Section 3: The Buses — How Components Communicate

The Big Picture: Roads Between Parts

Components don't just "magically" talk to each other. They communicate via buses. A bus is a shared pathway for data and signals.

Think of a city. Some roads are wide motorways (fast, short, busy). Some are small side streets (slow, but fine for light traffic). A computer has both kinds, and each part uses the road that suits it.

Bus = A highway for data and signals between components

Speeds are given in GB/s (gigabytes per second), MB/s (megabytes per second), Mbps and Gbps (megabits / gigabits per second). Bigger number, faster road.

3.1 The System Bus / Memory Bus

What it is: The fastest bus, connecting the CPU directly to RAM and chipset.

CPU ←→ [System Bus] ←→ RAM
        (very fast)

Characteristics:

• Fastest bus (hundreds of GB/s)
• Short distances (the parts sit very close together)
• Directly connected to CPU

What travels on it: Data from RAM, address signals (which memory spot to use), control signals (read or write?).

3.2 PCIe (PCI Express)

What it is: A modern bus for connecting expansion cards. An expansion card is a board you add to give the PC a new ability.

Common uses:

• GPU (graphics card)
• NIC (network card)
• Sound card
• Storage controller

Speeds:

Multiple "lanes": A lane is one two-way path, like one lane of a motorway. A PCIe slot can have 1, 4, 8, or 16 lanes depending on the slot. More lanes = more traffic at once.

Figure 1.6 — Look at the row of slots. They are different lengths: the longer the slot, the more lanes it has. From the labels you can find x1, x4 and x16 PCIe slots, plus one old white PCI slot for comparison. Photo: snickerdo, CC BY-SA 3.0, via Wikimedia Commons.

┌──────────────────────────────────────┐
│  PCIe Slot (16 lanes)                │
├──────────────────────────────────────┤
│  [GPU]                               │
│  ↓                                   │
│  16 lanes = 64 GB/s (PCIe 5.0)      │
└──────────────────────────────────────┘

3.3 SATA

What it is: An older bus for storage devices.

Used for:

• Hard drives
• SSDs (older ones)
• Some CD/DVD drives

Speed: ~600 MB/s max

Characteristic: Simple and reliable, but slower than newer options.

3.4 NVMe

What it is: A newer, faster bus for storage devices. It runs over PCIe lanes.

Used for: Modern SSDs

Speed: Up to 14 GB/s (varies by PCIe generation)

Characteristic: Much faster than SATA.

3.5 USB Bus

What it is: A bus for external devices, the ones you plug in from outside.

Controlled by: USB Controllers (special chips on the motherboard)

Types of USB:

Connected devices: Keyboards, mice, external drives, printers, etc.

Figure 1.7 — This is the back of a PC, where the buses meet the outside world. Look for the rectangular USB ports, the wide Ethernet (network) socket, the video sockets (VGA, DVI, HDMI), and the row of round audio jacks. Photo: Hitachi-Train, public domain, via Wikimedia Commons.

3.6 Thunderbolt

What it is: A high-speed bus combining PCIe and DisplayPort (a video connection).

Speed: 40–80 Gbps depending on version

Uses: External storage, monitors, docking stations

3.7 I2C and SPI

What they are: Slower buses for low-speed components. They are the quiet side streets.

Used for:

• Temperature sensors
• Power management chips
• Memory on smart devices

Summary