to a location where SCREEN+$3f8 are pointing to an empty sprite. Since there is no output of characters, the other content of the screen area does not matter. I used $0000 as screen address during blanking and had the empty sprite at $340 (so address $3f8 to $3ff contain $0d):

lda #$05
sta $d018   ;change sprite pointers by switching screen

That's it to start the blanking bar, two memory writes. To end the bar, I set these two addresses back after a couple of rasterlines. In my experiment, I used a rasterline IRQ to start blanking at line 150 (this is in the middle of a character) and ended the blanking bar in 162. The gray border showing in the picture is an additional effect that can be left out.

One disadvantage is that the bar created with ECM+BMM is always black, but for my purpose this is acceptable.

Answer 2

There may still be a way that you can use sprites to achieve this: If you have raster interrupt firing at the right time, then you can reprogram the sprites to cover the right area.

Then your next interrupt will fire to turn the "blanking" back off. At this time you set the sprites to be what you need in the top area.

This simple trick is known as sprite multiplexing.

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With judicious or clever graphical design, it might be unnecessary to do this. If you can set your background colour, attributes and the border colour all the same, and make sure that graphical data never gets written to this area, then you can safely switch to text mode when you need. But this depends on where your badlines are, exactly how tall you want the blank region, and so on.