PARTIAL SCREEN CLEAR
on Spectrum

This is a machine code program for the ZX Spectrum which allows you to clear
part of the screen.

Unfortunately, because of the way the Display File is arranged, it is only
possible to clear blocks of eight lines, so you can clear the top eight
lines, the middle eight lines, the bottom eight lines or any combination of
the three. By altering the second number in the data it is possible fill the
screen. This number is used like the numbers in user definable graphics so if
you change the 0 to 255 it will fill the screen with a solid black square.

The Z80 Mnemonics are as follows:
Ld C,0		14,0
Ld HL,"start"	33,0,64
Ld DE,"limit"	17,0,88
Ld(HL),C	113-Loop
INC HL		35
Ld A,D		122
SUB H		148
JRNZ Loop	32,250
Ld A,E		123
SUB L		149
JRNZ Loop	32,246
RETURN		301

As the program stands it will clear the complete screen so to clear part of
the screen change the numbers underlined. To clear the top third only change
the 88 to 72, to clear the bottom third change the 64 to 80 and to clear the
middle third change the 64 to 72 and the 88 to 80.

When you have run the program it should give you the stop report, so to run
the machine code enter Continue and the screen should fill with a solid black
square then clear.
