Why do we usually put the first memory card in the DIMM slot closest to the CPU

Press the RAM into the socket to engage the clasps on each end, which will snap tight around the edge of the RAM when plugged in.
Typically, you should start from the slot closest to the CPU and work your way from left to right.

This article explains what Motherboard RAM slots are, why they are used, and how to use them.

RAM slots are often found in pairs and are sometimes color-coded for identification. We should note that RAM slots in a desktop look different than RAM slots in a laptop. A RAM module is rectangular and has a connector on one of the long sides.

RAM slots or sockets on a PC motherboard are long channels, generally located close to the CPU. There are clasps on each end of the socket, which will snap tight around the edge of the RAM when plugged in. Pressing the RAM into the socket will engage these clasps, so they must be disabled before you can remove the currently installed RAM. You usually push the clasps away from the memory module, and they help disconnect the module from the motherboard.

Typically, motherboards have a total of 4 RAM slots or two pairs when they are dual-channel. Some high-end motherboards may contain as many as eight slots, and in supercomputers, there may be multiple motherboards per system, up to 32 slots total.

Consumer desktops rarely have more than 4 slots, however. As for how much RAM is supported per slot, it depends on the motherboard. Most current or modern motherboards support anywhere from 8GB to 32GB of RAM per slot, with the lower end of that threshold being more common.

There are a couple of reasons why motherboards come with 4 RAM slots, even if you’re not going to use them all right away.

First, it improves upgradeability as you can always expand total RAM capacity later by adding more memory.

Second, it allows you to have two slots per channel when they are run in dual-channel mode. More channels translate to wider bandwidth, offering faster data transfers and much-improved performance. It means that 2 low-capacity RAM modules running in dual-channel can run just as fast, if not faster, than a higher-capacity single module.

Essentially, you can plug RAM modules into any slot, but that’s not what you should be doing if you want to optimize performance. Typically, you should start from the slot closest to the CPU and work your way from left to right.

Modern motherboards allow two similar RAM modules—of the same speed and generation—to run in dual-channel for increased performance. However, to take advantage of dual-channel support, you must plug them into corresponding sockets, which are configured in pairs. Usually, the pairs are slots 1 and 3 for the first channel and slots 2 and 4 for the second channel. Sometimes, motherboard manufacturers will color-code the slots, or you will need to refer to the documentation (user manual) to understand the correct channel configurations.

It also means if you plug the RAM into the wrong slots, such as 1 and 2, you won’t be able to use dual-channel modes.

Even when modules are plugged into the proper RAM slots, you must enable dual-channel support within the motherboard’s BIOS settings.

Technically, yes, you can install RAM in any one of the four slots available on your motherboard.

As long as you've correctly plugged in the RAM and the slot is not defective, the computer will recognize the installed module(s). However, doing so means the RAM is not working to its full potential, especially if you've installed multiple modules.

This setup was not always possible in older computers, and the computer would not boot if the user plugged RAM into the wrong slot. The same thing can happen with today's computers when you try to use incompatible RAM. Therefore, it’s important to always refer to your motherboard’s documentation to know what RAM types and speeds are compatible.

If your computer seems to be slowing down or having trouble running multiple software applications simultaneously, increasing the total RAM capacity may help. Nearly all desktop and laptop computers can be upgraded, with some exceptions. Proprietary devices like tablets or 2-in-1 computers may not be upgradeable.

Installing RAM is a relatively simple process, but you will need to ensure you’re using compatible modules; otherwise, your computer may not boot. It’s also essential to identify what channels are paired up to take advantage of dual-channel support.

FAQ

How do you fix a dead RAM slot on a motherboard?

First, make sure the problem is with the RAM slot and not the RAM itself. If you're sure there's an issue with the slot, power down and unplug your computer, then open its case. Go to the RAM slot and gently remove the RAM module. Clean the RAM slot, paying attention to accumulated dust. Clean the RAM module and make sure it's free of dust, as well. Put everything back where it belongs and power up your computer again. If this doesn't work, ask a trusted computer repair person if you can replace the slot or if buying a new motherboard is a more cost-effective solution.
How can I tell how many RAM slots a motherboard has?

Open the Windows Task Manager and select the Performance tab to detect how many RAM slots your motherboard has. In the lower right, you'll see a Slots Used field, which will display the number of slots your motherboard has and how many are being used currently, for example, "2 of 4."
How do I test my motherboard's RAM slots?

The only way to test if a RAM slot is working is with patience and trial-and-error. Place a working RAM module into a slot, then see if your PC boots up properly. If it does, you'll know that slot is working correctly. Repeat this process for each slot.