Sorry my english is too poor so that I can't tell really what I want to say. I will not be happy if one of you turns against one other.

I'am 21, I have actually two MCU cores at home; the classic 8051 itself and the 8031 with is a romless one. I did not metion what chip I want to use because I don't only want to decoder the memory addresses for only the 8051 chip but with either the Microcontroller or Microprocessor chip. When I was in high school ( I'm graduated from high school with Electronic), the teachers I had was not experienced, but I tried myself to training with programing, the chip with work with was the 6502! Now I'm admited from an university an I do not start course yet but what I want now is to train myself with the decoding memory address. I know that the memory chip has the addresses pins and the data pins and others pins to be connected with the processor pins which can do the decoding I'm thinking about the 8051, but the way are differents if we use a microprocessor, this last one has not an embeded ports nor EEPROM. When we connect at least two EEPROMs or ROMs to a processor we need to select either that ROM or RAM chips without working with the other ROM or RAM chip(let say I want a memory map of 8k x 16 but I have on hand only 2k x 8 of chip memory RAM or ROM, now how to configure the hardware to choose to range on either the first two RAM or ROM chip or the last one?) sometimes I see ones use an OR guate to decoder and someone else use a NAND guate like Richard said before. . how do you reach at the conclusion that you will use A15 or A13... to decoder the address with the OR guate or NAND guate ? thanks all.