Hello Dear Raymond
If you continue this program step by step you find that:

row0->p0.0
row1->p0.1
row2->p0.2
row3->p0.3
column0->p0.4
column1->p0.5
column2->p0.6
column3->p0.7

For scanning keypad you must generate below sequence
11111110b,11111101b,11111011b,11110111b
0FEH,0FDH,0FBH,0F7H
So,you can load 0feh in one register and then
rotate it and at most this is 4 times.
therefore r3=4,r2=0feh
clearly,at first r4=0 that means if pressed key
is in first row?
r4 is incremented if pressed key is in second row ...
finally r4 is 0/1/2/3 and r6 is 0/1/2/3
if key 5 is pressed then r4=1 and r6=1
that means first row & first column
DYTA=1*4+1=5
and code 5 is returned from lookup table
I Hope it is Useful.
Mehdi N.Rahni