BIOS Boot Recap Previously, we saw that after the BIOS firmware is loaded, it searches for a bootable device from a list of storage options, such as a hard drive, SSD, USB, or network interface. The BIOS identifies a valid bootable device by checking for the 0x55AA signature at the end of the first sector. Once found, it loads the 512 bytes from this sector (LBA 0), which is known as the Master Boot Record (MBR). ...
1. Power‑On & Hardware Reset 1. Power‑Good Signal The power supply stabilizes voltages and asserts a “Power‑Good” (PWR_OK) line to the motherboard. All devices receive power and begin to initialize themselves. The Central Processing Unit (CPU) is initially held in a reset mode, meaning it’s not yet executing instructions. The memory layout is powered up, although the RAM itself has no content since it’s volatile. 2. CPU Reset Vector The reset vector is a predetermined memory address where the CPU begins execution after being powered on or reset. On x86 processors, this address is typically 0xFFFFFFF0 (near the top of the 4GB address space). When the CPU comes out of reset, its program counter (instruction pointer) is automatically set to this address. The motherboard’s memory mapping ensures that this address points to the BIOS/UEFI firmware ROM chip, so the very first instruction the CPU executes comes from the firmware. ...