Mottzilla's Stealth Unlocker

MottZilla's Stealth Unlocker (archived site) is a ROM designed for cheat cartridges that connects to the parallel I/O port (up to SCPH-750x models), allowing backup or imported discs to boot without the usage of a modchip. It accomplishes this by leveraging secret unlock commands found by nocash to unlock the CD-ROM controller and circumvent region checks at boot-up. It supports nearly all console regions and models, the inclusion of anti-modchip patches via game ID detection and re-enables the early CD player swap trick.

Features

Support for American, European and Net Yaroze consoles.
Japanese and Asian non-Japanese (SCPH-XXX3).
Anti-Mod patches to bypass 'modchip detection' protection mechanisms by checking for patches within its ROM using the game ID.
CD Player swap trick restored for patched models which have this method of booting backup and imported discs disabled.
Swap trick performance improvements including correct CDDA track playback and laser calibration.
- Note: Japanese and Asian consoles don't have the 'secret unlock' in the CD-ROM controller.

List of Supported Protected Games

USA (American)

Cool Boarders 2001 (USA)
Crash Bash (USA)
Dino Crisis (USA)
Disney's The Emperor's New Groove (USA)
Grind Session (USA)
Legend of Dragoon (USA)
MediEvil II (USA)
MLB 2002, 2003, 2004, 2005 (USA)
Resident Evil: Survivor (USA)
Spyro: Year of the Dragon (USA) (Rev 0, Rev 1)
Strider 2 (USA)
Tomba! 2 - The Evil Swine Return (USA)
Vandal Hearts II (USA)
Wild Arms 2 (USA)
Yu-Gi-Oh! Forbidden Memories (USA)

Japan

Biohazard 3 (Japan)
Dino Crisis (Japan)
Koneko mo Issho (Japan)
Pocket Muu Muu (Japan)
Robbit mon Dieu (Japan)
Rockman 1, 2, 3, 4, 5, 6 (Japan)
Rockman X5 (Japan)
Rockman X6 (Japan)
Seiken Densetsu - Legend of Mana (Japan)

Installing Stealth Unlocker

Disc Method (Swap Trick)

Download & Install MottZilla's Stealth Unlocker from the official source (requires sign up) or otherwise from the mirror.
Extract the archive's contents using 7-Zip to a new folder, then burn the (.cue) sheet file (the same way you burn an ISO file) using ImgBurn to a blank CD-R.
Attach your cheat cartridge to the parallel I/O port of your console and insert a licensed disc for your console's region. Use a piece of white tack or a spring to keep the tray sensor pressed down with the lid open.
Power on the console whilst keeping the lid open. Wait for the PlayStation logo to appear, and then quickly swap the game over for your CD-R. Wait for the disc to spin up, and then when it stops spinning, swap back to your pressed disc. Then when the NXFlash logo appears, you can swap back to your CD-R and close the lid.
Press the START button on the first controller to where it says in the menu "Flash EEPROM from CD".
Select the first ROM highlighted using the X button to flash Stealth Unlocker to the cartridge. Once it has finished updating the EEPROM, you can power off the system.

Using Stealth Unlocker

Normal Use (Secret Unlock)

Note: This only applies to most American and European models of the PS1. Japanese and Asian (non-Japanese) models don't have the 'secret unlock'.

Attach the cheat cartridge to the parallel I/O port of the console and insert any disc; whether that would be an unlicensed game, backup or an imported disc.
Power on the console with the tray closed and the disc should boot after the PlayStation logo.

CD Player Swap Trick

Early models of the PlayStation with a BIOS version of 'VC1(a)' or older supports the CD Player menu swap trick by default, but the Stealth Unlocker allows for Japanese and Asian consoles of newer versions to use it, too.

Power on the console with the lid open and the cheat cartridge attached. After the Sony logo appears, wait for the Main Menu and select the CD Player option.
Insert a licensed disc into the drive and hold down the lid sensor switch and wait for it to spin up and then stop spinning.
Keep the lid sensor switch held down and place a backup or imported disc into the console.
Exit the CD Player menu and the disc should boot once it gets past the PlayStation logo.