Computer Memory

Introduction Random Access Memory or RAM is the temporary storage in a computer system. This means that once a computer is turned off, everything stored in it is lost. When an application is opened, it is copied into RAM and therefore having adequate computer memory is important. Having a fast processor, but without enough memory can result in a slower running computer. Just as motherboards are designed for certain CPUs, motherboards are also designed for certain types of RAM.

Types of RAM The earliest type of memory was known as Dynamic RAM (DRAM) and Synchronous Dynamic RAM (SDRAM) but the current prevailing standard is the, Double Data Rate (DDR) and Static RAM (SRAM).  

SDRAM It is called SDRAM because its speed is synchronized with the motherboard's speed. In other words, the speed of the RAM matches the speed of the motherboard. Therefore, if a motherboard's speed was 133 MHz, the memory ran at 133 MHz (called PC133 memory). If the motherboard speed is 180 MHz, the memory ran at 180 MHz (called PC180 memory), and so on. This type is called Single Data Rate and is basically found on old motherboards. Its dynamic because its has to be refreshed all the time or it will forget what its holding

SDRAM

DDR As processors got faster, there was the need to speed things up hence Double Data Rate RAM (DDR) was developed. Practically every motherboard today supports some type of DDR RAM. As the name implies, it was designed to run at twice the speed of the motherboard speed i.e. it basically doubles the rate of data. So for example where a motherboard has a speed of 133MHz, DDR memory would run at 266MHz (called DDR266 or PC 266 memory). DDR2 and DDR3 were later developed, with each running at twice the clock speed of its predecessor. DDR2 running at speeds from 400 to 1066 MHz and DDR3 from 800-2000MHz. 

DDR In addition to speed, DDR2 and DDR3 were developed to run at lower voltages than regular DDR RAM, resulting in less power consumption. DDR runs at 2.5v while DDR2 runs off 1.8v and DDR3 1.5v

DDR

DDR3

SRAM Static RAM (SRAM) is a lot faster because it doesn't have to constantly be refreshed like Dynamic RAM (DRAM). But since it is expensive, SRAM is limited to being used in small amounts as CPU cache. CPU cache are small amount of memory that store information the CPU accesses frequently.  

Physical Characteristics SDRAM has 168 pins and two notches (holes). Regular DDR memory has 184 pins and DDR2 and DDR3 have 240 pins. Each type of DDR has one notch that is in a different location to prevent installing the wrong type.  Just like processors fit into certain sockets, different types of computer memory fit into certain slots called banks. Banks are designed to match the number of pins on a stick of RAM, much like CPU sockets are designed to match the number of pins on different processors.

Physical Characteristics (Modules) Memory module is another name for the RAM chip. They come in different sizes and have different pin configuration All types of SDRAM and DDR RAM are DIMM modules and fit into DIMM slots. DIMM stands for Dual Inline Memory Module. Older memory used the SIMM module. Laptops use a smaller version of DIMMS called SODIMM (Small Outline DIMM) and come in 72, 144, or 200 pin configuration. 

DIMM Slots

Memory Speed Memory clock Speed and Timings (latency) are the two main factors that determine how fast your memory is. The clock speed is how fast memory can read and write to data and is measured as frequency which is expressed in Hertz, i.e. "cycles per second”. It is the  rate at which it performs internal operations Memory requires a fixed number of clock ticks (or clock cycles) to execute each instruction. The faster the clock, the more instructions the memory can read and write per second. Timing also known as latency refers to the time it takes for the memory to react to a request for data

Clock Speed So, the DDR3 2000 will perform 2000 cycles a second while the DDR3 1600 will perform 1600 cycles per second while a CAS9 RAM will take 9 cycles to respond and the CAS6, 6 cycle Putting it together: the DDR3 2000 CAS9 will take 9/2000 , which is equal to 0.0045 seconds to respond while the DDR3 1600 CAS 6 will take 6/1600, which is equal to 0.0038 seconds, to respond. Thus, the 1600 one is faster.

