Because the cpu is involved in generating the video display,
the zx81 is somewhere around half as fast as the spectrum
when running these examples.  To compensate, the sprite
movement rates have been doubled.  I decided not to reduce
the number of moving sprites in order to keep the example
programs as nearly identical as possible across platforms,
though doing this would improve the smoothness of the result.

POST 1.8 PATCHES:
* sp1/zx81hr/tiles/sp1_ClearRectInv.asm
