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Commodore RAM expansion units

The 6502-family CPUs in Commodore 8-bit computers famously used 64K of RAM at a time. But in 1985, Commodore introduced a cartridge that added up to 512K of RAM to the 128. Commodore followed up soon after with a 256K cartridge for the 64. How did Commodore RAM expansion units work?

Blame marketing

Commodore advertised the 128 as having expandable memory, but somebody forgot to tell the engineers who designed it. So they had to come up with a way to expand the memory after the fact.

In many ways, the Commodore RAM expansion units worked like expanded memory worked on the IBM PC. You defined a block of memory and you swapped it with the contents of the extra RAM.  A special chip in the cartridge, the 8726 REC chip, did the work. The REC could move memory around at a rate of 1 megabyte per second, which was incredibly fast for anything in the Commodore 8-bit world.

Commodore RAM expansion units - model 1764

The Commodore 1764 RAM expansion unit came with a replacement power supply and a demo disk.

Commodore made three models: the 128K model 1700, the 256K model 1764, and the 512K model 1750. The 1764, as the name suggests, was designed for the C-64. It came with a heavy-duty power supply big enough to power the 64 and extra RAM. The 128’s stock power supply could already handle the additional memory. Commodore intended the 1700 and 1750 for use with the 128. There was a small timing difference between the two machines, so the 1700 and 1750 had a resistor at position R4, in the upper left corner of the board, that the 1764 lacked. In practice, people used whichever expansion unit they could get with whichever computer they had.

In 1989, Andrew Mikeski published plans for expanding the units to 1 or 2 megabytes of RAM online. It was possible to download them from Quantumlink, Compuserve, and many private BBSs.

What was it good for?

Not enough. GEOS, the graphical user interface for Commodore computers, used them as a RAM disk. Loading your fonts to the RAM disk was a good way to make GEOS run much faster. Most post-1985 disk copy programs would use the extra RAM to reduce disk swaps if you only had a single drive.

But aside from using it as a fast disk drive, not much 1980s and 1990s software used it. Commodore wrote a spinning globe demo to show off the peripheral but these devices found themselves in a bit of a catch-22. There wasn’t a lot of demand due to lack of software, and software makers didn’t want to put a lot of effort into supporting them due to lack of demand. But they were pretty difficult to get, too. More on that in a minute.

Today there are some pretty impressive graphics demos that use the extra RAM for animation, but those are relatively recent developments. I had one, and I knew a few others who had one, and we just basically used them as fast disk drives.


The biggest problem with Commodore RAM expansion units was availability.  Commodore didn’t make enough of them. I can speak from experience on this one. When we ordered one in 1987, it took months to finally get one. Few suppliers advertised them, and those who did advertise them didn’t actually have them in stock.

The lead editorial in the March 1989 issue of Run magazine addressed the issue. Commodore owners and dealers were contacting the magazine asking about availability. Yes, even Commodore’s own dealers weren’t getting answers from Commodore.

After speaking with some Commodore execs, Run technical manager Lou Wallace explained that RAM chips were in short supply, and it was natural for Commodore to build eight C-64s instead of a single 1750 512K RAM expansion unit. Commodore’s profit on eight C-64s was more than the total cost of the single RAM expansion unit.

Wallace suggested that if Commodore didn’t want to make the peripherals, it ought to sell the REC chip to someone who did. It took a couple more years but eventually Commodore did exactly that.

Enter GeoRAM

By late 1990, supplies of Commodore’s RAM product were dwindling. Berkeley Softworks, maker of GEOS, introduced its own RAM expansion. Dubbed GeoRAM, it worked only with GEOS and it wasn’t as fast as Commodore’s product, but it was cheaper and you could actually buy one.

Berkeley didn’t use Commodore’s REC chip. Commodore did sell the chip to a company called Chip Level Designs so they could produce clones of Commodore’s own units. It’s unclear whether Commodore was unwilling to sell the chips to Berkeley, or if Berkeley even asked. The way Commodore operated, it’s entirely possible they found a supply of chips in 1991 and opted to sell them to Chip Level Designs rather than use them. In the 1990s, Creative Micro Designs marketed a 1750XL, which paired up the Commodore REC chip with 2 megabytes of RAM.

RAM expansion today

The 1541 Ultimate emulates a RAM expansion unit, in addition to a lightning fast 1541. Of course the VICE emulator also emulates Commodore’s RAM expansion unit. So even if you can’t find an original 1700, 1750, or 1764, you can still use the software that takes advantage of one thanks to modern alternatives.

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