Category:REU RAM Expansion Units

RAM Expansion Unit also known as REU was 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. The REU is capable of initiating DMA transfers on request allowing storing, fetching and swapping memory with the C64 main memory. The C64 is not able to natively access the REU additional memory and programs need to utilise it especially.

So far 128, 256, 512, 2048 and 16384 kb (yes, that's 16MB using a 1541u) were spotted at the expansion port.

The REU is using up all of the IO-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 config and Super Snapshot while Expert Cartridge and Kernel ROM replacements (incl. ExOS and StarDOS cartridges) should work as well.

Emulation

Unfortunately emulation on several emulators yields problems in combination with common cartridges and other I/O devices. Results may vary here.

Tech

Technical Reference for CBM REU chip CSG8726 by Wolfgang Moser (local copy)
A draft VHDL description of the Commodore RAM Expansion Controller chip used in the Commodore RAM Expansion Unit (REU) (local copy) Reverse engineered and written by Rainer Buchty in September 19, 2001. Some syntax errors were corrected on September 17, 2004, but the design has not been tested.

Binaries

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 by email.