VDU code	                            Description					Notes

VDU 0	NUL, do nothing, it is ignored.
VDU 1,c	Send a character directly to the printer, if it has been enabled with VDU 2.	Not currently implemented in BBCSDL
VDU 2	Enable the printer, subsequent console output goes to the screen and printer.	Not currently implemented in BBCSDL
VDU 3	Disable the printer, reverses the effect of VDU 2.				Not currently implemented in BBCSDL
VDU 4	Restore the normal 'print at text caret' mode, reverses the effect of VDU 5.	VDU 4 text has an opaque background 
VDU 5	Cause text to be written at the current graphics position, except in MODE 7.	VDU 5 text has a transparent background
VDU 6 	Enable output to the screen, reverses the effect of VDU 21.
VDU 7	Send a Ctrl+G character to the console, which may result in a short 'beep'.	Not currently functional in BBCSDL
VDU 8	Move the text caret one character in the negative X direction (normally left).	Average width if proportional-spaced
VDU 9	Move the text caret one character in the positive X direction (normally right).	Average width if proportional-spaced
VDU 10 	Move the text caret one row in the positive Y direction (normally down).	'Line feed'
VDU 11 	Move the text caret one row in the negative Y direction (normally up).
VDU 12	Clear the text viewport to the current text background colour & home the caret.	Equivalent to CLS
VDU 13	Move the text caret to column 0 in the current row (normally the left edge).	'Carriage return'
VDU 14 	Enable auto-paging mode, scrolling pauses until <Shift> or <Esc> is pressed.
VDU 15	Disable auto-paging mode, reverses the effect of VDU 14.
VDU 16 	Clear the graphics viewport using the current background GCOL action & colour.	Equivalent to CLG
VDU 17,n	Set the text foreground (n < 128) or background (n >= 128) logical colour.	Equivalent to COLOUR n
VDU 18,m,n	Set the graphics foreground or background logical colour and plotting action.	Equivalent to GCOL m,n
VDU 19...	Set the colour palette, i.e. the mapping between logical and physical colours:
VDU 19,n,p,0,0,0	    Set logical colour n to physical colour p in the standard palette.
VDU 19,n,-1,r,g,b	   Set logical colour n to the specified RGB colour (each 0-63).
VDU 19,n,16,R,G,B	   Set logical colour n to the specified RGB colour (each 0-255).	Equivalent to COLOUR n,R,G,B
VDU 19,n+128,A,R,G,B	Set logical colour n to the specified ARGB colour (A = alpha, 0-255).
VDU 20	Set the text and graphics colours, and the colour palette, to their defaults. 
VDU 21 	Disable the VDU drivers until a VDU 6 is received, other output is discarded.
VDU 22,n	Select the specified display mode and reset VDU variables to their defaults. 	Equivalent to MODE n
VDU 23...	A multi-purpose command, VDU 23 is always followed by exactly 9 more bytes:
VDU 23,n,a,b,c,d,e,f,g,h	Redefine character n (32-255) to the specified 8x8 dot pattern.
VDU 23,0,10,s,0;0;0;	    Set the start line of the text caret (0 = top).
VDU 23,0,11,e,0;0;0;	    Set the end line of the text caret.
VDU 23,0,18,w,0;0;0;	    Set the width of the text caret in pixels.
VDU 23,1,0;0;0;0;	       Hide the text caret (cursor).				Equivalent to OFF
VDU 23,1,1;0;0;0;	       Show the text caret (cursor).				Equivalent to ON
VDU 23,7,m,d,0;0;0;	     Control how the screen scrolls. 				See the main documentation
VDU 23,16,x,y,0;0;0;	    Control how the text caret moves after a character is output.	See the main documentation
VDU 23,18,3,0,0;0;0;	    In MODE 7, disable black text & mosaics, and alt character sets. 
VDU 23,18,3,1,0;0;0;	    In MODE 7, enable black text & mosaics, and alt character sets.
VDU 23,22,w;h;cx,cy,n,bg	Select a custom display mode (size, number of rows & cols etc).	See the main documentation
VDU 23,23,t;0;0;0;	      Set the thickness in pixels when drawing lines and curves.
VDU 24,l;b;r;t;	Set a graphics viewport: left, bottom, right and top respectively.	Coordinates are graphics units
VDU 25,p,x;y;	  A general-purpose plotting command, where p = code, x & y = coordinates.	Equivalent to PLOT p,x,y
VDU 26	Initialise the text and graphics viewports, home the caret & graphics position.
VDU 27,n	Send the code n to the screen without interpreting it as a control character.
VDU 28,l,b,r,t	 Set a text viewport: left, bottom, right and top respectively.	Coordinates are row and column numbers
VDU 29,x;y;	Move the graphics origin to the specified graphics coordinates.	Equivalent to ORIGIN x,y
VDU 30	Home the text caret (in VDU 4 mode) or the graphics position (in VDU 5 mode).	Default is the top-left corner
VDU 31,x,y	Move the text caret (cursor) to the specified coordinates.	Equivalent to PRINT TAB(x,y);
VDU 127	Delete the character to the left of the caret and backspace to this position.	Works in both VDU 4 and VDU 5 modes
