64 Each character cell is 32 graphic units high and, in a 40 character mode such as MODE 4 , 32 units wide. Suppose we want to subscript a letter to produce for example the chemical formula H 2 . This can be done as follows 10 MODE 4 20 VDU 5 30 MOVE 500,500 40 PRINT "H"; 50 MOVE 532,484 60 PRINT "2" 70 VDU 4 Note that the letter H is positioned with its top left corner at 500,500. The 2 is then printed one character to the right (532) and a half a character down (484). Again the top left of 2 is at 532,484.
A neater way of achieving the same effect is to replace line 50 with PLOT 0,0,-16 One further feature of the BBC Microcomputer which is not normally available on ‘personal’ computers is the ability to superimpose characters. One obvious use of this facility is to generate special effects such as accents and true underlining. The short program below prints the word rфle with the accent correctly placed. 10 MODE 4 20 VDU 5 30 X=500 40 Y=500 50 MOVE X,Y 60 PRINT "role" 70 MOVE X+32,Y+16 80 PRINT "^" 90 VDU 4 Once in VDU5 mode the screen will not scroll up when you reach the bottom of the page, instead the writing will start from the top of the screen again.
In addition characters will be superimposed on those already on the screen. When in VDU5 mode you can only print things in the graphics window and not in the text window, and colour is selected with the GCOL statement. VDU5 will not work in text-only modes such as MODES 3 , 6 and 7 .