Motherboards: form factors, headers, and the firmware underneath

Why this matters

The motherboard is the component that decides what else is possible. Its form factor decides what case it fits, its chipset decides which processors and how many expansion lanes, its headers decide what can be connected, and its firmware decides whether any of it starts.

The exam asks for form factors by name and size, for expansion slot types, and for the firmware settings a technician configures. That last group is the practically important one: boot order, virtualisation support, secure boot and the system clock account for a large share of the "it will not boot" and "the hypervisor will not start" calls in the field.

Lesson 35 covers a machine that will not start at all. This lesson is where the settings and connections that make it start are described.

The lesson

Form factors by size and mounting, and what each one gives up

• ATX — the full-size standard, 305 × 244 mm. The most expansion slots, the most memory slots, the most headers. The default for a desktop build.
• microATX — 244 × 244 mm. Shorter, so fewer expansion slots, usually four memory slots. Fits ATX cases as well as its own, which makes it the flexible choice.
• Mini-ITX — 170 × 170 mm. One expansion slot, usually two memory slots. Enables genuinely small machines, at the cost of everything that needs space.
• ITX more broadly is a family; Mini-ITX is the one commonly met.

Two facts that matter practically. First, mounting holes are shared: a smaller board fits a larger case, so a microATX board goes in an ATX case, but never the reverse. Second, smaller boards concede in predictable order — expansion slots first, memory slots second, then headers, then cooling clearance.

Proprietary form factors exist in pre-built machines from large manufacturers, and they are the reason a replacement board for such a machine often has to come from that manufacturer. A scenario describing a failed board in a branded desktop, where a standard replacement will not fit the case or the front-panel connector, is describing exactly this.

Expansion slots, lanes, and why a card can fit a slot and still run slowly

PCI Express is the current expansion interface. Slots are described by their physical size and their electrical lane count, and those are not always the same:

• x1, x4, x8, x16 are the common sizes.
• A slot that is physically x16 may be electrically wired for x4 or x8.
• A card runs at the lower of what it wants and what the slot provides, so an x16 graphics card in an x16 slot wired for x4 works, at a quarter of the bandwidth.

Lanes are a finite resource provided partly by the processor and partly by the chipset, which is why populating an M.2 slot sometimes disables a SATA port or drops a graphics slot from x16 to x8. The board manual documents these trade-offs in a table, and a technician who reads it avoids a confusing afternoon.

Generations double the per-lane bandwidth each time, and they are backward and forward compatible — a newer card in an older slot works at the older speed.

PCI, the older parallel standard, still appears in legacy equipment and is worth recognising. Riser cards turn slots through 90 degrees for low-profile cases, and low-profile brackets are the corresponding physical adaptation.

Front-panel headers, fan headers and the connectors nobody labels clearly

The front-panel header is a block of small pins carrying the power switch, reset switch, power LED, drive activity LED and often the speaker. It is the most fiddly connection in a build and the one most often wrong.

The facts that make it manageable:

• Switches have no polarity. Power and reset work either way round.
• LEDs do have polarity. Backwards means the light does not come on, which is cosmetic and looks like a fault.
• The board prints the layout beside the header, and the manual has a diagram. Some boards ship an adapter block that lets the connectors be assembled off the board and fitted in one piece.
• A machine that does nothing at all when the button is pressed may simply have the power switch on the wrong pins — which is testable by briefly shorting the two power-switch pins with a screwdriver.

Other headers to know: USB 2.0 and USB 3 front-panel headers (different sizes, and USB 3 has a keyed block that can be forced in wrongly with enough determination), front-panel audio, and the fan headers.

Fan headers are three-pin or four-pin, where the fourth pin carries the speed control signal. A four-pin fan on a three-pin header runs at full speed; a three-pin fan on a four-pin header is usually controlled by voltage instead. Boards distinguish the processor fan header from case fan headers, and some refuse to start if nothing is connected to the processor fan header — which is a useful protective behaviour and a confusing one if you do not know it.

Firmware settings that matter: boot order, virtualisation, secure boot and the clock

The firmware — historically BIOS, now almost always UEFI — initialises the hardware and hands over to an operating system loader. UEFI added support for large disks with GPT partitioning, a graphical interface, faster startup, and secure boot.

The settings a technician actually changes:

• Boot order. Which device is tried first. This is the setting behind "it will not boot from the USB stick" and behind a machine that boots to a network prompt because a network boot entry came first.
• Virtualisation support — Intel VT-x or AMD-V, often with a second setting for directed I/O. Disabled by default on many machines, and its absence is why a hypervisor refuses to start or a virtual machine will only run 32-bit guests. Lesson 29 returns to this.
• Secure boot. Verifies that the bootloader is signed by a trusted authority. It protects against boot-level malware, and it also blocks some older or unsigned operating systems and some bootable tools — which is why it sometimes must be disabled temporarily and, importantly, re-enabled after.
• UEFI against legacy or CSM boot mode. The mode must match how the disk was prepared: a GPT disk installed in UEFI mode will not boot in legacy mode, and the machine reports no bootable device despite a perfectly good installation.
• The system clock. A wrong date causes certificate validation failures everywhere, so a machine that cannot reach any HTTPS site and reports certificate errors on all of them has a clock problem, not a network problem.
• Firmware passwords, which restrict access to settings and sometimes to booting at all. A supervisor password nobody knows is a genuine obstacle, and on business machines it is often not clearable by the usual methods.
• Fan and thermal profiles, and storage mode (AHCI against RAID), which must match what the operating system was installed under or the disk becomes unbootable.

Firmware updates fix compatibility and security problems and carry real risk: an interrupted update can leave the board unusable. Mains power, no interruptions, and the manufacturer's own file for that exact model.

The coin cell, what it actually holds, and the symptoms when it dies

The CMOS battery is a coin cell, almost always CR2032, that powers the small amount of memory holding firmware settings and the real-time clock while the machine is unplugged.

What it holds: the date and time, and the configured firmware settings. What it does not hold: the firmware itself, which lives in flash and survives regardless.

The symptoms of a dead cell, in the order a user notices them:

1. The clock resets to a default date on every power-off. This is the signature.
2. Firmware settings revert — boot order, storage mode, enabled devices.
3. A message at startup about invalid configuration or checksum error, often requiring a keypress to continue.
4. Downstream effects that look unrelated: certificate errors on every secure site, failed domain logins because the time is outside the permitted skew, and scheduled tasks behaving strangely.

That fourth group is the examinable part, because a user reports "no websites work" and the cause is a battery worth very little. Anyone who has seen it once recognises it immediately, and the fix takes two minutes.

Deliberately removing the cell is also the traditional way to clear firmware settings, alongside the board's clear-CMOS jumper. On modern business machines that no longer clears a supervisor password, and expecting it to is a common misconception.