A lot of processors don't even map usable RAM at address zero is the real problem. For instance a lot of simpler RISC-V chips are copying the basic Cortex-M me

A lot of processors don't even map usable RAM at address zero is the real problem. For instance a lot of simpler RISC-V chips are copying the basic Cortex-M memory map which places ROM low in memory. Even on systems with MMUs, it's very common that the kernel won't let you map RAM at address zero for several reasons. And the sizes are being talked about are about the size of a single page.