PearPC Nightly Builds

The acronym ROM stands for Read-Only Memory. It is a small amount of memory inside the computer which is programmed to contain a special program when the computer was manufactured. It cannot usually be changed. The special program is started whenever the Mac is started. It is the program which tests the computer, displays a happy or sad Mac icon. It checks if there is a disk from which the computer can be started, and if so then it starts the MacOS from that disk.

Once the MacOS is running, the job of the ROM program is not finished. It acts as an interface between programs on your computer and the actual hardware of the Mac. Because different Macs are equipped with very disparate hardware, the ROMs provide a common interface to whatever hardware is in your particular Mac.

Different ROM software versions are kept track of through three different systems. You can just record the amount of storage in the ROM -- some are known as 128k, 256k, 512k or 1MB ROMs. There is also an internal Mac version system which is not often used with regards to emulation. The most common way is through the "checksum". This is a figure obtained by comparing all the bytes of the ROM, adding them together to get a figure which will identify the ROM, as well as to tell you whether it has somehow become corrupted (the internally recorded checksum will be different to the one that you calculate from the ROM).

Most emulators are not yet clever enough to emulate the function of the Mac ROM, so they have to use a copied 'image'. You run a program on your Mac which copies the software in the ROM into a file on disk, called a ROM image. This file is then transferred onto the computer on which you want to emulate the Mac. On emulators such as SoftMac, Fusion and Basilisk II, any 512k or 1MB ROM image should work. vMac and Worm use a 128k ROM image from a Mac Plus. Executor and EmMac do not require ROM images.

Because the ROM software is copyrighted and licensed for use only on a Macintosh by Apple, the legality of using it on emulators is questionable. What is not questionable is the practise of distributing the software over the internet -- this is definately illegal and I will not give away copies of ROM images.