Editing Commodore REU (section)

==Hardware description==
[[File:Globe.gif|frame|right|Official performance demonstration]]
Although the C128 could access more than {{nowrap|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. Additionally, the C128's built-in [[Commodore BASIC|BASIC 7.0]] had three statements, <code>STASH</code>, <code>FETCH</code>, and <code>SWAP</code>, for storing and retrieving data from the REU.

Officially, only the 1700 and 1750 were supported on the C128. The {{nowrap|256 KB}} model, the 1764, was released for the C64 at the same time. There were only minor differences between the three models. The original C64 power supply could not support the additional load presented by the 1764 so a 2.5 ampere C64 power supply was included.

The 1700 uses sixteen 4164 64kx1 [[dynamic random-access memory|DRAM]] ICs, the 1764 has eight 41256 256kx1 DRAMs, and the 1750 has sixteen 41256 DRAMs.

In practice, the difference between the 1764 and the earlier units had little effect on compatibility, and people used 1700s and 1750s successfully with the C64, and 1764s successfully with the C128, although the C64's stock power supply was inadequate to reliably handle the power load of any of them. Some dealers unbundled the 1764 and the power supply in order to sell the power supply to C64 users, and/or upgrade the 1764 to {{nowrap|512 KB}}.

The REU model 1750 can be modified to support up to 2MB on memory by installing 2 additional ICs for bank switch. There is only one source of software incompatibility in this modification: software that doesn't set the bits at $DF06 to the same value at all times, or sets the bits to a bank of 512 kb that isn't installed yet, won't work.<ref>{{Cite web |last=DrWatson |date=2022-11-24 |title=Beyond 512 kb: The Two Megabyte REU |url=https://www.neperos.com/article/rlut8ce90fbb7701 |access-date=2022-11-25 |website=Neperos |language=en}}</ref>

During the late 1980s, there was a shortage of DRAM thus 1750s were rare and expensive. However it was comparatively easy to upgrade a 1700 or 1764 to {{nowrap|512 KB}}. Several firms did this commercially, either selling upgraded units or upgrading customer-supplied units.

In the early 1990s, [[do it yourself|DIY]] modification schemes to increase the capacity of an REU to one [[megabyte]] or higher appeared on various online services.