PC Gear Lab

What are PCIe Slots? – PCIe Simplified

One of the many things that make computers so great, particularly desktop computers, is the high degree of customization that they afford to the users.

If you want to add an expansion card that can provide you with additional features, or just upgrade your PC, a PCIe slot will come in handy.

You may have definitely come across PCIe slots in your search for motherboard. So what are PCIe Slots? To understand what are PCIe slots, you will need to understand what is PCIe.

Also Read: What does PCIe X16 Mean?

What is PCIe

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

With a PCIe bus slot, users can add dedicated graphics cards, sound cards and even some SSDs for storage with RAID cards among many other things.

PCIe slots come in many different sizes as will see below.

There are also many version of the PCIe with v4 being the latest, but that is a topic for a different article. Here are talking specifically about the slot and how its works.

Also Read: Best Motherboards for SLI

A Brief History of the PCIe Slot

PCI vs PCI Express (PCIe). 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 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.

PCIe standard was created to alleviate the shortfalls of the previous buses that were at the time much slower and hence inefficient for fast performance environments and applications.

In other words, newer expansion slots required faster transfer speeds and older buses were just not capable.

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

Difference between PCI vs PCIe

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.

The speed differences between the two were phenomenal. While PCI topped at 533 MB/s with a 64 bit bus type, the PCIe v3.0 has a transfer speed of 32 GB/s.

Also Read: Best Motherboards for Photo Editing

PCIe Slot Configuration

ATX size comparison
Different form factors follow a different PCIe slot configuration.

All modern motherboards come with a PCIe slot that will allow you to hook up extra devices, or expansion cards, like VGA cards, 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.

The number of PCIe slots depends a lot upon the motherboard form factor.

The slots come in different flavors that we’ll discuss shortly. The benefit of this is that it lets you assign a slot to a device that can make full use of it.

Like an X16 slot can be great for a graphics card and likewise, the smaller ports like the X4 can be great for a network card or a sound card.

Note on PCIe Lanes

PCIe Lanes Core i7-9th
Number of PCIe lanes provided by the Intel Core i7-9700K. Source: Intel.com

Understainding PCIe lanes is very important especially when building a custom PC.

Here the number after “X” tells you the number of PCIe lanes. Each lane has a send and receive link.

Therefore, a PCIe X16 slot has 16 PCIe lanes or it can support a device upto 16 PCIe lanes in bandwidth such a graphics card.

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

Having a large variety of slots with a large number of lanes is great, but only if the CPU and motherboard can support it.

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

Most mainstream processors have 16 dedicated PCIe lanes with only the extreme and premium processors reaching 40 or more dedicated PCIe lanes.

Motherboards also provide PCIe lanes, however, not all of them are dedicated (dedicated means avaialble to use). For instance, Intel Z170 motherboard has 20 PCIe lanes, however, only 4 are available for use the other 16 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 RAID card.

To add more graphics card you will ofcourse need to have a higher PCIe Lane count using expensive CPU and motherboards.

Again, you can find the number of CPU and motherboard PCIe lanes in their respective data sheets.

Also Read: Best MSI Motherboards for Gaming

PCIe Slot Sizes and Data Transmission

What are PCIe Slots
PCIe Slots come commonly in x1, x4, x8 and x16 sizes

The X1 slot is the smallest one and has the least bandwidth. Following up are the X4, X8 and X16 slot. X16 has the largest bandwidth for consumer space.

Industrial and commercial spaces also utilize the much large X32 bandwidth.

For there to be data transfer through a PCIe device, there needs to be a way to transmit the data. The PCIe slots have electrical connections that correspond to the number of lanes that they have for data transfers.

The lanes are the means through which data is transmitted across the slots from the devices to the CPU and how much data can travel through a PCIe slot at a given time will be determined by the number of lanes that the device has.

For example a graphics card typically has X16 bandwidth or lanes. Therefore, it would be ideal to fit it on a X16 slot. A network card, on the other hand, can work with a X1 slot.

X1 PCIe Slot

This is the smallest slot and hence has the smallest bandwidth. The speeds vary across the different generations of PCIe slots, with the first one i.e PCIe v1 posting 250MB/s, the PCIe v2 doubling the previous one to hit 500MB/s, the PCIe v3 at 985Mb/s and the PCIe v4 doing about 1969 MB/s.

Networks card and modems generally are ideal for X1 slots since they do not have a high bandwidth requirement.

X4 Slot

With the larger X4 slot, speeds are almost quadruple that of the X1 slot as expected. The first generation i.e PCIe v1 registers 1000MB/s and the PCIe v2 one does 2000MB/s. The PCIe v3 comes at about 3940MB/s while the PCIe v4 clocks 7876MB/s/.

These slots are great for RAID card or for SSD expansion since they require a higher bandwidth.

X8 Slot

X8 is the most peculiar slot of them all.

In many motherboard, the X8 slot looks almost similar to the X16 slot in both size and configuration, so a keener look may be necessary to tell them apart.  

In some motherboard, you may have full 2 x X16 slots. However, if you have a total of 24 PCIe Lanes only, then one PCIe X16 slot will have full X16 bandwidth, while the other PCIe X16 slot will only be able to operate at X8 bandwidth despite having the same size.

X8 slot can still support the full X16 slot albeit at weaker performance.

The first-generation PCIe v1 X8 PCIe slot has a bandwidth of 2000MB/s while the second is twice as first at 4000MB/s.

The third-generation PCIe X8 slot transfers 7880MB/s while the fourth PCIe v4 does 15752MB/s.

The X16 Slot

These are the largest available PCIe slots available for home users or consumer space.

These are distinctively used for graphics card.

At 4000 MB/s for the first generation, 8000MB/s, 15760MB/s and 31504MB/s for the second, third and fourth generations respectively, this is the fastest widely available PCIe slot yet.

It has 16 data lanes and is ideal for high bandwidth devices like graphics cards.

The X32 Slot

As part of the PCIe standard, there’s an X32 slot that has significantly increased bandwidths of up to 64GB/s.

It is rare in user-grade motherboards and it’s expected to be twice as large as the X16 slot to accommodate the extra data lanes. More information can be found on the specification.

Slot Compatibility

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

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 device into a high lane count slot will let the device work as intended albeit with the cumulative bandwidth of the slowest component.

Likewise, a high lane count device 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.

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.

Practical Example of Slot Compatability

MSI Motherboard PCIe
MSI MPG Z2390 Gaming Edge AC

The motherboard above is an excellent example of how cross compatbility with PCIe works.

You will notice that the motherboard above has 3 PCIe slots with X16 physical slot sizes (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 ealier, this motherboard has been specified to be used in x16/x0/x4 OR x8/x8/x4 modes.

X16 slot

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 a RAID card etc

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 with a RAID card etc

Here you will notice that this is configuration 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.

To get FULL 2 x X16 slots you will need to get higher end CPUs and Motherboards to give the higher number of PCIe lanes required to run the two X16 slots at full bandwidth.

The point to note here is that despite the size, the slots are compatible.


With more and more devices requiring high bandwidth transfers, a PCIe slot is essential for the modern computer.

Here we talked extensively about what are PCIe Slots.

Basically, these slots provide fast transfers and expandability to your motherboard which lets you customize it to your wildest desires.

PCIe slots are what connect your devices to the lanes for data transfers and they come in different sizes and capabilities. They have also evolved over the years with each new generation promising double the data transfer rates of the one before.

All this makes for a more efficient machine for both work and play.

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