SPIRAL CLEAR (16K ZX81)
by Gary Nugent, Churchtown, Dublin, Eire
from Your Computer (November 1982), Page 110

SPIRAL CLS was written for a 16K ZX-81. It clears a 22 by 32 screen. The
screen is first filled by a spiral of inverse spaces, and then by a spiral
of spaces. The screen is then ready for output, the PRINT position having
been reset to 0,0.

The routine is in machine code and is 91. The code is not relocatable.
Should you wish to move it, all the CALL addresses will have to be changed.

This is a novel way to clear the screen and is faster than the system CLS
when large amounts of memory and SCROLL are in use.

Poking address 16599 with a value less than 192 increases the speed of the
spirals. A larger value decreases the speed. To make the routine clear a 24
by 32 screen, do as direct commands:

    POKE 16535,23 (21 for 22 x 32 screen)
    POKE 16575,9  (11 for 22 x 32 screen)

The routine is called by RAND USR 16514.