Upon powering on the console, the first component that starts up is the System Management and Peripheral Control (SMPC), a 4-bit microcontroller that takes care of initialising nearby chips, such as switching on the two SH-2s and setting them in a 'master-slave' configuration .
The SMPC chip in my motherboard revision.
Afterwards, the reset vector of the master SH-2 is set to 0x00000000 , which points to a 512 KB ROM chip containing the Initial Program Loader (IPL).
Japanese version.
European and American versions.
The IPL performs the following functions as part of the boot sequence :
- Finish initialising the hardware.
- If a cartridge is inserted and contains executable code, continue booting from there.
- If a 'Video CD' card is inserted, boot it.
- If a disc is inserted, verify that it's genuine.
- While at it, the system displays the splash screen animation.
- If the disc is genuine, boot the game.
- If the disc is not genuine or there's no disc inserted, launch the interactive shell.
Interactive shell
Alternatively to playing games, the Saturn included a built-in music player called 'Multiplayer', from which users could also access a save data manager.
Interactive shell called 'Multiplayer' or 'Audio CD Control Panel'.
Memory Manager. Moves saves between the cartridge and internal memory.
If a Video CD card was inserted, Multiplayer could also reproduce MPEG video decoded from the card itself.
No BIOS?
Unlike the PlayStation, whose ROM chip bundles a BIOS that exposes APIs for programmers to use, the Saturn's ROM is often referred to as 'IPL' instead - presumably since its main job is to initialise the system, bootstrap the game, and run the shell.
However, the IPL ROM also stores some routines (called services) to manipulate the hardware, such as managing save data and power control. It even implements a semaphore mechanism for synchronising operations across multiple processors simultaneously. Hence, that part of the ROM is called System program.