What are PCIe Slots and Their Uses? – Beginners Guide

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PCIe slots allow you to add expansion cards on your motherboard in order to provide additional functionality to your PC. They come in various sizes, each designed for a specific category of expansion cards. Whether you want to add a graphics card for gaming, a WiFi network card, or a dedicated sound card, you use the PCIe slots.

There are two basic interfaces that a computer uses to add more components, SATA and PCIe. SATA is a slower interface and is used for spinning hard disks primarily these days and also for CD/DVD drives.

PCIe, on the other hand, is far superior in terms of transfer rate and thus supports high speed components ranging from NVMe SSD all the way to GPUs.

As mentioned earlier, there are different sizes of PCIe slots and knowing how they function and what differentiates one slot from the other is crucial for PC builders.

Here we will talk in depth about PCIe in general and PCIe slots in particular.

What Does a PCIe Slot Look Like?

In order to understand what PCIe slots look like, a picture would be the best guide.

What are PCIe Slots and Their Uses?
Image: Motherboard with two x1 slots (white) and two x16 slots (blue)

As you can see in the image above, PCIe slots come in different sizes. They are distinguished mainly by their PCIe lane count.

Also Read: What Does a PCIe Slot Look Like? – In Depth Look

What are PCIe Slots, Lanes, Generation?

There are a few important characteristics of PCIe interface you need to understand in order to understand and utilize the slots properly i.e

  • What PCIe is
  • PCIe Lanes
  • PCIe Slot Sizes and Motherboard Configuration
  • PCIe Generation

let us start from the beginning.

What is PCIe?

PCIe stands for Peripheral Component Interconnect Express, and it is an internal bus interface standard that is used to connect internal high-speed components to your desktop motherboard.

There is a list of things that you can attach to the PCIe slots. We have summarized them here: 10 Things That Can be Plugged in PCIe Slots here.

PCIe slots have PCIe lanes hardwired to them for transporting data.

PCIe Lanes in Brief

Understanding PCIe lanes is very important for understanding what PCIe slots are and how they work.

A PCIe lane is basically the path through, or the connection through, which data is transported to and from the slot and the CPU. Different slot sizes have different amount of PCIe lanes.

The more PCIe lanes a slot has, the higher is its throughput rate (speed) and the more demanding expansion cards it can support.

There are four types of PCIe lane configuration you can find on PCIe slots i.e x1, x4, x8 and x16.

In most cases, the number after “x” tells you the number of PCIe lanes the slot has. The x16 slot, however, is a bit confusing to understand as the number 16 here after “x” does not always mean 16 lanes; more on this below.

Therefore, a PCIe x4 slot has 4 PCIe lanes or it can support a device with a throughput rate requirement of upto 4 PCIe lanes such as an NVMe SSD expansion card.

You must note that the amount of PCIe lanes a PC has in total are generally dictated by the CPU and the Motherboard. A PC DOES NOT have unlimited lanes.

The total number of PCIe lanes a PC has can be found by looking at CPU specsheet and the motherboard CHIPSET specsheet.

Intel Core i7-10700K
Image: Number of PCIe lanes provided by the Intel Core i7-10700K. Source: Intel.com

Most mainstream processors have 16-24 PCIe lanes with only the extreme and premium processors reaching 44 or more dedicated PCIe lanes.

Motherboards also provide PCIe lanes, however, not all of them are user accessible. For instance, Intel Z490 motherboard has 24 PCIe lanes, however, depending on how the board is configured you could only have 4 to 6 lanes available for use and the rest would be reserved for internal components like SATA ports, USB ports etc.

Therefore, with a configuration of 16 CPU lanes + 4 provided by the motherboard chipset, you will have a total of 20 PCIe lanes to use in total.

With this you can, for example, add a single x16 graphics card and a single x4 NVMe SSD Card.

Or perhaps you could add a single graphcis card (x16), a WiFi Card (x1), a FHD video capture card (x1) and still have 2 PCIe lanes. This all depends upon how the motherboard is configured and what type of slots it provides.

Again, you can find the number of CPU and motherboard PCIe lanes in their respective data sheets. The following guides detail PCIe lanes in depth.

PCIe Slot Sizes and Configuration

what are pcie slots
Image: Different form factors follow a different PCIe slot configuration.

All modern desktop PC motherboards come with at least one PCIe slot that will allow you to hook up extra devices like VGA cards, Sound Card, RAID controllers etc to extend your PC’s functionality.

The only difference is that not all boards will come with the same number, type, or even version of PCIe slots.

