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/
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,
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