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).

The need for the REU came about when Commodore management decided to not use the final version of the custom Memory Management Unit (MMU) which then limited the size of memory in spite of early discussion of a larger memory map. Engineers traveling to the 1985 CES show were confronted with flyers and billboards advertising a memory size that was no longer supported and finally the top management asked where the additional memory (Up to 512K) would plug in.

By the time of the 1985 CES show in Chicago, the engineers were able to display a spinning globe of the earth as a demonstration of Direct Memory Access (DMA) by the new REU units.

The REU hardware was designed by Frank Palia and the dedicated Integrated Circuit (IC) was designed by Victor Andrade. Fred Bowen and Terry Ryan adapted the kernel and Basic to accommodate the REU natively and Hedley Davis wrote the globe spinning demo which was an impressive display of animation in the mid 1980s.

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. There were only minor differences between the three models. The factory unit could not support the 1764 and it bundled 2.5 ampere C64 power supply unit, which supported the computer and its devices and was defined by the level of need the device had for a regulated power source with enough RAM.

The 1700 utilizes sixteen 4164 64kx1 DRAMs, the 1764 has eight 41256 256kx1 DRAMs, and the 1750 has sixteen 41256 DRAMs.

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