Motherboard, buses and ports¶

Exercise cpu-motherboard-memory-annex.pdf · PDF

You plug your phone into a laptop to copy some photos. It takes forever. Your friend does the same with another cable, into another port of the same laptop, and it finishes in seconds. Same photos, same computer. What changed?

The answer lives on the big green (or black) board inside every computer. The CPU is the chef, but a chef alone does not make a restaurant: you need rooms, corridors and doors. This page is about the building.

The motherboard: the building¶

The motherboard is the main circuit board of a computer. Every other component is plugged into it, and it carries the wires — the buses — that connect them.

Keep the kitchen from the architecture page: if the CPU is the chef, RAM the countertop and the bus the corridor, then the motherboard is the building itself. It decides how many rooms there are, where the doors are, and how wide each corridor is.

A typical desktop motherboard has:

Buses: corridors of different widths¶

The architecture page showed one bus. Real computers have many buses, each designed for a different job and speed — like a building with a motorway to the kitchen, normal corridors to the offices, and a narrow staircase to the basement.

Here is how a modern PC is wired:

flowchart TB
    RAM["<b>RAM</b><br>the countertop"]
    GPU["<b>Graphics card</b><br>PCIe x16 slot"]
    NVME["<b>First M.2 SSD</b><br>PCIe x4"]
    subgraph CPU ["CPU (the chef)"]
        MC["memory controller"]
        PC["PCIe lanes"]
    end
    CH["<b>Chipset</b><br>the reception desk:<br>handles the slower doors"]
    subgraph SLOW ["Connected through the chipset"]
        direction LR
        USB["USB ports"]
        SATA["SATA disks"]
        NVME2["more M.2 slots"]
        NET["Ethernet, Wi-Fi,<br>audio"]
    end
    MC <-->|"memory bus<br>(the widest corridor)"| RAM
    PC <-->|"16 lanes"| GPU
    PC <-->|"4 lanes"| NVME
    CPU <-->|"one link, shared by<br>everything below"| CH
    CH <--> SLOW

Two things to notice:

• The fastest parts — RAM, the graphics card and the first SSD — connect straight to the CPU. No detours.
• Everything else goes through the chipset.

The chipset: the reception desk¶

The chipset is a chip on the motherboard that manages the slower connections — USB, SATA, extra M.2 slots, network, audio — and links them to the CPU.

Think of it as the building's reception desk. The chef cannot answer every doorbell, so the desk collects all the small deliveries and passes them to the kitchen through one door. That door is shared: if you copy between two disks that both hang from the chipset, they share its link.

The chipset also decides what a motherboard can do: how many USB ports, how many SATA disks, whether the CPU can be overclocked. That is why the same CPU is sold with cheaper and more expensive motherboards.

PCI Express: lanes and generations¶

PCI Express (PCIe) is the main high-speed bus inside a PC. Graphics cards, fast SSDs and network cards all use it.

PCIe is built from lanes. A lane is a small pair of connections: one to send, one to receive, both at once. A device can use several lanes side by side, like a motorway with several lanes of traffic:

Every new generation of PCIe roughly doubles the speed of each lane:

So a graphics card on PCIe 4.0 x16 has about 2 × 16 = 32 GB/s in each direction.

Good news: PCIe is backwards compatible. A PCIe 4.0 SSD works in a PCIe 3.0 slot — it just runs at PCIe 3.0 speed. And a smaller card fits in a bigger slot: an x1 card works in an x16 slot.

USB, SATA and M.2¶

USB: the universal door¶

USB (Universal Serial Bus) is the standard connection for peripherals: keyboards, mice, phones, memory sticks, external disks.

Its big idea, in 1996, was to replace a zoo of different ports with one. Since then it has kept getting faster:

Notice the ÷8 trap from the Binary page again: USB speeds are in bits per second. And real transfers are always a bit slower than the maximum, because part of the traffic is used to organize the transfer itself.

This is the answer to the hook: copying photos over a USB 2.0 port or cable is limited to 480 Mbps, while a USB 3.2 port can go 10 or 20 times faster. The port with a blue plastic tongue, or marked SS (SuperSpeed), is usually USB 3.

SATA: the classic disk cable¶

SATA (Serial ATA) is the interface used by hard disks and older SSDs, with a thin data cable plus a separate power cable.

Its latest version, SATA III, reaches 6 Gbps — about 600 MB/s in practice. That was plenty for spinning hard disks, but modern SSDs are much faster than that, so SATA is now the bottleneck.

M.2: a slot, not a speed¶

M.2 is a small slot and card format, mainly used for SSDs that lie flat on the motherboard with no cables.

The most common size is 2280: 22 mm wide and 80 mm long. Careful: M.2 is only the shape. An M.2 SSD can talk SATA (up to ~600 MB/s) or PCIe NVMe (many times faster) — and not every slot supports both. Check the manual. The differences between storage types are on the Storage page.

The ports on the back¶

The rear I/O panel is where the outside world plugs in. On a typical desktop:

Form factors: how big is the board?¶

A form factor is a standard for the size and shape of a motherboard — where the screw holes, slots and rear panel go — so it fits in any matching case.

A smaller board fits in a bigger case (a mini-ITX board in an ATX case), but never the other way round. Laptops and phones do not use these standards at all: each model has its own custom board, often with the CPU and RAM soldered on.

See it in your own computer¶

On Linux, every PCIe device the motherboard has found:

lspci

The same, as a tree — you can see what hangs from what:

lspci -tv

The speed and width each PCIe device actually got (look at LnkSta: link status):

sudo lspci -vv | grep -E '^[0-9a-f]|LnkSta:'

A line like LnkSta: Speed 16GT/s, Width x4 means PCIe 4.0 with 4 lanes — probably an NVMe SSD.

The USB tree, with the speed of each port (480M = USB 2.0, 5000M = USB 3):

lsusb -t

The motherboard's maker and model:

sudo dmidecode -t baseboard

Key takeaways¶

• The motherboard is the building: it holds every component and carries the buses that connect them.
• The fastest parts (RAM, graphics card, first SSD) connect straight to the CPU; the rest goes through the chipset, the reception desk.
• PCIe is built from lanes (x1, x4, x16), and each generation doubles the speed per lane. It is backwards compatible.
• USB speeds are in bits per second (÷8 for bytes), and USB-C is a connector shape, not a speed.
• SATA tops out at ~600 MB/s; M.2 is a slot shape that can carry SATA or much faster PCIe NVMe.
• Form factors (ATX, micro-ATX, mini-ITX) standardize the board's size so it fits in a case.

Quick check¶

We have the chef and the building. Next, the place where the chef actually works: the different kinds of memory.