Although Erik may opine otherwise, there is no such thing as immediately
naah, I agree. Just when someone states that milliseconds is "immediately" do I "opine otherwise".

Even with my current f130s an interrupt can take almost a microsecond to execute.

to the question by the OP
a way to do keys which I use is as follows

3 variables (since I have not seen if you use C or asm, I use byte and bit)
byte old key
byte new key
bit key is new

run a timer at 10ms (or whatever is needed for debounce)
timer ISR:
read keys to new key
if new key different from old key
{
old key = new key
set key is new
)

in the main loop:
if key is new is set
{
if old key = 0 - just release of a key
else process key
clear key is new
}


Erik