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Example - 05keyb-in.asm - Keyboard Input


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Example - 05keyb-in.asm

Waiting for Keyboard Input Image


; --------------------------------------------------------------
;  Input key presses from the keyboard until Enter is pressed.
; --------------------------------------------------------------
	CLO		; Close unwanted windows.
	IN	00	; Wait for key press - Store it in AL.
	CMP	AL,0D	; Was it the Enter key? (ASCII 0D)
	JNZ	Rep	; No - jump back.  Yes - end.

; --------------------------------------------------------------

11)	Easy!  Display each character at the top left position of the 
	VDU by copying them all to address [C0].

12)	Harder  Use BL to point to address [C0] and increment BL after 
	each key press in order to see the text as you type it.

13)	Harder!  Store all the text you type in RAM when you type it in.  
	When you press Enter, display the stored text on the VDU display.

14)	Difficult  Type in text and store it.  When Enter is pressed, 
	display it on the VDU screen in reverse order.  Using the stack 
	makes this task easier.

You can copy this example program from the help page and paste it into the source code editor.

IN 00

Input from port zero. In this simulator, port zero is wired to the keyboard hardware. The simulator waits for a key press and copies the ASCII code of the key press into the AL register. This is not very realistic but is easy to program. There is a more realistic keyboard on port 07 and interrupt 03 but this is for more advanced programmers.


Compare the AL register with the ASCII code of the Enter key. The ASCII code of the Enter key is 0Dhex.

CMP AL,BL works as follows. The processor calculates AL - BL. If the result is zero, the 'Z' flag in the status register SR is set. If the result is negative, the 'S' flag is set. If the result is positive, no flags are set. The 'Z' flag is set if AL and BL are equal. The 'S' flag is set if BL is greater then AL. No flag is set if AL is greater than BL.


JNZ stands for Jump Not Zero. Jump if the 'Z' flag is not set. The program will jump forwards or back to the address that Rep marks.

A related command is JZ. This stands for Jump Zero. Jump if the zero flag is set. In this program, the CMP command sets the flags. Arithmetic commands also set the status flags.


This will copy AL to address [C0]. The visual display unit works with addresses [C0] to [FF]. This gives a display with 4 rows and 16 columns. Address [C0] is the top left corner of the screen.


This copies AL to the address that BL points to. BL can be made to point to the VDU screen at [C0] by using MOV BL,C0. BL can be made to point to each screen position in turn by using INC BL. This is needed for task 2.

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© C Neil Bauers 2003