Commodore REU
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).
Read more about Commodore REU: Hardware Description, Model Differences, REU Software Support