I want to take a Hard Drive and memory chips from an old 486 and use them in an old Pentium 100. Can I just put the old hard drive in and let the computer auto detect it? I’m hoping it will fit inside with the existing HD but there may not be room, what happens in that scenario? As for the memory chips, I remember that older pentiums needed memory chips in pairs. Does this just mean that for eg an 8mb upgrade I just need two 8mb chips of any kind or do they have to be physically identical in some way?
Don’t assume older hardware will be auto-detected by a “newer” system, including a hard drive.
If the hard drive is still in a functioning system, check the CMOS settings and write them down (on paper!) before removing the drive and installing in the “newer” system. But before you install it, read the information on the hard drive case - manufactuer, model number, etc. - just so you can surf over to the manufacturer’s web site for any information they may still have.
As for the memory, what was that again?
Oh yeah. Head over to http://www.tomshardware.com/ (or similar) to see if the memory from the old MB will function in the “newer” MB.
The HD should work but you have to have a free IDE port. Such an early model may only have 2 IDE ports (for one HD and CD lets say, or 2 HD’s) you can get a IDE card to give you 4 which is the standard today.
If you are lucky that pentuum has a PCI motherboard making adding components easy to find. If you are unlucky that motherboard has vesa local bus slots and it’s going to be hard to find those cards. YOu could fall back and just use the ISA slots - but they are hard to find parts for now too.
If the HD dos not have a space inside you could look for a place to attach it and use zip-ties or such to secure it. If you are not moving the computer and are not in an earthquake prone area you may get away with just laying it on the bottom.
THe memory should be compatable. Yes you have to pair them - to get 8 megs you need two 4mb chips. THere is a slight chace that the Pentium uses 72? pin simms instead of 30? pin sims which you would need an adapter if you can still find one.
The pentium may have both types of mem slots. IF so you will most likely have to choose one or the other (not both)
It’s good if the mem matches but you may be able to get away w/ a mismatch.
YMMV.
I just did this a while ago, though it was from a 386 to a P75. Remember the jumper settings on both drives have to be correctly set or the board may refuse to recognize one or both. You may also have to go into BIOS and tell it the specs of the new drive (which, if they are not written physically on the drive, should be findable online) probably not though, it’s a PI, it should autodetect those.
Now if there’s nowhere to mount it inside the case, why are you adding it in the first place?
Don’t know the memory you’ve got. Eyeball it, see if it fits and recognizes it.
Thanks guys, I’ll take this on board, I just need some spare time to do it now 
You can put the 2nd HD on the same cable as the old HD or the CD-drive. One has to be jumpered "master’ and the other “slave”. See the drive maker’s sites for how to do this. (The CD-drive cable may have only one drive header though.) Since the PC will typically run the IDE interface at the speed of the slower drive, putting it on the CD-drive is a good idea if it’s alot slow than the 1st one.
The motherboard should be able to auto-detect the HD but follow the directions given before just in case.
As for memory, the SIMMS should generally be in identical pairs. But, I have a P-1 board that doesn’t require them. So the best thing to do is to ID your motherboard and check it’s specs. RTFM.
I’ve had some 486 motherboards that use 30 pin simms and some that use 72 pin simms (and I’ve seen oddball ones that don’t use either, so be careful). Every Pentium that I’ve used that is 120 MHz and less has used 72 pin simms but check to see what your motherboard uses. The pentium bus width is double that of a 486, so the 72 pin simms do have to be installed in IDENTICAL pairs. A 486 that takes 72 pin simms can have mismatched simms (i.e. an 8 meg and a 16 meg) but if you put that pair into a pentium it won’t work. The reason for this is that the 486 bus is only wide enough that it can accesss 1 simm at a time. The pentium bus is twice as wide and memory accesses go to pairs of simms, which is why they have to be identical. If you get two 8 MB simms, then your memory will show up as 16 MB (not 8, as the OP alluded to).
I’ve seen a lot of 486 motherboards that could handle 30 pin or 72 pin simms (some of which could actually do a mix of both at the same time). I’ve never seen a pentium motherboard with this setup, but it might exist.