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Photoshop CS5 64-bit and 32-bit Plugin Issues
Like the program itself, plugins must be 32-bit or 64-bit, to work with a 32-bit or 64-bit Photoshop CS5.
The workaround is to use CS5 in 32-bit mode (see below), but there is a downside: unlike Photoshop CS4 which could use up to 3072MB memory, CS5 limits itself to 2100MB. This limitation is fine for moderate size files, but will be a performance hit for large files.