Refers to a small auxillary microprocessor present on POWER8 and POWER9 CPU packages. The SBE is the first processor that runs when the CPU is powered up and is responsible for initializing the chip so that the main cores themselves can run. The SBE firmware is stored in a SEEPROM on the CPU module itself and executes out of a small dedicated SRAM (PIBMEM). The term is also used interchangeably to refer to the SBE firmware as well as the SBE. The final act of the SBE is to start Hostboot running on one of the cores.
The SBE firmware is open source. Notably, the POWER9 SBE runs on a dedicated Programmable PowerPC-lite Engine (PPE), in contrast to the POWER8 SBE which runs on a bespoke microprocessor with a custom ISA. This makes programming for the POWER9 SBE significantly easier.