Clock Speed DDR, DDR2 and DDR3 memories are classified according to the maximum speed at which they work as well as their timing. Their specs are written as (DDR 400, PC3200) The first number indicates the maximum clock speed that the memory chip supports. This means DDR400 memory chip work at 400MHz at the most, DDR2-800 can work up to 800 MHz and DDR3-1333 can work up to 1,333 MHz. However it is not the real clock speed of the memory. The real clock speed is half of the labeled clock speed

Clock Speed The second number indicates the maximum transfer rate that the memory reaches in MB/s. DDR400 transfer data at 3200 MB/s at the most and hence they are labeled PC3200. DDR2-800 transfer data at 6,400 MB/s and hence labeled PC2-6400. DDR3-1333 can transfer data at 10,664 MB/s and are labeled PC3-10600 OR PC3-10666. The number “2” or “3” are used after DDR or PC to indicate DDR2 or DDR3 not DDR

Timings Two memory modules with the same maximum transfer rate can achieve different performance levels due to their timings or latency. Column Access Strope (CAS)  also known as RAM timing measure the time the memory chip delays doing something internally. CAS latency is also expressed in “cycles per second. CL tells us how many clock cycles the memory module will delay in returning data requested by the CPU. In order word it is the time it takes between a command having been sent to the memory and when it begins to reply . A with a CL7 will delay 7 clock cycles to respond While two memory modules may run at the same clock speed, one will be faster and deliver data sooner .

Memory Specifications

Multi-Channel Memory Most desktop systems can offer improved memory performance when the memory is installed in pairs or triples. This is referred to as dual-channel when it is in pairs and triple-channel when in threes. This is a system that provides the memory with two (dual channel) or three (tri-channel) memory channels instead of one. This means that while one memory controller handles the reading and the writing, the other controller prepares the next access and this prevents any type of delays.

Dual Channel & Three Channel (tri-channel): Dual channel is a process that allows twice as much information to be sent to and from memory at the same time. As the name implies, two memory controllers are used for data transfer instead of one. Three channel moves data much faster with three memory controllers.  Memory sticks has to be installed in pairs (dual Channel) or in threes (tri channel) in the proper slots, which are usually colour coded. To avoid possible compatibility problems with motherboards, it's best to use identical memory sticks

Dual Channel & Three Channel (tri-channel): There is no such thing as dual channel or three channel memory. You may see matched sticks of RAM advertised as such, sometimes called memory "kits", but this is simply memory that has been tested to work on a dual or three channel board. Remember, the technology is on the motherboard, not on the memory itself. 

Dual Channel & Three Channel (tri-channel): Dual channel uses DDR, DDR2, or DDR3 RAM and is supported by most motherboards. Tri-channel only uses DDR3. 

Colour-Coded Dual Channel Slots

Memory Capacity Memory capacity is the total amount of RAM that can be used in a PC. It also refers to the amount of data an individual memory module can store. Some RAM module capacities are between 1GB and 64GB.  All the slots on a particular board have the same maximum capacity. The sum of these capacities determines the total amount of memory a system can support. For example, a motherboard may have four slots, each with a capacity of 1GB (gigabyte). If you add these then the total amount of memory that can be used for that PC is 4GB of memory. 

What Determines Amount of RAM Your Motherboard determines the maximum amount you can install (due to RAM slots available) . However your CPU and Operating System determine how much of that RAM can be utilized. A 32 Bit OS running a 64 bit processor can only utilize a max of 4 GB, regardless of the amount you have installed.

Virtual Memory Virtual memory increases the available memory your computer has by using hard disk space for additional memory allocation. However, since the hard drive is much slower than the RAM, data stored in virtual memory must be mapped back to real memory in order to be used.

Typical Memory Specification Northbridge Intel Haswell Southbridge Intel QM87 Memory Type DDR3 Memory Size 8 GBytes Channels Dual Memory Frequency 799.0 MHz CAS# latency (CL) 7.0 8GB (2 x 4GB) 240-Pin DDR3 1600 (PC3 12800) 1.5v, Desktop Memory

How to Install Desktop Memory One of the easiest things to do is install desktop memory. To put in a stick of RAM, look at the center of the slots and you will see a notch. On the ends of each slot you will see a small latch. Pull them down if they are not already.  Next, look at the memory and notice the hole in the connector. 

Place the ends of the RAM into the ends of the slot Place the ends of the RAM into the ends of the slot. Make sure the hole is over the notch. Push down on the ends of the memory until the latches on the slot attach themselves to the sides of the memory. You will hear a click when this happens. 

Conclusion That's all there is to it! When handling memory, try not to touch the contacts on the bottom. If your system supports dual channel and you want to use it, check the documentation to see which slots to populate. 

END OF LECTURE