Let us consider the operation of the EPROM device in more detail. Consider the pining details below again

 
Before we examine the interface means of the EPROM, it is worth giving the operation a little thought. Each memory cell in the EPROM  could store 1 byte of data i.e. 8 1, 0's. In order to access each location the EPROM has to assign each location a unique address, this is used to access the data. This is similar to the concept of a telephone number  each house has its own number. The device is constructed to request this address from external pins namely: A0 -> A15. The data pathway to the device is by means of the data bus i.e. DQ0 -> DQ7.  The remaining pins of the device are concerned with selecting the device i.e. power on (CE) and reading the data (OE).  These additional pins are often termed 'control bus'. The operation of the device to read data is simple in that we:

    1) Turn on the device CE =0
    2) Present the address on the address pins A0 -> A15
    3) Output enable the device to read the data (OE =0)
    4) Read the data of the data bus (D7 -> D0)
 
The timing of these operations is sometimes critical since it takes a finite time to access the data, therefore we must be careful not to read the data too quick. This time is termed the access time of the device, for this EPROM it is 150ns. RAM devices are similar in operation besides they have a read/write pin which enables the user to input and output data. The remaining digital systems employ VLSI methods ' Very Large scale integration', these devices are the heart of the modern PC, i.e. microprocessors, complex function devices. Before we discuss microprocessor we really should look at base systems and binary, hexadecimal and deanery systems