In order to use the optional duplex accessory, you must have a minimum of 12MB of RAM in your LaserWriter 12/640 PS printer. The table below shows the options available depending on the amount of RAM you have installed. The LaserWriter 12/640 PS printer uses industry standard 72-pin SIMMs (70ns or faster). The Tech Info Library article titled "LaserWriter 12/640 PS: Memory Configurations" contains a complete list of possible memory configurations.
| RAM Installed | Options Available |
| 4MB (Standard) | None. You cannot access either PhotoGrade or duplex printing. |
| 12MB | You can enable either PhotoGrade OR duplex printing. |
| 20MB | You can enable both PhotoGrade and duplex printing. Duplex printing is available up to US letter. |
| 24MB and greater | You can enable both PhotoGrade and duplex printing. Duplex printing is available for all paper sizes including US legal. |
Note: Additional memory also enables you to download additional fonts and provides greater document caching.