Types of RAM,ROM and Cache memory

Different types of RAM:

Primary memory is the main memory of computer. It is used to store data and instructions during the processing of data. Primary memory is semiconductor memory. Primary memory is of two kinds—Random Access Memory (RAM) and Read Only Memory (ROM).RAM is volatile. It stores data when the computer is on. The information stored in RAM gets erased when the computer is turned off. RAM provides temporary storage for data and instructions.RAM stores data and instructions during the execution of instructions. The data and instructions that require processing are brought into the RAM from the storage devices like hard disk. CPU accesses the data and the instructions from RAM, as it can access it at a fast speed than the storage devices connected to the input and output unit.The input data that is entered using the input unit is stored in RAM, to be made available during the processing of data. Similarly, the output data generated after processing is stored in RAM before being sent to the output device. Any intermediate results generated during the processing of program are stored in RAM.RAM provides a limited storage capacity, due to its high cost.

• RAM is used to store data and instructions during the operation of computer.

o The data and instructions that need to be operated upon by CPU are first brought to RAM from the secondary storage devices like the hard disk.

o CPU interacts with RAM to get the data and instructions for processing.

• RAM loses information when the computer is powered off. It is a volatile memory. When the power is turned on, again, all files that are required by the CPU are loaded from the hard disk to RAM. Since RAM is a volatile memory, any information that needs to be saved for a longer duration of time must not be stored in RAM. 

• RAM provides random access to the stored bytes, words, or larger data units. This means that it requires same amount of time to access information from RAM, irrespective of where it is located in it.

• RAM can be read from and written to with the same speed.

• The size of RAM is limited due to its high cost. The size of RAM is measured in MB or GB.

• The performance of RAM is affected by—

o Access speed (how quickly information can be retrieved). The speed of RAM is expressed in nanoseconds.

o Data transfer unit size (how much information can be retrieved in one request).

• RAM affects the speed and power of a computer. More the RAM, the better it is. Nowadays, computers generally have 512 MB to 4 GB of RAM.

• RAM is a microchip implemented using semiconductors.

There are two categories of RAM, depending on the technology used to construct a RAM—

 (1) Dynamic RAM (DRAM), and (2) Static RAM (SRAM).

·         DRAM is the most common type of memory chip. DRAM is mostly used as main memory since it is small and cheap.

o     It uses transistors and capacitors. The transistors are arranged in a matrix of rows and columns. The capacitor holds the bit of information 0 and 1. The transistor and capacitor are paired to make a memory cell. The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

o     DRAM must be refreshed continually to store information. For this, a memory controller is used. The memory controller recharges all the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back.

o     DRAM gets its name from the refresh operation that it requires to store the information; otherwise it will lose what it is holding. The refresh operation occurs automatically thousands of times per second. DRAM is slow because the refreshing takes time.

o     Access speed of DRAM ranges from 50 to 150 ns.

·         SRAM chip is usually used in cache memory due to its high speed.

o     SRAM uses multiple transistors (four to six), for each memory cell. It does not have a capacitor in each cell.

o     A SRAM memory cell has more parts so it takes more space on a chip than DRAM cell.

o     It does not need constant refreshing and therefore is faster than DRAM.

o     SRAM is more expensive than DRAM, and it takes up more space.

o     It stores information as long as it is supplied with power.

o     SRAM are easier to use and very fast. The access speed of SRAM ranges from 2– 10 nanosecond.

·  The memory chips are available on a separate Printed Circuit Board (PCB) that is plugged into a special connector on the motherboard. Memory chips are generally available as part of a card called a memory module. There are generally two types of RAM modules—Single Inline Memory Module (SIMM) and Dual Inline Memory Module (DIMM).

1.   SIMM modules have memory chip on one side of the PCB. SIMM modules can store 8 bits to 32 bits of data simultaneously.

2. DIMM modules have memory chips on both sides of the PCB. DIMM format are 64–bit memories. Smaller modules known as Small Outline DIMM (SO DIMM) are designed for portable computers. SO DIMM modules have 32–bit memory.

Different types of ROM:

·         ROMs are of different kinds. They have evolved from the fixed read only memory to the ones that can be programmed and re-programmed. They vary in the number of re-writes and the method used for the re-writing. Programmable ROM (PROM), Erasable Programmable ROM (EPROM) and Electrically Erasable Programmable ROM (EEPROM) are some of the ROMs. All the different kinds of ROM retain their content when the power is turned off.

o     PROM can be programmed with a special tool, but after it has been programmed the contents cannot be changed. PROM memories have thousands of fuses (or diodes). High voltage (12 V) is applied to the fuses to be burnt. The burnt fuses correspond to 0 and the others to 1.

o     EPROM can be programmed in a similar way as PROM, but it can be erased by exposing it to ultra violet light and re-programmed. EPROM chips have to be removed from the computer for re-writing.

o     EEPROM memories can be erased by electric charge and re-programmed. EEPROM chips do not have to be removed from the computer for re-writing.

·         Flash Memory is a kind of semiconductor-based non-volatile, rewritable computer memory that can be electrically erased and reprogrammed . It is a specific type of EEPROM.

o     It combines the features of RAM and ROM. It is a random access memory and its content can be stored in it at any time. However, like ROM, the data is not lost when the machine is turned off or the electric power is cut. Flash memory stores bits of data in memory cells.

o     Flash memories are high-speed memories, durable, and have low-energy consumption. Since flash memory has no moving part, it is very shock-resistant. Due to these features, flash memory is used in devices such as digital camera, mobile phone, printer, laptop computer, and record and play back sound devices, such as MP3 players.

                  Cache Memory

·         The data and instructions that are required during the processing of data are brought from the secondary storage devices and stored in the RAM. For processing, it is required that the data and instructions are accessed from the RAM and stored in the registers. The time taken to move the data between RAM and CPU registers is large. This affects the speed of processing of computer, and results in decreasing the performance of CPU.

·         Cache memory is a very high speed memory placed in between RAM and CPU. Cache memory increases the speed of processing.

·         Cache memory is a storage buffer that stores the data that is used more often, temporarily, and makes them available to CPU at a fast rate. During processing, CPU first checks cache for the required data. If data is not found in cache, then it looks in the RAM for data.

·         To access the cache memory, CPU does not have to use the motherboard’s system bus for data transfer. (The data transfer speed slows to the motherboard’s capability, when data is passed through system bus. CPU can process data at a much faster rate by avoiding the system bus.)

Cache memory is built into the processor, and may also be located next to it on a separate chip between the CPU and RAM. Cache built into the CPU is faster than separate cache, running at the speed of the microprocessor itself. However, separate cache is roughly twice as fast as RAM.

·         The CPU has a built-in Level 1 (L1) cache and Level2 (L2) cache. In addition to the built-in L1 and L2 cache, some CPUs have a separate cache chip on the motherboard. This cache on the motherboard is called Level 3 (L3) cache. Nowadays, high-end processor comes with built-in L3 cache, like in Intel core i7. The L1, L2 and L3 cache store the most recently run instructions, the next ones and the possible ones, respectively. Typically, CPUs have cache size varying from 256KB (L1), 6 MB (L2), to 12MB (L3) cache.

·         Cache memory is very expensive, so it is smaller in size. Generally, computers have cache memory of sizes 256 KB to 2 MB.