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For dual-channel, the most common memory configuration, a pair of RAM modules should be inserted into the first and third, or second and fourth slots. If you're only inserting one module, it can go in any slot.

One of the easiest mistakes you can make when building or upgrading a computer is placing the memory modules in the wrong RAM slots, resulting in suboptimal performance. So which RAM slots should you use, and why does it make any difference where the memory modules go?

What is a RAM Slot?

A RAM slot, also known as a RAM socket or Memory Socket, is a long, slim socket on the motherboard of a computer, usually arranged in a bank of two or four. They allow RAM (random access memory) modules of different speeds and capacities to be added to the computer.

The slots for RAM are commonly located close to the CPU slot. Each has a small, hinged clamp at each end to hold the memory module firmly in place. A riser, partway along the memory slot, ensures that only compatible memory can be inserted. Compatible memory modules have a cutout that matches the position of the riser.

How Many RAM Slots Do Motherboards Have?

Most consumer motherboards will have either two or four RAM slots. High-spec boards, or those aimed at gamers, will almost certainly have four slots, sometimes as many as six or eight. Anything more than eight slots is usually reserved for servers or very high-end system boards.

Each slot has a maximum RAM capacity, generally between 8 and 32GB. A 16GB memory module may fit in an 8GB slot, but in the best-case scenario, will only register 8 GB. Worst case, it won’t work at all. By multiplying the capacity by the number of slots, you can work out the total amount of random access memory a motherboard supports.

Why Are There Different Slots for RAM?

The RAM slots on all modern motherboards are numbered so you know which to use when inserting the memory modules. The numbering system varies between manufacturers but is usually A1, A2, B1, B2, etc. RAM slots are often also colored differently, and sometimes slightly offset, from each other. Again, this is to help you correctly place the modules according to the motherboard manufacturer’s specifications.

colored RAM slots on a motherboard

Older motherboards, which might only support single-channel memory, didn’t need this distinction because it mattered less where the RAM was placed.

RAM slots in a laptop are different from those on a desktop computer. Laptops most commonly use SODIMM (Small Outline Dual In-line Memory Modules.) These are shorter and smaller than the standard desktop DIMM modules. Most laptops have just two RAM slots, which are canted over to the side to allow the modules to lay flat. If a laptop supports dual-channel memory, there should already be a module in each slot.

Does It Matter Which RAM Slots You Use?

Assuming your motherboard supports dual-channel memory, and most do, choosing the correct RAM slot order can significantly impact how well the memory performs. To understand why, it helps to understand a little more about how multi-channel memory works.

The CPU accesses the memory in a RAM module through a memory controller. Depending on the motherboard, the memory controller can have one, two, four, six, or eight channels. In dual-channel memory, the CPU can access the memory across two channels simultaneously, increasing memory bandwidth. In a quad-channel system, the CPU can access four memory channels, and so on.

The performance boost achieved by dual-channel memory is the reason why most RAM is sold in module pairs. To take full advantage of multi-channel memory a pair of RAM modules must be placed in different memory channels. And this is where the mistake often occurs. RAM slots are fitted in pairs, so to access separate channels the modules need to go in the first and third slot, or the second and fourth, and not side by side.

It is always best to check the motherboard documentation to find the recommended RAM slot order. Here are some common dual-channel configurations for varying amounts of slots and modules.

  • Two slots, one module – Insert in any slot
  • Three slots, two modules – Insert in first and third slots
  • Four slots, one module – Insert in the first or fourth slot
  • Four slots, two modules – Insert in the second and fourth slots
  • Four slots, four modules – A matched pair in one and three, and a matched pair in two and four.

Single vs. Dual-Channel RAM Performance

For a long time, single-channel RAM was all you needed. Games and other software couldn’t efficiently use dual-channel architecture, so any extra memory bandwidth was wasted. But that changed with the emergence of more advanced game engines and multi-core processors that require access to multiple memory channels.

In theory, dual-channel RAM should perform twice as well as single-channel RAM. In practice that is rarely the case due to inevitable bottlenecks elsewhere in the system. It also depends on how well a game or app is programmed to use that extra memory bandwidth.

Most of the benchmarks we have looked at suggest an improvement of 20-30% when using a dual-channel memory configuration rather than a single-channel setup. These are based on core performance metrics such as gaming frame rate.

Frequently Asked Questions

RAM slots or memory slots—what’s the difference?

RAM slot and memory slot are two different names for the same motherboard socket. Although the term “memory slot” can refer to other ports that are used to add memory to a system, when talking about computer motherboards, a memory slot is where you install RAM modules.

Should RAM be in 1 and 3 or 2 and 4?

There is no difference in speed between these two pairs, but it is often suggested that slots 2 and 4 should be used first. This leaves the maximum gap between the first module and your CPU cooler, so is better for heat dissipation.

Is it OK to mix RAM capacity?

You can mix RAM modules of different capacities (a 16GB and 8GB module, for example), and assuming they are the same speed, frequency, and latency, you shouldn’t have any problems. If you use RAM modules with different speeds, the system will default all of them to the lower speed.

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Russ is a freelance writer who specializes in writing about technology. He loves exploring and figuring out how complex things work, and sharing that knowledge with others, something he has been doing online and in print for more than 15 years. When not writing for How-To Geek, Russ can usually be found planning his first novel or taking something apart to see how it works.
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