So, this is the inside of a PCjr RAM sidecar, which normally adds 128 kiB of RAM to a PCjr. In fact, this is the official IBM version, released after several other companies released aftermarket RAM sidecars.

A base PCjr had 64 kiB, it could be internally expanded with another 64 kiB (with improvements in video capability), and then you needed sidecars after that - think similar to an ISA card, but in a plastic enclosure that attached to the side of the case, so that you didn’t have to open the case to add them, and the case could be smaller. IBM supported running three of these sidecars on one machine to get a total of 512 kiB, and running a fourth to get 640 kiB was trivial (I mean, it had four switches, they told you turn on switch 1 for the first, switch 2 for the second, and switch 3 for the third... so switch 4 for the fourth is only logical.)

However, note that there’s 16 RAM chips in there. By default, they were 4164s - 1 bit wide, 64 kilobit RAM chips, organized in two 8 bit wide banks. I bought this off of eBay, intending to add sockets and upgrade it to 41256s, which are 1 bit wide, and 256 kilobit. (Really attentive people will have already seen what’s up.)


But, I noticed that all four switches were turned on... so I put it on my PCjr, and noticed that it passed POST, stating 640 KB! That’s when I cracked it open, and found out that someone had already done the mod that I was going to do, and the seller didn’t even say that in the auction. Score! (If you zoom in, you’ll notice socketed chips, and that they’re 41256s already.)