If all else fails, it should issue an INT 18h BIOS interrupt call (followed by an INT 19h just in case INT 18h would return) in order to give back control to the BIOS, which would then attempt to boot off other devices, attempt a remote boot via network. Many modern systems (Intel Macs and newer PCs) use UEFI. Unlike BIOS, UEFI (not Legacy boot via CSM) does not rely on boot sectors, UEFI system loads the boot loader (EFI application file in USB disk or in the EFI System Partition) directly, and the OS kernel is loaded by the boot loader. Other kinds of boot sequences Many modern CPUs, SoCs and microcontrollers (for example, TI OMAP) or sometimes even digital signal processors (DSPs) may have boot ROM integrated directly into their silicon, so such a processor can perform a simple boot sequence on its own and load boot programs (firmware or software) from boot sources such as NAND flash or eMMC. It is difficult to hardwire all the required logic for handling such devices, so an integrated boot ROM is used instead in such scenarios. Also, a boot ROM may be able to load a boot loader or diagnostic program via serial interfaces like UART, SPI, USB and so on. This feature is often used for system recovery purposes, or it could also be used for initial non-volatile memory programming when there is no software available in the non-volatile memory yet. Many modern microcontrollers (e.g. flash memory controller on some USB flash drives) have firmware ROM integrated directly into their silicon. Some embedded system designs may also include an intermediary boot sequence step. For example, Das U-Boot may be split into two stages: the platform would load a small SPL (Secondary Program Loader), which is a stripped-down version of U-Boot, and the SPL would do some initial hardware configuration (e.g. DRAM initialization using CPU cache as RAM) and load the larger, fully featured version of U-Boot. It is also possible to take control of a system by using a hardware debug interface such as JTAG. Such an interface may be used to write the boot loader program into bootable non-volatile memory (e.g. flash) by instructing the processor core to perform the necessary actions to program non-volatile memory. Alternatively, the debug interface may be used to upload some diagnostic or boot code into RAM, and then to start the processor