This chapter describes the CD-ROM subsystem of the PlayStation hardware such as specifications, capabilities and limitations of the hardware. This chapter also teaches the basics of using the CD-ROM subsystem in your PlayStation projects, common optimization methods and more.
Before delving into how to use the CD-ROM subsystem in your projects, it is best to first understand the hardware first. You should know by now that the CD-ROM subsystem uses CD's and that it can read either CD-ROMs and CD-Rs. But getting the latter to read reliably requires a bit of tweaking of the hardware to make such discs readable, though a mod-chip is mandatory for being able to do anything useful with the CD-ROM subsystem as far as homebrew is concerned which will be described in greater detail later.
You should also know by now that the total capacity of a CD-ROM is about 700MB. Whilst pretty much any modern CD-ROM and CD-R disc is 700MB, the original 'standard' capacity of CDs is actually 650MB. Reason for bringing this up is because the PS1 hardware could supposedly only access up to the first 650MB of the disc and trying to access anything past it will result in problems. This is something that may be worth considering when packing a lot of content on a disc.
The CD-ROM hardware is capable of 2x read speed with transfer rates of up to around 300KB/s. The hardware also supports ADPCM compressed 'XA' audio with interleaving capability of up to eight 37.8KHz Stereo audio streams simultaneously, but only one of eight audio streams can be 'listened' from at a time. More channels is possible when using mono and lower sample rate XA audio which may be covered in future chapters. XA sectors can also be interleaved with data sectors and is usually used for FMV movies. Obviously, the hardware can also play plain CD audio tracks.
PS1 game discs are typically formatted in mixed Mode 2/Form 1 containing a standard ISO9660 version 1 file system, so opening a PS1 disc in any computer wouldn't look much different to a typical PC CD-ROM, except perhaps files containing a data+XA interleave would be unreadable. XA audio data is stored on the disc as Mode 2/Form 2 in the same track containing Mode 2/Form 1 sectors.
An inherent problem in regards to producing homebrew on the PS1 is the requirement of a mod-chip. The reason for this is to get around the console's copy protection mechanism within the HC05 microcontroller that drives the CD-ROM subsystem as a way to prevent unauthorized copies from working on the console.
The copy protection mechanism works by storing a simple four character license string inside the so-called wobble groove situated at the innermost part of the disc. Normally, this special groove would contain ATIP information used to identify CD-R medium with the wobble pattern used for speed calibration prior to burning. CD-ROMs normally don't contain this wobble groove but Sony repurposed it as an effective copy protection mechanism, as it is impossible to replicate the wobble pattern with any CD-R burner without special disc pressing equipment. Producing special CD-Rs containing the license data in the wobble groove is also not possible as replacing the ATIP information with the license string would render the disc impossible to burn.
Perhaps as an extra level of security in the HC05 microcontroller, the CD-ROM subsystem will not read data sectors properly if the disc is not identified as a licensed PS1 disc. So even if you got your homebrew running on the console through other means, such as a cheat cartridge, you won't be able to do much with the CD-ROM subsystem as you won't be able to read or stream data from the disc properly. However, a simple swap trick with a licensed disc can be used to get around this, but it has it's limitations. Namely, the table of contents read from the last disc will persist, preventing the use of CD audio tracks and updating the TOC is impossible wihtout clearing the licensed disc status in the HC05. It is also not very convenient when testing homebrew as you'll have to swap discs around in every turnaround.
The easiest and simplest solution to this problem is by installing a mod-chip in the console, either a 12C508P microcontroller programmed with MM3 or a compatible Arduino programmed with PSnee. The function of a mod-chip is to basically fool the HC05 microcontroller into thinking that the disc inserted is a licensed disc, by feeding the appropriate license string into it at the right time. If you live in an area where bootleg PS1 discs were prominent during the relevancy of the console (often Asian territories), there's a good chance your console already has a chip. Do not remove the chip even for 'legality' reasons if you intend to develop homebrew, unless you want to be counter intuitive to yourself.
There is one more alternative to getting CD-Rs to read properly on the PS1 that doesn't involve the use of a mod-chip or swap method and that is by a backdoor in the HC05 firmware through the use of special unlock commands. Once these commands are issued the HC05 would behave like as if a licensed disc is inserted and data sectors will be read properly. However, this only works in US and EU region consoles and the only bootloader that supports this feature currently is Sicklebrick's UniROM.
There are two ways of accessing the CD-ROM in the software side. One is through the BIOS CD-ROM subsystem and the other is through a CD-ROM library (libcd for PsyQ/Programmers' Tool or psxcd for PSn00bSDK). The CD-ROM library is most recommended as it gives full control over the CD-ROM hardware which the BIOS subsystem does not offer, but using the BIOS yields a much smaller executable size and permits listing of directory contents useful for utility style homebrew. PSn00bSDK however provides directory querying facilities not available in PsyQ/Programmers' Tool SDK in it's CD-ROM library.
This chapter will only cover using the CD-ROM subsystem through the libraries, as it offers greater functionality and performancec over the BIOS subsystem.
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