The motherboard (also called mainboard, system board, or logic board) is the primary printed circuit board that physically and electrically interconnects all components of the computer. It is the substrate on which the entire system is built — every component either plugs directly into the motherboard or connects to it via cables. Modern motherboards are multilayer PCBs (Printed Circuit Boards) typically with 6–16 copper trace layers. The visible green, black, or white solder mask covers the outer layers; the inner layers carry power planes (solid copper fills for 12V, 5V, 3.3V, and ground) and signal traces connecting every component. High-speed traces (PCIe, memory buses) are impedance-controlled — their width, spacing, and distance to ground planes are precisely calculated to maintain 50–100 ohm impedance, preventing signal reflections at multi-gigabit speeds. The VRM (Voltage Regulator Module) is the most critical power delivery subsystem on the motherboard. It converts the PSU's 12V rail to the precise CPU core voltage (0.7–1.5V) demanded by the CPU's power management firmware. A VRM consists of phases: each phase uses a high-side MOSFET, low-side MOSFET, inductor, and capacitors forming a buck converter. More phases = smoother voltage delivery at high current draws = less thermal stress per component. Premium boards have 12–24 power stages; budget boards may have 4–8. Inadequate VRMs throttle or damage CPUs under sustained all-core loads. The PCH (Platform Controller Hub) — on modern Intel/AMD platforms a single chip versus the old North Bridge + South Bridge pair — manages all slower peripheral communication. It connects to the CPU via the DMI (Direct Media Interface) link at ~16 GB/s total bandwidth. Behind the PCH sit: USB controllers, SATA controllers, PCIe x1 and x4 slots for expansion cards, additional M.2 slots, audio codec, Ethernet controller, and Thunderbolt controller. The DMI link is a shared bus — saturating it with simultaneous high-speed USB, NVMe, and PCIe transfers creates contention. BIOS/UEFI firmware stored in a dedicated SPI NOR flash chip (32–256MB) initializes all hardware components at power-on during POST (Power-On Self-Test). Modern UEFI provides a full graphical interface with mouse support, XMP/EXPO memory profile activation, fan curve configuration, overclocking tools, CPU power limits (PL1/PL2 on Intel, PPT on AMD), and secure boot management. BIOS updates (delivered as .ROM files) can add support for new CPU generations without hardware changes — the AM5 platform has received Ryzen 7000, 8000, and 9000 series support through BIOS updates.