Commodore's RAM Expansion Unit (REU) range of external RAM add-ons for their home computers was announced at the same time as the Commodore 128. The REUs came in three models, initially the 1700 (128KB) and 1750 (512KB), and later the 1764 (256KB, for the C64).

Hardware description

Although the C128 could access more than 64KB of RAM through bank switching, the memory inside the REU could only be accessed by memory-transfers (STORE/LOAD/SWAP/COMPAREs) between the main memory and the REU memory, thus, giving an equivalent to a (slow) small memory window. Additionally, the C128's built-in BASIC 7.0 had three statements, STASH,FETCH, and SWAP, for storing and retrieving data from the REU.

Officially, only the 1700 and 1750 were supported on the C128. The 256KB model, the 1764, was released for the C64 at the same time. However, aside from a bundled 2.5 amp C64 power supply unit (the factory unit could not support the 1764), there were only minor differences between the three models.

In practice, the difference between the 1764 and the earlier units had little effect on compatibility, and people used 1700s and 1750s successfully with the C64, and 1764s successfully with the C128, although the C64's factory power supply was inadequate to reliably handle the power load of a 1700 or 1750. Some dealers unbundled the 1764 and the power supply in order to sell the power supply to C64 users.

Because of memory chip shortages in the late 1980s, the 1750 was only produced in small quantities. However, due to Commodore's longtime practice of filling unused chip sockets with solder, it was difficult to upgrade a 1700 or 1764 to 512K. Several firms did this commercially, either selling upgraded units or upgrading customer-supplied units.

In the early 1990s, DIY modification schemes to increase the capacity of a REU to 1 megabyte or higher appeared on various online services.

Model differences

The 1700's circuit board was identical to that of the 1750, and a trace marked J1 indicated the size of the chips used. On the 1750 and 1764, this trace was cut.

The 1700 and 1750 had a resistor at position R4 that, according to Commodore engineer Fred Bowen, compensated for subtle differences in the expansion port on the C64 and C128. The 1764 lacked that resistor. Bowen and other CBM engineers recommended against using a 1764 with a C128 it unless the resistor was added, or a 1700/1750 with a C64 unless the resistor was removed.

It was possible to check for the presence of a 1750 by reading memory address $DF00's bit 4, which was 1 on a 1750, and 0 on a 1700 or 1764. However, since this procedure would not distinguish between a 1700 and a 1764, many programmers wrote to the RAM itself to find out the amount of memory installed.

REU software support

Very little software made use of the REUs. Like other add-on products from Commodore International, their relatively small installed base relative to the huge installed base of the C64 made software developers hesitant to invest much time and effort in supporting it, and the lack of commercial support kept sales lower than they otherwise might have been.

The REUs came with software to utilize the extra memory as a RAM disk, but the RAM disk's compatibility with commercial software was spotty, as some commercial software relied heavily on various quirks of the Commodore 1541 floppy drive. Additionally, many commercial programs simply overwrote the memory space occupied by the RAM disk software.

The GEOS operating system had built in support for the REU as a RAM disk, as did the C128's version of CP/M, and some disk copy programs used the REU to facilitate high-speed copying with a single disk drive. GEOS as well as other programs used the REU even for quick memory transfers in the main machine's main memory by storing memory into the REU and fetching it back from there to another location. This was much faster than doing it with the main CPU, since only the data to be transferred had to go on the machine's bus.

Due to its high speed relative to Commodore's floppy drives or even the commercially available hard drives, the REU also became popular with BBS operators.