AZБК blitter demo #6 The ball bounces off the screen under background with rotating

Demonstration of the graphic capabilities of the PDP-11 compatible computer BK on the processor KR1801VM1 using AZBK.
Working with the blitter in 256 color video modes. Demonstration of the capabilities of the blitter and the use of various AZBK ​​blitter commands.
Features
the code is universal - it will work on any BK
the code is a demonstration - I did not do any optimization or beauty of the code
code with a correct return to the operating system
Demo #6. Putting under the background (commands 11 33 and 42) + rotation. This is a complication of example 4, here too
first the background is saved to memory, then the ball is drawn, in the next iteration the background is restored, the new position of the ball is calculated, from it the background is saved to memory and the ball is drawn.
The difference is in the overlay mathematics - here one of the background color codes is declared as transparent and the ball is overlaid only on it, the other codes are opaque and the sprite is not overlaid. Rotation is implemented by switching the sprite that is "underlaid".
The source codes are posted on the MAXIOL forum: forum.maxiol.c...
This video is a direct video capture from the VGA output of AZBK ​​in a resolution of 1024x768 60fps.