Is it possible to use only expansion RAM on IBM PC?

[pearce_jj]

The BIOS will initialise it. But we might be able to tweak the hardware to not generate it in the first place perhaps.

 

[jwt27]

Okay, I've added two jumpers to the mainboard, one on U12 to disable the on-board data bus, and one on U99 to disable parity check:

This seems to work so far. I'll pull out the on-board memory and report back.


edit: Machine boots fine without its on-board memory now, IBM diagnostics disc confirms all memory is good.

In case anyone here wants to perform this modification too:
On U12, cut pin 19, bend it upwards. Glue a 3-pin header on top. Connect left pin to U12 pin 20, middle pin to U12 pin 19, right pin to the hole where pin 19 used to be. Left position now disables the on-board memory.
On U99, cut pin 5, bend it upwards. Glue a 3-pin header on top. Connect right pin to U99 pin 14, middle pin to U99 pin 5, left pin to the hole where pin 5 used to be. Right position now disables parity check.

Thanks to pearce_jj and modem7 especially for the hints and schematics!

 

[pearce_jj]

Thanks for posting this. When I get chance, I'll add this to the product wiki page, if you are happy for me to do so?

 

[jwt27]

Sure, go ahead! Would make it easier to find for other users of your card.



And, um, here I thought I could drop these chips in a PC-7000. They looked fairly similar from a distance. Turns out, there's quite a difference between 4164 and 41464... three entire bits, to be precise. :\