Originally Posted by
Adobe Support
Allocating memory above 2 GB with 64-bit processors
Photoshop CS3 is a 32-bit application. When it runs on a 32-bit operating system, such as Windows XP Professional and some versions of Windows Vista, it can access the first 2 GB of RAM on the computer.The operating system uses some of this RAM, so the Photoshop Memory Usage preference displays only a maximum of 1.6 or 1.7 GB of total available RAM. If you are running Windows XP Professional with Service Pack 2, you can set the 3 GB switch in the boot.ini file, which allows Photoshop to use up to 3 GB of RAM.
Important: The 3 GB switch is a Microsoft switch and may not work with all computers. Contact Microsoft for instructions before you set the 3 GB switch, and for troubleshooting the switch. You can search on the Microsoft support page for 3gb for information on this switch.
When you run Photoshop CS3 on a computer with a 64-bit processor (such as a, Intel Xeon processor with EM64T, AMD Athlon 64, or Opteron processor) running a 64-bit version of the operating system (Windows XP Professional x64 Edition or Windows Vista 64-bit) and with 4 GB or more of RAM, Photoshop will use 3 GB for it's image data. You can see the actual amount of RAM Photoshop can use in the Let Photoshop Use number when you set the Let Photoshop Use slider in the Performance preference to 100%. The RAM above the 100% used by Photoshop, which is from approximately 3 GB to 3.7 GB, can be used directly by Photoshop plug-ins (some plug-ins need large chunks of contiguous RAM), filters, or actions. If you have more than 4 GB (to 6 GB), then the RAM above 4 GB is used by the operating system as a cache for the Photoshop scratch disk data. Data that previously was written directly to the hard disk by Photoshop is now cached in this high RAM before being written to the hard disk by the operating system. If you are working with files large enough to take advantage of these extra 2 GB of RAM, the RAM cache can speed performance of Photoshop. Additionally, in Windows Vista 64-bit, processing very large images is much faster if your computer has large amounts of RAM (6-8 GB).
The default RAM allocation setting is 55%. This setting should be optimal for most users. To get the ideal RAM allocation setting for your system, change the RAM allocation in 5% increments and watch the performance of Photoshop in the Performance Monitor. You must quit and restart Photoshop after each change to see the change take effect.
The available RAM shown in the Performance preference automatically deducts an amount that is reserved for the operating system from the total RAM in your computer. You shouldn't set the percentage of RAM to be used by Photoshop to 100% because other applications which run at the same time as Photoshop (for example, Adobe Bridge) need a share of the available RAM. Some applications use more RAM than you might expect. For example, web browsers can use 20-30 MB of RAM, and music players can use 20-50 MB RAM. Watch the Performance Monitor to view the RAM allocations on your computer.
Watch your efficiency indicator while you work in Photoshop to determine the amount of RAM you'll need to keep your images in RAM. The efficiency indicator is available from the pop-up menu (choose Show > Efficiency) on the status bar of your image and from the Palette Options on the Info Palette pop-up menu. When the efficiency indicator goes below 95-100%, you are using the scratch disk. If the efficiency is around 60%, you'll see a large performance increase by changing your RAM allocation or adding RAM.