How to jump if there is no keyboard key pressed

Question

Is there any value or code I can use to jump to another procedure when there is no key pressed?

cmp ah,(value of no pressed key)
je (next procedure)

or is there any interrupt I can use so that if it sees that there is no keyboard key pressed it will ump to the next procedure?

Answer 1

Example with polling the keyboard controller status register:

in   al,64h  ; get status register
test al,1    ; output buffer
je  NOKEY
;-----------

;-----------
NOKEY:

http://wiki.osdev.org/%228042%22_PS/2_Controller

Edit: In this example we are checking only the first bit of the status byte and if it is set, the output buffer is filled with one or more bytes, that we can get from the data port (60h) with following port instruction(s).

Additional we can check the status byte if the byte(s) comes from a PS2-mouse (movement or mouse-click):

test al,20h
jne NOKEY

Edit 2: The bytes that we can receive from the data port 60h are so called "scancodes" and they can be divided into make-codes if a key is pressed, brake-codes if a key is released and additional communication protocol codes.

http://www.win.tue.nl/~aeb/linux/kbd/scancodes-1.html#ss1.1

A keyboard interrupt service routine(ISR) can be used IRQ1 and if an IRQ1 occur, then it simple have to read from IO Port 60h (there is no need for to check bit 0 in the Status Register first, because we know that the data came from the first PS/2 if we receive an IRQ1), translate and echo an ASCII or something else, sending an EOI to the interrupt controller and finally return from the interrupt handler.

But the example above is only for to show how to poll the keyboard ports. And for this we have to dissable the IRQ1 before, for to prevent that a running keyboard ISR is getting the bytes of the output buffer of the data port 60h before we can do it.

Answer 2

Replace:

test al, 1
je nokey

by:

or al, al
jz nokey

This way, your program checks, whether the register is actually zero. Using the "test al, 1" would only check whether your number is odd.

Answer 3

You can read the input from port 60h. If a key is pressed AT THE MOMENT, the port will contain a number (not the ascii code) of the key. Else it's zero. Another way is to use function ah=1 of BIOS interupt 16h. If a key has been pressed, which hadn't been read, the ascii code is in al and the scancode in ah. If a key has been pressed, ZeroFlag isn't set if no key had been pressed ZeroFlag is set. If no key had been pressed, ax contains no useful value. After calling this function the buffer won't be flushed. If the user pressed two keys, the first key will be read first. Another way is the function ah=0bh of DOS-interrupt 21h. Al will be 0 if no key had been pressed. al will be 255 if a key had been pressed.