I read in a forum somewhere of a fellow who was able to install two 1GB DIMMs in an MDD and have it recognized as 2GB of RAM. Additional sticks did not get him any additional capacity. As someone wrote earlier, that's probably a memory map limitation.
It would probably depend on the DIMMs and what kind of memory chips they are made out of. The limitation is usually the row and column addresses. Usually, later memory chips have higher capacities and DIMMs made out of these chips use more row and/or column address lines and often the older machines don't have physical support for the extra line(s).

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