make fast colour changes to get sky effects, change layer priorities, enable or disable screen modes etc. all that from a simple list of commands.

On older Spectrum models, you would have needed some very clever use of the Interrupt system to do these sort of tricks with some being completely impossible or just too slow to be of any practical use. Even with the ZX Spectrum Next's ability to generate interrupts on each raster line, setting that up (especially in NextBASIC) and then trying to get the timing right for nice clean effects is very complicated (or impossible) and yet simple to accomplish by using the Copper.

We'll jump ahead a bit and introduce a special command; REG (which will be covered in full in Chapter 22). For now take REG n,v to be the same as OUT 9275, n: OUT 9531,v. Let's see our example:

  10 BORDER 0: PAPER 0: INK 7:CLS
  20 REG 98,0: REM make sure Copper is
     stopped
  30 REG 97,0
  40 REM Select the Copper data
     register
  50 FOR x=0 TO 6: REM Increase this
     if you add more data lines.
  60 READ m,l
  70 REG 96,m: REG 96,l: REM write the
     Copper list from DATA statements
  80 NEXT x
  90 REG 97,0: REM low part of address
 100 REG 98,%@11000000: REM high part
     of address and start Copper,
     repeat on VBlank
1000 DATA 128+(45*2),0:
     REM WAIT for line zero horizontal
     45
1010 DATA 64,16,65,BIN 11100000:
     REM WRITE Palette Index 16 (Paper
     and Border), then WRITE RED
1020 DATA 128+(45*2),100:
     REM WAIT for line 100 horizontal
     45
1030 DATA 64,16,65,BIN 00000000:
     REM WRITE Palette Index 16 and
     WRITE contents back to BLACK.
1040 DATA 128+1,128
1050 REM Last line waits for a bit of
     the screen that does not exist
     1*256+128 = 386 (STOP)

You can try changing the BIN statements in lines 1010 and 1030 to use different colours – this is the 8 bit Palette value so RRRGGGBB

Now remember this list is still running in the background but, it is changing ULA palette 0 paper colour. NextZXOS uses palette 1 so you do not see it when editing NextBASIC. Just type CLS and you will see that it comes back until you press a key!

WAIT commands (where the top bit is 1 i.e. bytes >128) will pause processing until a certain point on the display (to a fixed resolution).