The original PlayStation was one of the first consoles to have a modchip available for it. This was primarily due to the rise of cheap CD burners and blank discs that allowed game backups to be easily produced. That and the rising popularity of downloadable game backups online.
PlayStation 1 modchips were produced as early as 1996, not very long after the console was first released. These early mod chips were very expensive, but over time they were reverse engineered, cloned, and sold prices fell as a result. The original chips used the PIC16C54 microcontroller.
In 1997 a new modchip was released called “Old Crow”. This new modchip was another clone of the original modchip, but the source code was released to the public, reducing the cost of the chip even further. “Old Crow” was originally made for the Zilog Z8 microcontroller, but versions were released for the PIC16C54, and PIC16C84 as well.
Later in 1997 the “Old Crow” code was ported to the then new PIC12C508 microcontroller, which is still being used for modchips today. The new chip dropped the modchip prices to the price range they are still sold at today.
Currently there are a few options for the PS1 modchip. The three most popular chips today are the MM3, the Mayumi v4, and the ONEchip.
All three are based off of the PIC12C508, but there are ports that allow the code to run on other chips like the PIC12F629. These chips are based on the “Old Crow” code with additional features like support for anti modchip games.
MM3 and Mayumi v4 chips are very similar, but operate slightly differently. Because of this some consoles may work better with one than the other.
ONEchip modchips are designed specifically for PAL region PSone’s, and don’t work with anything else. They were designed to bypass additional protection inside the SCPH-102.
There is also newer code designed to be used with modern Atmel processors like the ATmega and ATtiny series of chips.