Chapter 7 COS Control Characters Screen Editor


	Five keys on the ASCII keyboard allow cursor movement and editing anywhere on the V.D.U. screen.  These are
	Control A	-	cursor left
	Control S	-	cursor right
	Control W	-	cursor up
	Control Z	-	cursor down
	Control Q	-	read character


	Pressing the control key and the A, S, W or Z keys move the cursor around the screen but do not send any new characters down the input channel.


	Once the cursor is positioned over the desired character, this character may be sent down the input channel using control Q, the cursor  is then moved one space to the right automatically.


For example, suppose it is required to edit a BASIC line:LIST
	10	REM EXAMPLE
	20	FOR I = 0 TO 50
	30	PRINT "ORANGES AND LEMONS"
	40	NEXT I
	50	END
	>


	After listing the program the screen is as above with the cursor at the bottom following the BASIC prompt.


First move the cursor to the start of the line to be edited using control A, S, W and Z :LIST
	10	REM EXAMPLE
	20	FOR I = 0 TO 60
	30	PRINT "ORANGES AND LEMONS"
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	40	NEXT I
	50	END



Now use control Q to re-enter the line number and first part of line 30 :LIST

10

20

30 PRINT "ORANGES AND LEMONS"

40

50



Next the new output required is typed:LIST

10

20

30 PRINT "APPLE OR PLUM "MONS"

40

50



	And as no more of the old line is required the return key is now pressed and the program may be listed again.



Printer and Screen Control


	The following ASCII control characters affect the V.D.U. screen when they are output.  They are listed together with their keyboard key and decimal value.  The operating system also provides for a printer which may be connected using an extra interface card.  Control characters for this printer stream are also listed: -





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STX (ctrl B, 2)	Start Printer

	This starts the printer output stream but is not sent out to the printer.  It also sets alphanumerics green on the

V.D.U.


ETX (ctrl C, 3)	End Printer

	This ends printer output stream.  It also sets alphanumerics yellow on the V.D.U.


ACK (ctrl F, 6)	Start Screen

	This starts the output stream to the V.D.U. screen, which is the normal condition initially set by the COS.


BD (ctrl H, 8) This moves


HT (ctrl I, 9) This moves


LF (ctrl J, 10) This moves


VT (ctrl K, 11) This moves
	Cursor Back

the cursor back one character position


Cursor Forward

the cursor forward one character position


	Line Feed the cursor down one line.


	Vertical Tab the cursor up one line.


FF (ctrl L, 12)	Form Feed

	This clears the screen and moves the cursor to the top left hand corner.


CR (ctrl M, 13)	Carriage Return

	This moves the cursor to the start of the current line.


NAK (ctrl U, 21)	End Screen

	This ends the output stream to the V.D.U.


RS (ctrl  , 30)	Home Cursor

	This moves the cursor to the top left hand corner of the screen.


DEL (DEL, 127)	Delete

	This backspaces the cursor and deletes the character at that position.
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	Printers connected to the system printer port will often respond to other control characters in the output stream so refer to the  relevant  printer manual.  Note that some of these other control characters may produce a response on the V.D.U. if its output stream is enabled.


	Because some printers will initiate the printing of a line in response to either a carriage return or a line feed character in the output stream the operating system prevents the character specified by location 254 in zero page from reaching the printer. This character is initialised on reset to be a line feed but it may be changed by altering the contents of location 254. Thus it is possible to stop any one character from being printed but note that if neither line feed nor carriage return are specified some printers will do a double line feed after printing.



































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