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the address decode for your BIOS is not so simple. Many many motherboards support shadowing your ROM into the much faster RAM for access. Also sometimes the CS/ doesn't come from the address bus of the ISA bus at all, but rather the internal address bus of the chipset. The CS/ may be derived from a CSBIOS/ pin from some chipset IC and if CSBIOS/ is active the external bus may not even be driven! If you are relying the address to appear on the ISA bus you may be in for a surprise.
I like the idea of cutting the CS/ pin and wiring up some small logic:
CS(old) = CS(mb) AND BIOS_SEL CS(new) = CS(mb) AND /BIOS_SEL
where CS(old) = the CS/ pin on the orignal BIOS CS(new) = the CS/ pin on the new BIOS CS(mb) = the original CS/ signal from the motherboard.
This can be done cheaply with a few HC or F series gates and if you're feeling creative, pull BIIOS_SEL up to VCC and use the turbo switch to ground the input to select which one to use. The ICs will be happy and you won't have to worry about about long lead lengths causing ringing to the CS/ signal.
As far as mounting the new ROM -- why not just take a wirewrap socket, cut the CS/ pin short and solder the remainder of the pins directly ON TOP (i.e. in parallel) with the original ROM? (minus the CS/ of course...) Then wire up the CS signals as shown above.
whichever ROM is selected will follow the proper "rules" as far as the motherboard is concerned. they will follow shadowing and cacheing too, so long as you don't switch which BIOS is active while the computer is on all should be fine.
In addition, if you get tired of the project, you can remove the new BIOS, leave the socket on there but permanently activate the original BIOS and the computer is no worse for wear.
back to lurker mode, Andrew - To Unsubscribe: send mail to firstname.lastname@example.org with "unsubscribe openbios" in the body of the message