Random Access Memory: RAM Explained


Random access memory is an important thing to know about when building computers, or even when just buying a new computer. You will often hear RAM be simply referred to as ‘memory’. On the other hand, there is also ROM, which stands for Read-Only Memory. RAM and ROM should not be confused as they are two separate types of memory.  Here I will discuss only Random Access Memory, and save ROM for another day.

So, what is RAM? Physically, they are simply the thin rectangular sticks that are (in parallel) plugged in to your motherboard. What they do is hold information that you access, such as files and folders. This information will stay in the RAMs memory banks for as long as they are supplied power, but once power is lost – so is the information held inside RAM. Why does it constantly need power?  That is because RAM is volatile. And WTH is that?!

In order to really understand this, we need to learn about the two most types of RAM:

DRAM: Dynamic Random Access Memory

SRAM: Static Random Access Memory

Dynamic RAM is again, volatile, meaning that it needs to be refreshed many times in order to retain the information inside its memory banks. This ‘refreshing’ is more or less reminding itself what it is storing. Even while supplied power the memory cells will still weaken, until it is refreshed again. This happens over, and over, at very fast rates in order to retain this information. Once power is turned off, the information inside DRAM will be almost immediately lost. DRAM is used as the main memory for computers and is relatively cheap.

Like DRAM, Static RAM is also volatile, but the difference with this kind of RAM is that it is able to hold information for a longer period of time and does not need to be refreshed. This is because the cells inside SRAM require four to six transistors, where DRAM only has one. Simply, there is more help to retain the information for longer periods. Once power is turned off, the information inside SRAM will over time be lost. SRAM is much faster (and much more expensive) than DRAM, and for that reason is used as cache memory inside CPUs.

Now that we know what DRAM and SRAM are, we can talk about the different kinds of DRAM. As you will see, as RAM technology is improving and able to process data faster, it is also requiring less voltage to do so.

First – imprint this equation in to your mind:

Transfer rate (or MT/s) x 8 bytes = Bytes per second (data throughput)

(8 bytes being the width of all DDR SDRAM)

Note: MT/s is mega transfers per second. Or simply, a million transfers per clock cycle.

Later on these equations might get longer. And I know people might comment and say there are with simpler equations, and that mine are a little drawn out. But these are the original equations for determining data throughput, and in the end multiplying MT/s by 8 bytes will always give you your throughput. But I take the extra steps to hopefully make you understand how we came to the figures. Believe me, I am not a wizard at math and I have dreaded doing this blog on RAM because it can at times be a little complex to understand, but I believe it is rather important. That said, moving on.

SDRAM:  Synchronous Dynamic RAM. This type of RAM ‘syncs’ itself to the system clock speed. The CPU is also tied to the system clock speed. Meaning, SDRAM syncs itself with the same timing of the CPU. The CPU communicates to the Memory Chip Controller (also tried to system clock) telling the SDRAM when they are able to transmit data, and then transmits this data through independent banks on the memory chips simultaneously one time per clock cycle. If you relate it to taking breaths, SDRAM transmits one piece of data every time you take a deep breath. SDRAM also uses pipelining, which means the memory chips are able to accept new information before processing the old information, achieving higher data transfer rates.

Say you have a system clock speed of 100MHz. Your transfer rate would be 100 MT/s. And one stick of RAM which has 8 bytes. Multiplying the MT/s by 8 bytes will give you the rate (megabytes/second) in which your system processes data.

100 MT/s x 8 bytes = 800MB/second = PC800 = PC rating of SDRAM operating with a clock rate of 100 MHz

RAM Clock Rate I/O Bus Clock MT/s
SDRAM 100MHz 100 MHz 100 MT/s

SDR can also stand for Single Data Rate. The power consumption for SDRAM is 3.3V.

DDR-SDRAM: Double Data rate SDRAM. Where SDRAM transmits data one time per clock cycle, double-data rate will transfer two. This is referred to as “double pumping.” While maintaining the same clock frequency, DDR doubles the input/output speed. If we think of the breathing concept again, DDR-SDRAM will send data when you take a deep breath in, and then send data again when you fully exhale. That being said, DDR-SDRAM is sending data on the rise and fall of a full clock cycle.

Referring back to our equation of MT/s x 8 bytes = data throughput (MB/s), since DDR doubles the data rate of SDRAM (still using same clock rate of 100MHz), we would get the following equation:

200 MT/s x 8 bytes = 1600MB/second = PC1600 = PC rating of DDR SDRAM operating with a clock rate of 100 MHz

RAM Clock Rate I/O Bus Clock MT/s
DDR SDRAM 100MHz 100 MHz 200 MT/s

The power consumption is roughly 2.6V. Unfortunately, DDR is not backward compatible with SDRAM.