Motherboard Form Factor Matters in PCIe Slot Configuration

The number of PCIe slots you have also depends a lot upon the motherboard form factor in addition to the total amount of PCIe Lane count, as covered earlier.

An ATX motherboard, for instance, typically offers a whole lot of PCIe slots in different sizes. A Mini-ITX motherboard, on the other hand, being the smallest motherboard for desktop computers, offers only a SINGLE PCIe x16 slot despite supporting more than 16 PCIe lanes!

Different PCIe Slot Sizes and Their Uses

There are typically two PCIe slot sizes found on almost all commercial motherboards:

  • x1: This offers only a single PCIe lane. It is suitable for weaker and smaller cards like a WiFi card or a sound card.
  • x16: This is the largest slot found on a motherboard and also the most confusing to understand. More on this below.

Also Read:

The Curious Case of the x16 Slot

how to tell if pcie slot is bad w
Image: Gigabyte GA-P67A-UD3 has two x16 slots (Blue), but the second x16 slot only offers x4 lane.

Many time you will find an x16 slot wired to only x8 or even just x4 lanes! Always refer to the specsheet to understand the true lane count of the slot.

If you only have a single x16 slot on your motherboard, then this will almost definitely have 16 PCIe lanes coming directly from the CPU.

If you have MORE than one x16 slot, however, then the second x16 slot would either be operating in x8 or x4 mode. This is due to the limited nature of PCIe lanes you have available on your PC. A typical PC with 20 to 24 PCIe lanes cannot have two full x16 slots as that would take the lane count to 32!

In summary, there are three configuration for an x16 slot:

  • x16(x4): This slot has the physical size of an x16 slot but is hardwired to only 4 lanes. This slot is used for devices like an NVMe M.2 SSD expansion slot or 4k video capture card, that require a higher bandwidth than an x1 slot an offer.
  • x16(x8): These are found on premium motherboards for dual graphics card builds. Again, this slot has the physical size of an x16 slot, but only has four lanes connected to it.
  • x16(x16): A true x16 slot that connects to x16 lanes is found on almost all motherboards. They have the highest throughput rate and thus these are typically occupied with graphics cards. 

Also Read:

PCIe Generation and Slot Performance

The overall performance of the PCIe slot is significantly affected by its PCIe version.

Each newer generation of PCIe interface DOUBLES the per lane speed. So for instance while a single PCIe v3.0 lane (x1) has a throughput of 985 MB/s, a single PCIe v4.0 lane (x1) has double the throughput i.e 1,969 MB/s.

The following table summarizes PCIe speeds across varios generation.

Versionx1
(GB/s)
x2
(GB/s)
x4
(GB/s)
x8
(GB/s)
x16
(GB/s)
1.00.2500.500 1.0002.0004.000
2.00.5001.0002.0004.0008.000
3.00.9851.9693.9387.87715.754
4.01.9693.9387,.87715.75431.508
5.03.9387.87715.75431.50863.015
6.07.87715.75431.50863.015126.031

A newer PCIe version has a lot of ramification. A motherboard with a newer PCIe slot can support newer gen devices such as faster NVMe SSDs and more powerful graphics cards. 

Also Read: PCIe 3.0 vs 2.0 – What is the Difference?

Slot Cross and Backward Compatibility

The best part about the PCIe standard is the cross and backward compatibility of different slots with different PCIe devices and generation.

Basically, if you have an x4 slot and the device you want to install is an x1 device, you won’t need any special software to work around the hurdle.

Plugging in a lower lane count expansion card into a higher lane count slot will let the device work as intended albeit with the cumulative bandwidth of the slowest component.

In other words, installing a weaker x1 device into an x4 slot will work, however, the x1 device will NOT get any performance boost.

Likewise, a higher lane count expansion card can go into a small lane count slot as long as it can fit. So, you can plug your x16 graphics card into an x8 slot and it will still work, but at the speed of the x8 slot; therefore, there can be some performance reduction.

This compatibility also cuts across the versions of PCIe so a PCIe 3.0 device can work in a PCIe 2.0 slot and vice versa.

The only caveat is that the configuration will work at the lowest common speed between the slot and the component.

Can I Use a PCIe 3.0 Card in a 2.0 Slot
Image: PudgetSystems.com. Comparison of how RTX 3090 (PCIe 4.0 Device) and Titan RTX (PCIe 3.0 Device) performs on different PCIe generations. An impact can be seen on performance in DaVinci Resolve (video editing software).

Here you can see that using a graphics card on an older gen slot can result in some performance reduction. 

