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 . 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. 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 . Several firms did this commercially, either selling upgraded units or upgrading customer-supplied units. In the early 1990s, DIY modification schemes to increase the capacity of an REU to one megabyte or higher appeared on various online services. Model differences The 1700s circuit board was identical to that of the 1750, and a trace marked J1 indicated the size of the chips used. On the 1750 and 1764, this trace was cut. The 1700 and 1750 had a resistor at position R4 that, according to Commodore engineer Fred Bowen, compensated for subtle timing differences in the expansion port on the C64 and C128. The 1764 lacked that resistor. Bowen and other CBM engineers recommended against using a 1764 with a C128 unless the resistor was added, or a 1700/1750 with a C64 unless the resistor was removed. It was possible to check for the presence of a 1750 by reading memory address $DF00's bit 4, which was 1 on a 1750, and 0 on a 1700 or 1764. However, since this procedure would not distinguish between a 1700 and a 1764. To solve this, programmers wrote to, then read from, the REU's RAM to find out the amount of memory installed. REU software support A small percentage of software made use of the REUs. Like other add-on products from Commodore, their relatively small installed base relative to the huge number of C64s & C128s made software developers hesitant to invest much time and effort in supporting REUs. The lack of commercial support kept sales lower than they otherwise might have been. Due to its high speed relative to Commodore's floppy drives or