Boot sector
The first 512 bytes of a disk: just enough code to fetch the rest of the operating system.
When the BIOS finishes its self-test it reads the very first sector of the boot device, places it at address 0x7C00 and jumps there. On a PC that sector holds both the startup code and the partition table, which leaves a little over 440 bytes for the program itself — enough to find and load a real boot loader, and no more.
That small space was for a long time among the most attacked places in a computer. Boot sector viruses such as Brain (1986), Stoned and Michelangelo spread on floppies, loaded before the operating system and were therefore invisible to antivirus software running afterwards. UEFI with GPT and signed boot has made the technique irrelevant on modern machines, though the structure survives for compatibility.