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mem= doesn't control the amount of RAM. It actually just specifies the largest physical address the kernel should try to use. The first 4GB of physical memory typically has some non-RAM things in it like the part of the physical address space that's reserved for talking to PCI devices.

If you have 2GB of RAM, 2GB of "other stuff" in the first 4GB, and specify mem=4096m, you'll end up with a kernel that sees 2GB of RAM.

Just a mistake, should be 2048.

2gb user + 2gb kernel?

2 gb user + 1 gb swap + 1 gb rootfs unpacked in ram perhaps. They discuss a decompression operation and the only portion of the sequence that would occur in normally is unpacking a rootfs in RAM.

The kernel itself is compressed, and among the first things that happen after the bootloader hands off to the kernel is that the stub at the beginning of the kernel decompresses the rest of the kernel.

And in this case they don't appear to be using any initrd for the root filesystem, just hurrying to bring up access to the full filesystem over eMMC.