Category:REU RAM Expansion Units

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1750 REU
1750 REU
Technical Info
RAM 128 - 2048 kbyte originally
ROM None
Hardware features DMA
CRT ID -105 (VICE only)

RAM Expansion Units also known as REU were initially produced and sold by Commodore. This rather popular device is well covered on the net currently and also emulated in software and hardware (e.g. by the 1541 Ultimate) pretty well. The large Commodore version was followed by a CLD version and the CMD clones which all used a CSG8726 as controller chip also called REC. There are apparently two mounting case forms of the REC. The square one is said to be more common.

The REU is capable of initiating DMA transfers on request allowing storing, fetching and swapping memory with the C64 main memory. While the C64 is not able to natively access the REU additional memory and programs need to utilise it especially the REU DMA is able to transfer memory extremely quick.

So far 128, 256, 512, 1024, 2048 and 16384 kb (yes, that's 16MB using a 1541u) were spotted at the expansion port. The sold devices usually went by an obscure numbering representing the RAM size:

  • 1700 REU = 128 kbyte
  • 1764 REU = 256 kbyte
  • 1750 REU = 512 kbyte

The (usually 1750) clones are often smaller and match the size of a common cartridge while keeping the 512 kbyte RAM. These REUs are also consuming a little less power from the C64 expansion port and are far better to handle on port expanders.

The REU is using up all of the I/O-2 area from $DF00 - $DF0F and is mirroring that register range each $20 bytes. Cartridges operating at $DF00 will collide with the REU. Known to work in parallel using a port expander are Retro Replay when using a ROM supporting the REU memory configuration and Super Snapshot while Expert Cartridge and Kernel ROM replacements (incl. ExOS and StarDOS cartridges) should work as well.


Unfortunately emulation on several emulators yields problems in combination with common cartridges and other I/O devices but should work ok as standalone cartridge and usually allows defining the REU size as well. Results may vary here.

RAM Upgrading

There are several documents describing on how to extend the RAM of a REU up to 2 MBytes which is the maximum capacity the REC can handle. Newer implementations as found on the 1541 Ultimate support addressing of more RAM (and usually _have_ that RAM as well).



As you can see below the REU topic is well covered all around. The binary archive for the REU is under construction. Please send anything we may miss to us by email.