Also Read:

Practical Example of Understanding PCIe Slot Configuration

MSI MPG Z490 Gaming Carbon WiFi

Image: MSI MPG Z490 Gaming Carbon WiFi

The motherboard above is an excellent example of how PCIe slots are configured on a typical mid range full ATX motherboard.

You will notice that the motherboard above has 3 PCIe x16 slots (two in silver color and there is one black colored x16 slot at the very bottom that may be hard to see).

Due to the Lane limitation, as mentioned earlier, this motherboard has been specified to be used in x16/x0/x4 OR x8/x8/x4 modes.

In other words, you can use the motherboard in two modes.

Mode 1: ( x16/x0/x4)

  • Top slot working in x16 mode i.e with a single Graphics Card.
  • Middle slot empty
  • Bottom slot working in x4 mode i.e for an NVMe SSD expansion card for instance.

Mode 2: (x8/x8/x4)

  • Top slot working in x8 mode i.e with a Graphic Card
  • Middle slot working in x8 mode i.e with another Graphics Card
  • Bottom slot working in x4 mode i.e for an NVMe SSD expansion card for instance.

Here you will notice that mode 2 is great for SLI or Crossfire setup where you have two graphics cards. The only issue is that both graphics card will work at reduced performance due to half the supported bandwidth. The difference as tested, however, between running the card in x16 or x8 is only marginal.

This motherboard also has two smaller x1 slots that can be used for smaller cards as well.

To get FULL 2 x X16 slots you will need to get a workstation grade CPU from the Intel X-series or AMD Threadripper series and also a workstation grade motherboard to give the higher number of PCIe lanes required to run the two or more x16 slots at full bandwidth.

A Brief History of the PCIe Slot

PCI vs PCIe
Image: PCIe (Left) vs PCI (Right). Source: How Stuff Works

Several other slots were used to connect expansion cards to the motherboard before the introduction of the PCI slot in 1992 by Intel.

For example, before the introduction of the original PCI standard, ISA (Industry Standard Architecture) bus slots were the main interface for connecting expansion cards.

ISA slots, unfortunately, were longer and had slower data transfer speeds.

PCI standard was created to alleviate the shortfalls of the previous buses that were at the time much slower and hence inefficient for the growing throughput demand of the expansion cards.

The PCI standard became more widespread towards the end of the 90s and saw some improvements. However, it was replaced by the more efficient PCI Express standard that has been in use since 2002.

Difference Between PCI vs PCIe

Difference Between PCie and PCI
Image: Shows the difference between the older PCI standard using shared bus interface as compared to the newer PCIe that uses serial direct interface.

PCIe makes use of independent transfer lanes instead of the shared bus that the older PCI standard uses. The ability to transfer data in multiple lanes means an overall increase in bandwidth.

The main difference between the two typologies was that the PCI used Parallel where as the PCIe used Serial architecture. Basically with the PCI parallel architecture, each connected device shared the same bus.

The limitation here was that since every device shared the same bus, when a higher number of devices where to be connected, they would have to wait for the bus to free up to send and receive data.

PCIe, on the other hand, completely changed the topology to a serial architecture where each and every device has its own dedicated bus now.

The speed difference between the two interfaces was phenomenal. While PCI topped at 2.13 GB/s (bi-directional bandwidth) with its final PCI-X 2.0 version released in 2002, the PCI Express v1.0 on the other hand, released in 2002, had a total bi-directional bandwidth of 8GB/s. Thus drastically improving on its predecessor.

The current v4.0 has a throughput of around 32 GB/s and bi-directional bandwidth of around 64 GB/s!

Also Read: What are USB Headers?

Final Words – Do You Need to Use PCIe Slots?

The key takeaway from here is that a PCIe slot is the primary interface through which you connect high speed expansion cards that add functionality to your PC.

Here we talked extensively about what are PCIe slots by looking into PCIe lanes, slot configuration and sizes, and PCIe version.

There are, of course, chances that you may never have to use PCIe slots at all for your system.

For instance, if your CPU comes with an integrated graphics card and if your motherboard comes with display output ports, a soundcard, and a network card – like most motherboard these days do – then chances are that you may never have to add another expansion card anyways.

This is particularly true if you are an average home or office user. Basic PC for home use have everything you need installed onboard the motherboard and the CPU itself. Only professionals and gamers may require the use of PCIe slots for adding more functionality to their system.

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Atif Qazi
Atif Qazi is the founder of PCGuide101. He is a digital nomad who loves everything PC. He is a PC builder, tech enthusiast, engineer, and a lover of single player lore-rich RPG games.

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