An 8 Kbyte ROM with an active low Chip Select input (CS') is to be used in an 8085 microprocessor based system. The ROM should occupy the address range 1000 H to 2FFFH. The address lines are designed as A15 to A0 , where A15 is the most significant address bit. Which one of the following logic expressions will generate the correct CS signal for this ROM?
(A) A15 + A14 + (A13.A12+A13'.A12')
(B) A15.A14.(A13 + A12)
(C) A15' + A14'.(A13.A12' + A13'.A12)
(D) A15' + A14'+ A13.A12
(Question credit: EC-Gate 2016 SET-2)
I am trying to learn digital circuits on my own. Above is a question I am unable to solve.
According to me, the aim is to choose an option which would make the chip select signal to be 0. So both option A and B should be correct, but that is not the case. Moreover, A12 is address line for the ROM so how can we use it for CS signal.
Edit: Thanks @AJN and @Brian Drummond. I understood why option A is correct and not B but I have one more confusion, according to what I know some of the address lines coming out of the 8085 are used as address lines for the memory (ROM in this case) and others (the higher order address lines) are used as the chip select signal. My question is that when A12 is used as address lines for the ROM then can we use it for CS signal also. According to me A15, A14 and A13 should go to chip select signal and others should go to the address lines.