What's the differance between the flash chips and the rom simm?
This is a strange Mac. It has one part of the ROM stored in flash chips and a different part of the ROM stored in a ROM SIMM. They aren't redundant; both are apparently necessary to boot the Mac. We're not really sure how they work together but we do know that the startup sounds are stored in flash.
My theory is that the flash chips contain open firmware (OF) and the very low-level code to get the machine to load up OF. This is why the OF version changes when I've put the Digibarn flash chips into the motherboard. As the ROM SIMM is only 3MB I think this doesn't contain any OF.
On NewWorld ROM machines the startup from is only about 1MB in size (such as the B&W G3). Also on NewWorld machines (which load the Mac OS toolbox ROM from file) the Mac OS ROM file is 3MB. On the Beige G3 (oldworld) the ROM SIMM is 4MB.
I suggest to boot into the classic Mac OS on the PEX the following process occurs:
1) OF loads from the flash chips,
2) Boots to the default device of OF APPL,ROM
3) In this case the start memory location of APPL,ROM is within the flash chips.
4) This then plays the second startup sound and then points to a memory location which is the start of the Mac OS toolbox ROM from the SIMM.
In comparison the oldworld boot process is:
1) OF loads from the onboard ROM / ROM SIMM (in Beige G3s)
2) Boots to the default device of OF APPL,ROM, which then loads the Mac OS toolbox ROM (stored on the onboard ROM/ROM SIMM)
The newworld boot process is:
1) OF loads from the flash chips on the motherboard
2) OF locates the Mac OS ROM file on the chosen boot device
3) This is stitched into the OF memory and APPL,ROM is defined in memory as the boot device.
4) Boots to the default device of OF APPL,ROM, which then loads the Mac OS toolbox ROM
Normally a ROM SIMM added into a board would complete disable an existing onboard ROM, but as people have noted they seems to coexist. It's almost a half-way house between the oldworld and newworld ROM architectures.
I found the documents below useful reading in understanding the classic Mac OS boot process. Mac OS X simply uses OF to load BootX and the Kernel, and does not use the Mac OS toolbox ROM at all.
http://developer.apple.com/documentation/Hardware/DeviceManagers/pci_srvcs/pci_cards_drivers/PCI_BOOK.26.html#pgfId=3296
http://support.apple.com/kb/TA29027?viewlocale=en_US