5.2 Stored-Program Concept 127 Recall that data and instructions reside in main memory and are treated alike. This means that an instruction could be changed while a program is executing. How can this happen? There could be an instruc-tion to take the contents of the location that contains an instruction, add or subtract a value from it, and return it to the same location. There are times that you might actually want to do this. However, inadvertently changing a program can be very costly. ROM memory solves this problem. ROM stands for R ead O nly M emory. The contents in locations in ROM cannot be changed. Their contents are permanent and cannot be changed by a stored operation. Placing the bit pattern in ROM is called burning . The bit pattern is burned either at the time the ROM is manufac-tured or at the time the computer parts are assembled. RAM and ROM are differentiated by another very basic property. RAM is volatile; ROM is not. This means that RAM does not retain its bit
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