Hardware · Topic 4 of 13
RAM Characteristics
Compare DDR3, DDR4 and DDR5 memory, DIMM and SODIMM sizes, ECC, buffered memory, channels, speed and latency, and know how to match RAM to a system.
- 4 min
- DDR3, DDR4 and DDR5 are not compatible with one another.
- DIMM is for desktops; SODIMM is for laptops.
- DDR3 DIMMs have 240 pins, and DDR4 and DDR5 DIMMs have 288.
What it is
Random access memory (RAM) is the fast, temporary memory that the central processing unit (CPU) uses to hold the programs and data a computer is using. It loses its contents when power is removed.
RAM comes on small circuit boards called modules. Choosing the right type, size and speed means matching the module to the motherboard and the job.
How it works
Generations
Double data rate synchronous dynamic RAM (DDR SDRAM) has gone through several generations. Each is faster and uses less power, but they are not interchangeable. The notch on each module sits in a different place, so a wrong generation will not fit the slot.
| Generation | DIMM pins | SODIMM pins |
|---|---|---|
| DDR3 | 240 | 204 |
| DDR4 | 288 | 260 |
| DDR5 | 288 | 262 |
A dual inline memory module (DIMM) is the full size stick used in desktops and servers. A small outline DIMM (SODIMM) is the shorter stick used in laptops and small computers. DDR4 and DDR5 DIMMs both have 288 pins, but their notches differ, so they cannot be swapped.
Speed and latency
Memory speed is shown as a transfer rate (such as DDR4-3200) or a module name (such as PC4-25600). A bigger number is faster. The motherboard and CPU set the top speed they support, and extra speed is lost if the board cannot use it.
Latency is the delay between a request and the data. One common measure is the column address strobe (CAS) latency, written CL. At the same speed, a lower CL number is faster.
Capacity
Capacity is measured in gigabytes (GB). A 32 bit operating system can use about 4 GB at most, while a 64 bit one can use far more. Each motherboard also has a maximum total and a maximum per slot.
More RAM helps when many programs run, when a virtual machine runs or when big files are edited. When RAM runs out, the system uses a paging file on the drive, which is far slower.
Error correction and buffering
| Feature | Meaning |
|---|---|
| Non ECC | Regular memory with no error checking; used in most home and office PCs |
| Error correcting code (ECC) | Extra bits let the module find and fix some memory errors; used in servers |
| Unbuffered | Standard; the controller talks straight to the chips |
| Registered or buffered | A register chip reduces electrical load, so more modules can be used; seen in servers |
ECC memory needs a CPU and motherboard that support it. Registered and unregistered memory cannot be mixed in one system.
Channels
Dual channel memory lets the memory controller use two modules at the same time, which raises bandwidth. Boards may also support single, triple or quad channel. Slots for a pair are usually colour coded, and the manual shows which ones to fill.
For best results fit matching modules of the same size, speed and type in the paired slots.
Single and double sided
A single sided module has chips on one side only, while a double sided one has chips on both. Sides do not always match the internal organisation, so use the system's compatibility list.
Installing RAM
- Power off, unplug and use electrostatic discharge (ESD) protection.
- Open the clips at the ends of the slot.
- Line up the notch on the module with the slot key.
- Press down evenly until the clips snap up and lock.
- Start the computer and check that the full amount shows up.
If new RAM does not fit, check the notch. A different generation will not fit, and forcing it damages the slot.
What to remember for the exam
- DDR3, DDR4 and DDR5 are not compatible with one another.
- DIMM is for desktops; SODIMM is for laptops.
- DDR3 DIMMs have 240 pins, and DDR4 and DDR5 DIMMs have 288.
- ECC detects and corrects errors; it is common in servers.
- Registered memory is for servers and cannot be mixed with unregistered memory.
- Dual channel uses matching modules in paired slots for higher bandwidth.
- A lower CAS latency number is faster at the same speed.
- A 32 bit operating system sees about 4 GB at most.
Common mistakes
Buying the wrong generation. Check the motherboard manual for DDR3, DDR4 or DDR5 before buying.
Mixing speeds and expecting the faster speed. Mixed modules usually run at the slowest speed.
Using ECC in a board that does not support it. It may not work, or it may run without error correction.
Filling the wrong slots. Modules in the wrong slots may run in single channel.
Not seating a module fully. A half seated stick can cause a failure to start or beeps.
Check yourself
What type of memory module do most laptops use?
SODIMM.
What does ECC memory do?
It uses extra bits to detect and correct some memory errors, which is why servers use it.
Why do you fit matching modules in the paired slots?
To enable dual channel mode, which raises memory bandwidth.
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