Before we dive into the "661" specifically, we need to understand the container. A .PBP file (PlayStation Portable Binary) is the native executable format for the PSP. Think of it as the equivalent of a .EXE file on Windows.
Note: Sony sometimes reorganizes their support pages. If that link breaks, searching "PlayStation Support PSP System Software 6.61" will get you there. Psp 661.pbp Download
: To launch the PSP environment on a PS Vita, Adrenaline requires the official 6.61 firmware file to extract the XMB (Cross Media Bar). Before we dive into the "661" specifically, we