ZX Spectrum Screen Driver
-------------------------

The new screen driver is still fairly minimal but it does feature 64
column text and 32 column text (switchable) and can be easily extended.

Here are the byte sequences for it:

1,64	  - Switch to 64 column mode
1,32	  - Switch to 32 column mode
2,ll,hh   - Set the address of the 32 column font. ll is low byte, hh = high
	    byte. By default this points to the ROM font at 15616
3,ll,hh   - Set the address for the UDGs by default this points to
	    65368

8,9,10,11 - Move in x and y as you would expect
12	  - Form feed - clears the screen and moves print posn to 0,0
13	  - Carriage return - advances y and sets x to 0

16,x	  - Set the ink colour  (x = 48-55 ('0'-'7'))
17,x	  - Set the paper colour (x = 48-55 ('0'-'7'))
18,x	  - Turn flash on/off (x=49/48 ('1'/'0')
19,x	  - Turn on/off bright 
20,x      - Turn on/off inverse (x=49/48 ('1'/'0')

22,y,x	  - Move to position y,x on the screen (0<=y<=23, 0<=x<=63)
	    NB. y and x are displaced by 32 eg to move the print position
	    to (0,0) use 22,32,32. 32 column mode also uses 64 column
	    coordinates.


All the commands, except for the udg address/font address  can be embedded 
in strings using either octal or hexadecimal representation. Address changes
can be sent either using fputc() or using printf with the address as
parameters.

Characters greater than 127 are interpreted as udgs. These are always
printed using the 32 column mode. This gives a potential of 128 udgs
though obviously, it's possible to swap between banks in the middle of
printing a string.

The screen now scrolls when line 24 is "hit", the routine used is in the
48k ROM.

dom
24/1/2002