GPU memory in megabytes. This sets the memory split between the CPU and GPU; the CPU gets the remaining memory. Minimum value is 16; maximum value is 192, 448, or 944, depending on whether you are using a 256M, 512MB, or 1024MB Pi. The default value is 64.
Setting gpu_mem to low values may automatically disable certain firmware features, as there are some things the GPU cannot do if it has access to too little memory. So if a feature you are trying to use isn't working, try setting a larger GPU memory split.
Using gpu_mem_256, gpu_mem_512, and gpu_mem_1024 allows you to swap the same SD card between 256MB, 512MB, and 1024MB Pis without having to edit config.txt each time:
The gpu_mem_256 command sets the GPU memory in megabytes for the 256MB Raspberry Pi (it is ignored if memory size is not 256MB). This overrides gpu_mem. The maximum value is 192 and the default is not set.
The gpu_mem_512 command sets the GPU memory in megabytes for the 512MB Raspberry Pi (it is ignored if memory size is not 512MB). This overrides gpu_mem. The maximum value is 448 and the default is not set.
The gpu_mem_1024 command sets the GPU memory in megabytes for the 1024MB Raspberry Pi 2 (it is ignored if memory size is not 1024MB). This overrides gpu_mem. The maximum value is 944 and the default is not set.
Setting this to 1 disables the CPU's access to the GPU's L2 cache, and requires a corresponding L2 disabled kernel. Default value is 0.
This article uses content from the eLinux wiki page RPiconfig, which is shared under the Creative Commons Attribution-ShareAlike 3.0 Unported license