Category:REU RAM Expansion Units: Difference between revisions

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.

Emulation

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).

• 1750 1764 Service Schematics.pdf Includes passages on upgrading the 1764 and also includes the beyond 512k article linked below
• Projects directory at zimmers.net has several documents explaining upgrades

Tech

• Technical Reference for CBM REU chip CSG8726 by Wolfgang Moser (local copy)
• 1750/1764 Service Schematics
• 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 us by email.

Weblinks

• Commodore REU at Wikipedia
• Super 1750 Clone at Wikipedia
• Demodisks
• http://www.zimmers.net/anonftp/pub/cbm/documents/chipdata/programming.reu
• http://www.zimmers.net/anonftp/pub/cbm/documents/chipdata/reu.registers
• http://www.zimmers.net/anonftp/pub/cbm/documents/projects/memory/reu/index.html
• REU registers at AAY64
• GEOS Technical Manual has the support routines of GEOS for REU documented
• Advanced Computing Environment supports the REU
• http://www.zimmers.net/anonftp/pub/cbm/c64/programming/restrings.prg and http://www.zimmers.net/anonftp/pub/cbm/c64/programming/restrings.readme
• German cover scan of the 1700/1750 manual at Lemon64