i) Asynchronous DRAM:
This type of DRAM is not synchronized with the CPU clock. So, the drawback with this RAM is that CPU could not know the exact timing at which the data would be available from the RAM on the input-output bus. This limitation was overcome by the next generation of RAM, which is known as the synchronous DRAM.
ii) Synchronous DRAM:
SDRAM (Synchronous DRAM) began to appear in late 1996. In SDRAM, the RAM was synchronized with the CPU clock. It allowed the CPU or to be precise the memory controller to know the exact clock cycle or timing or the number of cycles after which the data will be available on the bus. So, the CPU does not need for the memory accesses and thus the memory read and write speed can be increased. The SDRAM is also known as the single data rate SDRAM (SDR SDRAM) as data is transferred only at each rising edge of the clock cycle. See the image in the following description.
iii) DDR SDRAM:
The next generation of the synchronous DRAM is known as the DDR RAM. It was developed to overcome the limitations of SDRAM and was used in PC memory at the beginning of the year 2000. In DDR SDRAM (DDR RAM), the data is transferred twice during each clock cycle; during the positive edge (rising edge) and the negative edge (falling edge) of the cycle. So, it is known as the double data rate SDRAM.
There are different generations of DDR SDRAM which include DDR1, DDR2, DDR3, and DDR4. Today, the memory that we use inside the desktop, laptop, mobile, etc., is mostly either DDR3 or DDR4 RAM. Types of DDR SDRAM:
a) DDR1 SDRAM:
DDR1 SDRAM is the first advanced version of SDRAM. In this RAM, the voltage was reduced from 3.3 V to 2.5 V. The data is transferred during both the rising as well as the falling edge of the clock cycle. So, in each clock cycle, instead of 1 bit, 2 bits are being pre-fetched which is commonly known as the 2 bit pre-fetch. It is mostly operated in the range of 133 MHz to the 200 MHz.
Furthermore, the data rate at the input-output bus is double the clock frequency because the data is transferred during both the rising as well as falling edge. So, if a DDR1 RAM is operating at 133 MHz, the data rate would be double, 266 Mega transfer per second.
ii) DDR2 SDRAM:
It is an advanced version of DDR1. It operates at 1.8 V instead of 2.5V. Its data rate is double the data rate of the previous generation due to the increase in the number of bits that are pre-fetched during each cycle; 4 bits are pre-fetched instead of 2 bits. The internal bus width of this RAM has been doubled. For example, if the input-output bus is 64 bits wide, the internal bus width of it will be equal to 128 bits. So, a single cycle can handle double the amount of data.
iii) DDR3 SDRAM:
In this version, the voltage is further reduced from 1.8 V to the 1.5 V. The data rate has been doubled than the previous generation RAM as the number of bits that are pre-fetched has been increased from 4 bits to the 8 bits. We can say that the internal data bus width of RAM has been increased 2 times than that of the last generation.
iv) DDR4 SDRAM:
In this version, the operating voltage is further reduced from 1.5 V to 1.2 V, but the number of bits that can be pre-fetched is same as the previous generation; 8 bits per cycle. The Internal clock frequency of the RAM is double of the previous version. If you are operating at 400 MHz the clock frequency of the input-output bus would be four times, 1600 MHz and the transfer rate would be equal to 3200 Mega transfer per second.
Difference between Static RAM and Dynamic RAM:
|It is a static memory as it does not need to be refreshed repeatedly.||It is a dynamic memory as it needs to be refreshed continuously or it will lose the data.|
|Its memory cell is made of 6 transistors. So its cells occupy more space on a chip and offer less storage capacity (memory) than a DRAM of the same physical size.||Its memory cell is made of one transistor and one capacitor. So, its cells occupy less space on a chip and provide more memory than a SRM of the same physical size.|
|It is more expensive than DRAM and is located on processors or between a processor and main memory.||It is less expensive than SRAM and is mostly located on the motherboard.|
|It has a lower access time, e.g. 10 nanoseconds. So, it is faster than DRAM.||It has a higher access time, e.g. more than 50 nanoseconds. So, it is slower than SRAM.|
|It stores information in a bistable latching circuitry. It requires regular power supply so it consumes more power.||The information or each bit of data is stored in a separate capacitor within an integrated circuit so it consumes less power.|
|It is faster than DRAM as its memory cells don’t need to be refreshed and are always available. So, it is mostly used in registers in the CPU and cache memory of various devices.||It is not as fast as SRAM, as its memory cells are refreshed continuously. But still, it is used in the motherboard because it is cheaper to manufacture and requires less space.|
|Its cycle time is shorter as it does not need to be paused between accesses and refreshes.||Its cycle time is more than the SRAM’s cycle time.|
|Examples: L2 and LE cache in a CPU.||Example: DDR3, DDR4 in mobile phones, computers, etc.|
|Size ranges from 1 MB to 16MB.||Size ranges from 1 GB to 3 GB in smartphones and 4GB to 16GB in laptops.|