The PSP uses ELF (Executable and Linking Format) as the executable format. These are packaged into a EBOOT.PBP along with other files for easy distribution and loading. When loaded, the MIPS cpu can run the code in kernel or user mode. Most of the XMB and drivers for the hardware boot in kernel mode. Most of the kernel code and device drivers for the PSP are coded in a relocateable executable format with the extension .prx. This format is a proprietary format made by Sony. Note that .prx format code need not run in kernel mode, but a vast majority of them are because Sony provided them to control the PSP hardware.