A+ Core 1 3.5: Motherboard Expansion Slots

In one line: an expansion bus lets you add cards to a board — PCI is the old parallel bus (32/64-bit), PCIe is the modern serial bus measured in lanes (x1–x16).

Companion to: Professor Messer A+ Core 1 (220-1201), Section 3, lesson 5. The full version is the Motherboard Expansion Slots class notes; the section overview is the Section 3 sheet.

Buses

Term	What it is
Bus	Shared pathway carrying data between board components
Memory bus	Connects the CPU to the RAM slots
Expansion bus	Connects the expansion slots — how you add cards
Trace	The fine copper line on the board forming a bus

PCI vs PCIe

	PCI	PCIe
Signalling	Parallel (all bits at once)	Serial (one bit at a time)
Unit of width	Bus width: 32-bit or 64-bit	Lanes: x1, x2, x4, x8, x16
More speed by	Wider bus (more wires)	More lanes
Duplex	—	One path each way per lane (full duplex)
Notch/keyway	Set further back from edge	Closer to the board edge
Slot length	64-bit longer than 32-bit	x1 shortest, x16 longest
Retention latch	No	Often yes (x16 especially)
Era	Older boards (early 1990s on)	Modern boards

• “PCIe x4” is read “PCIe by 4” = 4 lanes ≈ 4x the throughput of x1.
• 64-bit PCI card = longer, with an extra keyed section for the extra 32 bits.
• Keying enforces card type/orientation — a card only drops in when it matches.

Installing a card

1. Align the card’s notch with the slot’s key.
2. Press straight down, steady even pressure — never force it.
3. Seated right = no copper contacts visible above the slot.
4. Screw the bracket to the case.
5. Removing a PCIe card: undo the screw and release the retention latch first.

🔐 Security notes

• A card in a slot is on the system bus, not held at arm’s length by the OS — an unexpected add-in card is an incident, not a curiosity.
• PCIe supports DMA: a rogue PCIe card (PCILeech class) can read/write RAM directly and lift keys, credentials and decrypted data — even on an encrypted disk, because it’s unlocked while running. Same reason Thunderbolt (tunnelled PCIe) is treated carefully.
• Control is physical: lock the case, account for every slot, know what should be installed. Full-disk encryption protects a powered-off machine, not live memory.

Practice drills

1. Parallel or serial: PCI? PCIe?

PCI is parallel; PCIe is serial.

2. What does "PCIe x16" tell you, and how is it read?

Sixteen lanes (“PCIe by 16”) — the widest common slot, used for graphics cards.

3. On a board with both, how do you tell a PCIe slot from a PCI slot?

PCIe notch sits closer to the board edge (PCI’s is set further back), and PCIe slots vary in length (x1 short, x16 long). Many PCIe slots also have a retention latch.

4. A card won't drop into the slot whichever way you turn it. Cause? Action?

The keying doesn’t match — wrong card/slot. Do not force it.

5. You've seated a card but a strip of gold contacts still shows. What now?

It isn’t fully home — press it down a little further until no copper is visible, then screw the bracket.

6. Two things to release before pulling out a PCIe x16 graphics card?

The case bracket screw and the retention latch at the far end of the slot.

7. Why is a rogue PCIe card dangerous even on a full-disk-encrypted machine?

DMA lets it read live RAM, and the disk is decrypted while the system runs — so keys and data sit in memory in the clear.

Key takeaways

• Bus = shared pathway; the expansion bus is how you add cards.
• PCI = parallel, 32/64-bit; PCIe = serial, measured in lanes (x1–x16), more lanes = more throughput.
• PCIe notch is nearer the edge; PCI notch is set back. Slot length and a retention latch also give PCIe away.
• Install: match the key, press straight down until no copper shows, screw it in. Remove PCIe: screw and latch.
• Security: a slot is direct bus access; PCIe DMA can read live memory — physical control is the defence.