Commodore 1750 [Commodore 64/128]

In this article, I am revisiting an item from my collection that I haven't showcased before: the Commodore 1750 module. This piece of hardware has the abbreviation REU, which stands for RAM Expansion Unit, and was a memory expansion for the Commodore computers C64 and C128.

History of creation and publication

The Commodore 1750 was presented for the first time at the CES (Consumer Electronics Show) in Chicago in 1985. A specific release date and the start of availability need to be researched in more detail, but it is known that the hardware came onto the market relatively soon after the launch. The REU series was created in response to the growing need for additional memory for the home computers that were popular at the time.

Differences between the versions

The REU is available in three configuration levels:

Commodore 1700 with 128 KB (price: 199 DM).
Commodore 1764 with 256 KB, specifically for the C64.
Commodore 1750 with 512 KB, the most powerful variant.

The 1700 and 1750 versions were primarily intended for the C128, but could also be used with the C64. However, when used on the C64, a more powerful power supply was required due to the increased power consumption. The REU 1764, which was developed specifically for the C64, was therefore delivered directly with an adapted power supply (C128 power supply with plug for the C64).

Technical similarities: All expansion modules are identical in design; only the size and number of memory chips differ.

1750 REU - Anschluss — 1750 REU – Connection

Technical details

Unfortunately, the Commodore 1750 could not simply be used as additional addressable RAM. The reason for this lies in the limitations of the 6510 processor, which can address only 64 KB of RAM. Instead, the REU—utilizing the REC (RAM Expansion Controller), a specialized chip—enabled the rapid exchange of data between main memory and the REU via DMA (Direct Memory Access).

For programs to utilize the hardware, they had to be equipped with appropriate routines. One example of this is GEOS, which used the REU as an external RAM disk. The C128, in particular, offered expanded capabilities, as BASIC V7.0 included the commands FETCH, STASH, and SWAP for directly accessing the REU. However, even in this case, the memory could not be used directly for BASIC.

Potential applications at the time

The REU was primarily developed for memory-intensive applications. Examples include:

GEOS: Use as a RAM disk for faster operation.
CP/M 3.0 on the C128: Some versions recognized the REU and could mount it as a fast RAM drive (drive letter M:).
Programmers used the REU for their own routines in assembly language.
Games and Demos: Some ambitious developers were able to use the REU to achieve impressive graphical effects or faster loading times.

1750 REU - Anleitung deutsch — 1750 REU – Anleitung deutsch

1750 REU - Anleitung englisch — 1750 REU – Anleitung deutsch

Today's possibilities with the 1750

Nowadays, the REU can still be put to a wide range of uses, particularly by retro-computing enthusiasts:

Use with emulators such as VICE that virtually replicate REU functions.
Utilization by modern tools that fully exploit the REU's DMA function.
Hardware expansion: It is theoretically possible to upgrade the 1700 and 1764 models to 512 KB by replacing or adding memory chips. Furthermore, capacities of up to 2 MB are realistic and feasible. However, with more than 512 KB of memory, the REU is no longer 100% compatible.

One example of alternative memory expansions is the GeoRAM (512 KB), developed by Berkeley Softworks as a successor to the REU for GEOS. Nevertheless, the original REU was faster and more versatile due to its DMA access.

1750 REU – Connection
1750 REU – Metallkäfig
1750 REU – Platine
1750 REU – Platine – Unterseite

Conclusion

The Commodore 1750 was an impressive piece of hardware for its time, offering the C128 and C64 entirely new possibilities—ranging from faster load times to expanded memory applications. Although its utility was limited back then, today's retro community demonstrates the true potential of this expansion.

I received my 1750 REU with English and German manuals. The previous owner used a C128 primarily with GEOS, which remains one of the most popular applications for the REU to this day.