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from Wikipedia:

Commodore’s RAM Expansion Unit (REU) range of external RAM add-ons for their Commodore 64/128 home computers was announced at the same time as the C128. The REUs came in three models, initially the 1700 (128 KB) and 1750 (512 kB), and later the 1764 (256 kB, for the C64).

Although the C128 could access more than 64 kB 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 256 kB model, the 1764, was released for the C64 at the same time. However, aside from a bundled 2.5 ampere 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 stock power supply was inadequate to reliably handle the power load of any of them. Some dealers unbundled the 1764 and the power supply in order to sell the power supply to C64 users, and/or upgrade the 1764 to 512 kB.

Because of memory chip shortages in the late 1980s, the 1750 was only produced in small quantities. However it was not difficult to upgrade a 1700 or 1764 to 512 kB. 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 an REU to one megabyte or higher appeared on various online services.

source: wikipedia