Working with image files in Photoshop can use enormous amounts of memory, especially when you have high resolution images or use multiple layers. Photoshop provides several methods for managing your memory, including altering cache settings and manually purging individual caches when your memory runs low. Keep in mind that while lowering memory use or emptying a cache will lighten the load on your system, it may slow down the program or prevent you from undoing as many actions.
Clearing Active Memory
Photoshop stores several elements of your work in active memory, including your clipboard, data for using the "Undo" feature and your history, which makes it possible to step backward in your workflow to an earlier point. Manually clearing one or more of these items recovers memory but may impact your work. For example, if you clear your history, you can't revert your image to an earlier state. To empty the memory, open the "Edit" menu, click "Purge" and select a specific item to purge, or pick "All."
Photoshop Scratch Disks
When Photoshop needs to store more data than can fit in active memory, it writes to a cache file on your hard drive. This scratch disk can use a significant amount of space, but you don't need to delete it manually -- Photoshop erases it every time you quit the program. If the application crashes, preventing the cache from emptying, just reopen Photoshop and quit normally. You can move the scratch disk to a different hard drive with more space on the "Performance" tab of the preferences, or pick multiple drives. Adobe recommends using the fastest connected drives other than the boot drive.
In addition to changing how much RAM Photoshop uses, you can modify three aspects of caching in the performance preferences. "History States" determines how many steps back you can take in your work. "Cache Levels" are used to improve speed -- you should increase the number when working with large images with few layers, and decrease it when using smaller images with many layers. Similarly, increase the "Cache Tile Size" to improve performance with large images and lower it for small images with many layers. Restart Photoshop for setting changes to take effect.
Moving Without the Clipboard
To move or copy part of an image once, use the Move tool. This method does not store the data in the clipboard, saving memory. To move a selection, drag it with the Move tool. To copy, hold "Alt" while dragging. You can also use this method to copy between files: Open both files in Photoshop, make a selection in one file and use the Move tool to drag it onto the second file's tab. Once the screen switches to show the second image, move the cursor onto the image and release the mouse button.
Information in this article applies to Adobe Photoshop CC. It may vary slightly or significantly with other versions or programs.