GPU memory in megabytes, sets the memory split between the CPU and GPU; the CPU gets the remaining memory. The minimum value is 16; the maximum value is 192, 448, or 944, depending on whether you are using a 256MB, 512MB, or 1024MB Pi. The default value is 64. For the Raspberry Pi 4, which is available in 1GB, 2GB and 4GB versions, the minimum and maximum value are the same as for a 1GB device.
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 Raspberry Pi devices with 1024MB or more of memory. (It is ignored if memory size is smaller than 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 on BCM2835 is 0. On BCM2836, BCM2837, and BCM2711, the ARMs have their own L2 cache and therefore the default is 1.
The standard Pi kernel.img and kernel7.img builds reflect this difference in cache setting.
This article uses content from the eLinux wiki page RPiconfig, which is shared under the Creative Commons Attribution-ShareAlike 3.0 Unported license