MAMEWorld Forums - EmuChat - Re: OK I have some good question.

The PS2 runs PS1 games using a combination of hardware and software emulation. The sound and I/O CPU in the PS2 is the same CPU type as the main (and only) CPU in the PS1. This CPU and the PS2 sound and I/O interfaces (to controllers, memory cards and the CD drive) can be switched into a mode where they emulate the PS1 CPU and its corresponding hardware interfaces, hiding away all the PS2-specific registers and capabilities. Meanwhile, the PS2 main CPU (aka the Emotion Engine) runs a software emulator that emulates the PS1 graphics hardware. The emulator is built into the PS2 BIOS.

In short: graphics are software-emulated, everything else is running directly on the bare hardware. That's why you need a PS1 memory card to save PS1 games.

This is somewhat similar to how Sega Master System games run on the Genesis: the games run on the Genesis sound CPU which is the same as the Master System main CPU, and all the hardware is switched into backward-compatibility modes. The difference is that on the Genesis the graphics are also done using hardware backward-compatibility and (as far as I know) the main CPU is simply turned off when playing a Master System game.