How can I distinguish which key is pressed in one interrupt handle routine?

I need to write a bare board driver(no operating system) for 4x4 matrix
keyboard.I use the S3C2410A MPU of samsung company.
The hardware circuit description are as following:
					VCC +3.3V
					   |
					Pull-up Resistance
					   |GPG3------------------Key1---------------Key2	
			 |		   |
			 |		   |
GPF1------------------Key3---------------Key4
			 |		   |
			 |		   |
			 |		   |
GPF2----------------------		   |
GPF0----------------------------------------

The port GPG3 and GPF1 has multi-function.I configure them as EINT11 and
EINT1 function.

When the key1 or key2 is pressed,It  trigger the interrupt handle
routine,but how can I distinguish which key is pressed ?

The same problem:
when the key3 or key4 is pressed,It  also trigger the inputer handle
routine,but I don't know how to distinguish which key is pressed.

Any suggestion?

Thank you.


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