I need to write a MIPS program will let the user to enter strings one per line representing valid decimal integers in the range 0 to 999999999999. I need to use 64 bits for each integer. and use some method to convert ascii to integer. here is an example that apparently does it.. but I don't understand it

addi	$t3, $t1, -48		# Convert from ascii to integer
	mul	$t2, $t2, $t4		# Multiply to put integer in 
					# right place

also I am not sure what is meant by using 64 bit integers?