The 512MB / 1GB isn’t directly available to the user, it is according to the Rikomagic forums:
“On the A10, the Mali GPU reserves about 200K (at least for 720p mode — perhaps more for 1080) for itself before the kernel gets its memory block. I believe this is reserved by uboot prior to loading the kernel, but haven’t got that far in studying the boot process yet.
Edit: note that the amount passed to the kernel by uboot doesn’t necessarily match the actual physical memory; however, in the amount reported is the total amount that linux thinks it has. This memory is divvied up in many ways, including the kernel’s executable code and runtime data, running programs’ code and data, file system caches, I/O buffers, and so on”
So far, my MiniX reports around 314MB free user memory, which is comparable to the MK802.