DDR is comprised of the following:

RAM Clock Rate I/O Bus Clock MB/s
DDR-200 100 MHz 100 MHz 1600
DDR-266 133.33 MHz 133.33 MHz 2133
DDR-333 166.67 MHz 166.67 MHz 2666
DDR-400 200 MHz 200 MHz 3200

DDR2-SDRAM: Double Data Rate 2nd Gen. SDRAM. The benefit of DDR2 is that it halves the speed of the internal clock to that of the data bus. Simply, it doubles the bus speed. With a higher bus speed it is able to transfer double that of DDR: four transfers per clock cycle. So our original 100 MHz doubles our internal bus speed to 200 MHz. And DDR2 quadrupled our original clock cycle speed (100 MHz) to give us our transfer rate of 400 MT/s.

Referring again back to the equation, we would get the following:

400 MT/s x 8 bytes = 3200MB/second = PC3200 = PC rating of DDR2 SDRAM operating with a clock rate of 100 MHz

RAM Clock Rate I/O Bus Clock MT/s
DDR2 SDRAM 100MHz 200 MHz 400 MT/s

Due to the internal clock at half speed, power consumption is reduced to roughly 1.8V. Like DDR, it is not backwards compatible.

DDR2 is comprised of the following:

RAM Clock Rate I/O Bus Clock MB/s
DDR2-400 100 MHz 200 MHz 3200
DDR2-533 133.33 MHz 266.66 MHz 4266.67
DDR2-667 166.67 MHz 333.33 MHz 5333.33
DDR2-800 200 MHz 400 MHz 6400
DDR2-1066 266.67 MHz 533.33 MHz 8533.33

DDR3-SDRAM: Double Data Rate 3rd Gen. SDRAM. With DDR3, we are tripling the clock rate to get our bus speed. And to get our mega transfers we are “doubling” yet again, from four data transfers to eight data transfers, per clock cycle, giving us 800 MT/s.

Again, back to the drawing board with our equation:

800 MT/s x 8 bytes = 6400MB/second = PC6400 = PC rating of DDR3 SDRAM operating with a clock rate of 100 MHz

RAM Clock Rate I/O Bus Clock MT/s
DDR3 SDRAM 100MHz 400 MHz 800 MT/s

Power consumption is reduced from 1.8V to 1.5V. And like the generations before it, DD3 is not backward compatible with DDR2 or DDR.

DDR3 is comprised of the following:

RAM Clock Rate I/O Bus Clock MB/s
DDR3-800 100 MHz 400 MHz 6400
DDR3-1066 133.33 MHz 533.33 MHz 8533.33
DDR3-1333 166.67 MHz 666.67 MHz 10666.67
DDR3-1600 200 MHz 800 MHz 12800
DDR3-1866 233.33 MHz 933.33 MHz 14933.33
DDR3-2133 266.67 MHz 1066.67 MHz 17066.67

DDR4-SDRAM: Double Data Rate 4th Gen. SDRAM. First off, systems that are using DDR4 SDRAM cannot operate at a clock rate of 100 MHz. DDR4 requires a minimum clock rate of 200 MHz, so that is where I will explain.  With DDR4, we are tripling the clock rate (now 200MHz) to get our bus speed of 800 MHz. And of course, our mega transfers are “doubled” that of DD3, from 8 to 16.

1600 MT/s x 8 bytes = 12,800/second = PC12800 = PC rating of DDR4 SDRAM operating with a clock rate of 200 MHz

RAM Clock Rate I/O Bus Clock MT/s
DDR4 SDRAM 200MHz 800 MHz 1600 MT/s

Power consumption is reduced from 1.5V to roughly 1.2V. And of course – not backward compatible.

DDR4 is comprised of the following:

RAM Clock Rate I/O Bus Clock MB/s
DDR4-1600 200 MHz 800 MHz 12800
DDR4-1866 233.33 MHz 933.33 MHz 14933.33
DDR4-2133 266.67 MHz 1066.67 MHz 17066.67
DDR4-2400 300 MHz 1200 MHz 19200

For a long time now people thought that DDR4 would be the finale when it came to DRAM, but DDR5 will soon be reaching our doorstep, with a release in the year 2020. And when I say soon, I mean like a year and a half after it is actually released. DDR5 is slated to be installed in to servers first before is it released to the major population. Since we are already at transfer rates of up to almost 20 GB/s (standard, without overclocking), I’m curious as to what DDR5 will have in store for us.

All I know is that it will not be backward compatible. Spoiler alert!


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