6.61 PRO
6.61 PRO CFW
6.61 PRO is a long-established custom firmware for the PlayStation Portable, built on top of Sony's final 6.61 official firmware. For years it was the default choice for PSP enthusiasts, and it remains a solid, well-understood option even though development has largely moved on to newer projects.
What it does
Like other CFW, 6.61 PRO unlocks the system so you can run homebrew, emulators, plugins, and backups of games you own. It is mature and stable, and a huge amount of community documentation references it directly, which can make troubleshooting easier.
Temporary by default
Running the PRO installer gives you a live custom firmware. This boots you into CFW immediately, but the effect lasts only until you reboot or fully power down the console. After a restart you would need to launch the installer again.
Making it stick
To convert a temporary PRO session into a permanent one, install 6.61 Infinity. Infinity patches the boot sequence so the CFW reloads automatically, removing the need to re-run the installer each time.
PRO or ARK-4?
Both serve the same role as the step-two CFW installer. ARK-4 is the more actively maintained option and is generally recommended for new installs. Choose 6.61 PRO if you specifically want its long track record or are following an older guide that assumes it. Either pairs with Infinity for permanence.