This Parallel RAM Expansion Adds 128kB to a Vintage IBM PC — Spread Across Five Breadboards

Pseudonymous retrocomputing enthusiast "GoldNPotato" has put together a prototype expansion designed to add 128kB of parallel RAM to a vintage IBM PC 5150 — complete with parity checking.

"On a commercial RAM expansion board of that era, DRAM [Dynamic RAM] was mostly used, and it was actually nine bits of data being stored: 8 data bits + 1 parity bit," GoldNPotato explains of what makes his prototype particularly unusual. "The approach I used to implement parallel RAM included using 128 kiloBYTE RAM IC for data, then a 128 kiloBIT RAM IC to store parity bits."

Built atop a number of breadboards with a concerning amount of spaghetti wiring between, plus a link to an ISA prototyping board for connection to the PC itself, the resulting 128kB RAM "card" should support being pushed further — in theory, at least. "The goofy design I made is pretty easily scalable to larger sizes," GoldNPotato writes. "Granted, at larger sizes I think DRAM would probably be more cost effective."

The way the IBM communicates with the expansion board is also somewhat unusual, at least from a modern perspective: Each time a single bit of parity is written, a full byte needs to be read from parity RAM and stored in holding; as the data is read, the bit to be written is flipped before the whole byte is written back to the parity RAM from whence it came.

Sadly, the unusual expansion isn't likely to last long. "I was so excited to start this as a new project I didn’t realize that the after-market memory expansion card this PC came with actually maxed the computer out to 640kB," GoldNPotato explains.

"The only reason the computer reported less was a DIP switch was out of position on the motherboard. Oh well, time to tear it apart and build an IO multi card!"

More details on the project are available in GoldNPotato's Reddit thread.