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Terminal Services Client Registry Editor: Bitmap Cache tab

The bitmap cache stores bitmaps client side so they do not have to be resent over the wire.

Bitmap caching places commonly used bitmaps, like the one used to display the Start button, in a file on the client computer. This improves performance by minimizing the amount of display information that must be passed over a connection.

Bitmap caching is enabled on a per-session basis from the client software and requires 10 megabytes (MB) of free hard-drive space on the client computer.

Persistent bitmap caching allows the client to store bitmaps sent from the server on a persistent media like a disk or flash RAM. On connection, the client informs the server of the bitmaps it has so the server doesn't have to send them over again. The bitmap cache uses a simple virtual memory scheme to increase the size of the cache, without taking up more physical memory. This allows the client to advertise more bitmap cache entries than it has in RAM, and use the disk or file system as the swap space. This helps to reduce the number of cache evictions and improves the bandwidth utilization. Virtual memory bitmap caching is especially useful for bitmaps larger than 4 kilobytes (KB).

The RDP protocol supports a maximum of five bitmap caches. The cell sizes for cache 0 to 4 are 256 bytes (16X16 bitmaps), 1024 bytes (32X32 bitmaps), up to 64 KB (256X256 bitmaps). The number of cell entries depends on the client's configuration. Persistent bitmaps are stored under the directory where the client is installed. Within that directory, a "cache/" directory is created, and within that are numbered directories, one for each cache.