Please enter the complete URL - including the "http..." bit - so that the forum will automatically make it clickable:

http://files.dalsemi.com/microcont...ds1620.asm

So the original was:

CALL IN_DATA   ;Get LSB of temp.
MOV  R1,A      ;Save LSB.

CALL IN_DATA   ;Get MSB/MSb of temp.
MOV  R2,A      ;Save MSB.

and you changed it to:

CALL IN_DATA   ;Get LSB of temp. 
MOV  R1,A      ;Save LSB. 

CALL IN_DATA   ;Get MSB/MSb of temp. 
MOV  R2,A      ;Save MSB. 

MOV P0,A       ;<<<-------My change made to the original program!! 

So, do you now understand what "LSB" and "MSB" mean?

Do you now understand what is actually in the 'A' register at the point where you added your line? And, thus, can you now see why your extra line writes only the MSB (which contains only the sign bit) to the port?