In addition to main memory, there are small storage areas inside the CPU (Central Processing Unit) called registers. Some of these registers are visible to the software, while others are only used internally by the processor to keep track of its state. These registers also lose their contents when power is removed. Two important examples of registers (for the x86 family of processors, which are the processors used by Windows PCs) are the CS (Code Segment) register and the IP (Instruction Pointer) register. The combination of these two registers defines where the next instruction is to be found in memory.
When power is restored to these electronic devices (including both main memory and CPU registers) the contents are random and therefore useless.
The power on reset is triggered when a signal called "power stable" is received by the CPU from the power supply. The power on reset initializes all CPU registers to a specified state. Many registers are set to zero (for example the IP register). A few registers require special non-zero values to put the CPU into a reasonable state. For example, the CS register is set to 0xFFFF, that is all one bits.
The setting of other register values cause the following initial state to be set:
Once the power on reset sequence is finished, the CPU begins executing instructions in its normal fashion. The combination of the initial CS and IP values causes execution to begin in the BIOS or UEFI.
Other ways to initiate a system boot