System Configuration : System Setup : Configuring Pro Tools System Settings : Cache Size

Cache Size
(Pro Tools HD and Pro Tools with Complete Production Toolkit Only)
The Cache Size setting in the Playback Engine determines the amount of memory DAE allocates to pre-buffer audio for playback and recording. In most cases, the default setting of Normal is the optimum Cache Size for most sessions.
Pro Tools HD and Pro Tools with Complete Production Toolkit can also load audio files used in Pro Tools sessions into RAM for cached playback. Pro Tools prioritizes files closest to the current playhead location. This way, when you start playback, those files are already cached for playback. This is especially useful when working with shared media storage (such as with Avid Unity MediaNetwork and ISIS shared storage systems).
To determine the maximum amount of RAM available for the Disk Cache, Pro Tools polls the computer for the amount of RAM installed and subtracts 3 GB with Mac systems or 4 GB with Windows systems.
For example, if your computer has 12 GB of RAM installed, the total amount of RAM available for the Disk Cache will be 9 GB (Mac) or 8 GB (Windows).
Note that Windows systems reserve more RAM for the system than Mac systems.
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You can use the Cache Meters in the System Usage window to determine whether to assign more or less RAM to the Disk Cache for the current session. See Cache Meters.
To set the amount of RAM for use by the Disk Cache:
1 Choose Setup > Playback Engine.
2 From the Disk Cache selector, select the amount of RAM you want to allocate for Disk Cache.
3 Click OK.