Memory Devices – Notes – 1

1. All memory devices store binary logic levels (1s and 0s) in an array structure. The size of each binary word (number of bits) that is stored varies depending on the memory device. These binary values are referred to as data.

2. The place (location) in the memory device where any data value is stored is identified by another binary number referred to as an address.Each memory location has a unique address.

3. All memory devices operate in the same general way. To write data in memory, the address to be accessed is placed on the address input, the data value to be stored is applied to the data inputs, and the control signals are manipulated to store the data.To read data from memory, the address is applied, the control signals are manipulated, and the data value appears on the output pins.

4. Memory devices are often used along with a microprocessor CPU that generates the addresses and control signals and either provides the data to be stored or uses the data from the memory. Reading and writing are alwaysdone from the CPU’s perspective.Writing puts data into the memory, and reading gets data out of the memory.

5. Most read-only memories (ROMs) have data entered one time, and from then on their contents do not change. This storage process is called programming. They do not lose their data when power is removed from the device. MROMs are programmed during the manufacturing process. PROMs are programmed one time by the user. EPROMs are just like PROMs but can be erased using UV light. EEPROMs and flash memory devices are electrically erasable and can have their contents altered after programming. CD ROMs are used for mass storage of information that does not need to change.

6. Random access memory (RAM) is a generic term given to devices that can have data easily stored and retrieved. Data are retained in a RAM device only as long as power is applied.

7. Static RAM (SRAM) uses storage elements that are basically latch circuits. Once the data are stored, they will remain unchanged as long as power is applied to the chip. Static RAM is easier to use but more expensive per bit and consumes more power than dynamic RAM.

8. Dynamic RAM (DRAM) uses capacitors to store data by charging or discharging them. The simplicity of the storage cell allows DRAMs to store a great deal of data. Because the charge on the capacitors must be refreshed regularly, DRAMs are more complicated to use than SRAMs. Extra circuitry is often added to DRAM systems to control the reading, writing, and refreshing cycles. On many new devices, these features are being integrated into the DRAM chip itself. The goal of DRAM technology is to put more bits on a smaller piece of silicon so that it consumes less power and responds faster.

9. Memory systems require a wide variety of different configurations. Memory chips can be combined to implement any desired configuration, whether your system needs more bits per location or more total word capacity. All of the various types of ROM and RAM can be combined within the same memory system.

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