The main memory of the computer holds the data on which the processor is currently working. Main memory is also referred to as the primary memory of RAM (Random Access Memory). Whenever you need to execute any program, the program and related data are copied to the main memory from which the process can access it directly.
The created program and its relevant data are usually stored in secondary memory (your system’s hard drive). But the processor cannot directly access the secondary memory. Thus, the program and its relevant data are first copied to the main memory to execute a program. Then the processor can directly access the program elements from the main memory.
As main memory is directly connected to the processor, the movement of program elements, i.e. instructions and data, in and out of the processor is very fast.
Main Memory of Computer
What is Main Memory Made up of?
The elements of main memory are made up of semiconductor devices. The semiconductor devices are preferred to make the main memory as they are directly accessible by the microprocessors. Along with this, the access time to semiconductor memory is approximately equivalent to the operating time of the microprocessor.
Types of Main Memory
Based on the technology implemented, the main memory can be classified into two types:
- Dynamic RAM
- Static RAM
Dynamic RAM (DRAM)
Dynamic RAM is a semiconductor memory where data is stored on the memory cells in the form of charge on the capacitors. However, the presence of charge on the capacitor is interpreted as binary value 1. On the other hand, the absence of charge on the capacitor is interpreted as binary 0.
Now, the capacitor’s natural tendency is to get discharged in some time. So, dynamic Ram must be refreshed periodically to maintain the data stored in the memory cell. This is why it is referred to as dynamic, as the charge on the capacitor leaks away, even if there is a continuous power supply.
Let’s now discuss the structure of dynamic RAM. Well, DRAM is made up of multiple cells. The figure below shows you a single memory cell of DRAM that stores a single bit of data, either 1 or 0, i.e. depending on whether there is a charge on the capacitor.
You can notice the address line in the cell above; it gets activated whenever the processor reads or write to the cell. A transistor also acts as a switch that, when closed, allows voltage to flow through the address line and activates it. When the switch is open, it doesn’t allow voltage to flow through the address line.
How Read-Write Operation is Performed on the Memory Cell of DRAM?
The voltage is applied on the bit line to perform the write operation. If the voltage is high, it is interpreted as binary 1, and if the voltage is low, it is interpreted as binary 0. Accordingly, it activates the address line, transferring the charge to the capacitor.
When the address line is activated, the transistor is turned on to get the charge on the capacitor to be fed into the bit line and the sense amplifier to perform the read operation. The amplifier then senses the charge on the capacitor and compare it to a reference value, and identifies whether the charge on the capacitor is logical 1 or 0. Whenever the processor readout the value from the memory cell of DRAM, it gets discharged. So, they must be restored to complete the operation.
However, the memory cell of DRAM stores a single bit at a time (either 0 or 1). It is an analog device; thus, the charge value it stores lies within a range. Therefore, the threshold value determines whether the charge is 0 or 1.
Static RAM (SRAM)
Static RAM is a semiconductor memory that uses the same logical elements as used in a processor. It uses traditional flip-flop logic gates to store the binary value 0 or 1 in the memory cell of SRAM. SRAM is a volatile memory, which means it holds data as long as the power supply is on.
The figure below shows the individual cell of static memory. You can notice that the four transistors are cross-connected with each other. In this form, the transistors produce a stable logic state.
Now to denote the logic state 1, the transistor T1 and T4 are off, whereas T2 and T3 are on at this point, C1 is high, and C2 is low.
Now to denote the logic state 0, the transistors T1 and T4 are on, whereas T2 and T3 are off at this point C1 is low, and C2 is high.
Unlike DRAM, the charge here doesn’t get leaked, so we don’t need to refresh the memory cell periodically. Like DRAM, the address lines are responsible for opening and closing the switch. The address line controls the controllers of transistors T5 and T6. When the voltage is supplied to the address line, it turns the switches on, allowing the read-write operation.
To perform the write operation, bit line B is applied to the desired bit, and the complement of this bit is applied to the bit line. This compels the transistors T1, T2, T3 and T4 to be at the accurate logic state. However, the read operation is performed by reading the bit value at bit line B.
Advantages of Main Memory
- Main memory is faster than the secondary memory of the computer.
- It even consumes less power compared to secondary memory.
- Increased size of main memory enhances the speed of your computer.
- The processor can directly access it.
- Capable of reading and writing any kind of data.
Disadvantages of Main Memory of Computer
- Memory is volatile.
- It needs a continuous power supply for operating.
- Slower if compared to the cache memory of the computer.
Thus, we have discussed the computer’s main memory in detail.