Those wanting to take their original PlayStation to the next level usually needed to open up the system and install a modchip; that's been common knowledge since even the '90s. However, a user named Socram, previously known for creating amiitool, has released an exploit that makes modding your PS1 easier than ever before. Called "tonyhax", because it utilizes Tony Hawk's Pro Skater 2 or 3, it loads specific data off of the PS1's memory card that "unlocks" the system's disc drive, which then lets you run other region games or even backups.
tonyhax is a save game exploit that uses a specially crafted save game for the Tony Hawk's Pro Skater 2 and 3, in both PAL and NTSC-U versions, to load a custom backup loader that uses no$psx' secret CD unlock commands to enable loading backups on a totally unmodded and stock PS1.
After "extensive testing", Socram has decided to release their exploit to the public, source code and all. The full project is available on GitHub, while a writeup and documentation as to how tonyhax works is available on their website. The creator says that the exploit is possible because in either Tony Hawk's Pro Skater 2 or Tony Hawk's Pro Skater 3 (NTSC or PAL), the game doesn't check if a skater profile name has been edited or messed with in any way. Should you edit the skater name in a dramatic way, it overwrites the memory of the system, which in turn allows custom code to be run.
This first stage payload is about 144 bytes, and its sole purpose is to load the secondary program loader (or SPL for short) from an additional save file in the memory card using the PS1 BIOS calls. Once loaded, it jumps straight to it.
As the console is left in an inconsistent state, the SPL first reinitializes the system kernel (RAM, devices…), by using the very same calls the ROM executes during the booting of the console.
After that, the GPU is reset. Once the GPU is ready again, the sets up the video to a resolution of 320x240, unpacks the 1bpp font from the BIOS ROM into VRAM, and draws the basic border and program name to know everything is working fine until this point.
With a fully working screen, it then proceeds to unlocks the CD drive to accept discs missing the SCEx signature, leveraging the CD BIOS unlock commands found by Martin Korth. These unlock commands are a sort of backdoor, and the drive, probably in order to keep them secret, returns an error instead of a success message. The SPL is coded to expect a particular error to be returned, and will actually abort if the drive returns that it succeeded or if it returns another unexpected error code.
After unlocking it, it waits for the lid to be opened and closed, allowing the user to insert a new CD.
After that, the CD filesystem is reinitialized. It proceeds to read the SYSTEM.CNF configuration file, reinitializes the kernel with the parameters the game needs, and finally loads and runs the game’s main executable.
You'll need a PS1 memory card with tonyhax on it, to which Socram recommends using a PS2 and Free MCBoot to copy it. After loading the profile in-game, the exploit will boot up, and your CD drive will then accept games, even if they're burned CD-R backups, or games from other regions. Tonyhax works on all PAL PlayStation consoles, NetYaroze, and all NTSC-U systems except the original SCPH-1000.