Boot loaders may face peculiar constraints, especially in size; for instance, on the earlier IBM PC and compatibles, a boot sector should typically work in only 32 KiB (later relaxed to 64 KiB) of system memory and only use instructions supported by the original 8088/8086 processors. The first stage of PC boot loaders (FSBL, first-stage boot loader) located on fixed disks and removable drives must fit into the first 446 bytes of the Master boot record in order to leave room for the default 64-byte partition table with four partition entries and the two-byte boot signature, which the BIOS requires for a proper boot loader — or even less, when additional features like more than four partition entries (up to 16 with 16 bytes each), a disk signature (6 bytes), a disk timestamp (6 bytes), an Advanced Active Partition (18 bytes) or special multi-boot loaders have to be supported as well in some environments. In floppy and superfloppy volume boot records, up to 59 bytes are occupied for the extended BIOS parameter block on FAT12 and FAT16 volumes since DOS 4.0, whereas the FAT32 EBPB introduced with DOS 7.1 requires even 87 bytes, leaving only 423 bytes for the boot loader when assuming a sector size of 512 bytes. Microsoft boot sectors therefore traditionally imposed certain restrictions on the boot process, for example, the boot file had to be located at a fixed position in the root directory of the file system and stored as consecutive sectors,conditions taken care of by the SYS command and slightly relaxed in later versions of DOS.The boot loader was then able to load the first three sectors of the file into memory, which happened to contain another embedded boot loader able to load the remainder of the file into memory.When Microsoft added LBA and FAT32 support, they even switched to a boot loader reaching over two physical sectors and using 386 instructions for size reasons. At the same time other vendors managed to squeeze much more functionality into a single boot sector without relaxing the original constraints on only minimal available memory (32 KiB) and processor support (8088/8086). For example, DR-DOS boot sectors are able to locate the boot file in the FAT12, FAT16 and FAT32 file system, and load it into memory as a whole via CHS or LBA, even if the file is not stored in a fixed location and in consecutive sectors.

BIOS and UEFI not only load the operating system from a non-volatile device, they also initialize system hardware for operating system.

Examples of first-stage bootloaders include BIOS, coreboot, Libreboot and Das U-Boot.
What is First-stage boot loader
The first-stage bootloader (FSBL) is the first boot stage for the HPS. In FPGA Configuration First mode, the SDM extracts and loads the FSBL into the on-chip RAM of the HPS. The SDM releases the HPS from reset after the FPGA has entered user mode. After the HPS exits reset, it uses the FSBL hardware handoff file to setup the clocks, HPS dedicated I/Os, and peripherals. Typically, the FSBL then loads the SSBL into HPS SDRAM and passes the control to the SSBL.