In the RISC-V 32 bit ISA, the physical address space is 34 bit with a 32 bit virtual address space. When virtual memory is enabled in supervisor mode the 32 bit virtual address is translated by accessing the page table, yielding a 34 bit physical address. When virtual memory is disabled however, the 32 bit addresses still must be converted to a 34 bit physical address. In the RISC-V privileged ISA specification in section 4.1.12 it states:
When MODE=Bare,supervisor virtual addresses are equal to supervisor physical addresses
So, my question is: does this mean that only the low 4GB (bottom 32 bits) of memory are able to be accessed in supervisor mode with virtual memory disabled? If so, then how is the rest of the 16 GB (34 bit) physical memory supposed to be accessed in supervisor mode when virtual memory is disabled?
Someone asked a similar question in an issue on the Github repo for the ISA manual. It appears to be the case that when running with MODE=Bare with RV32, you can only access the bottom 4GiB of the 34-bit physical address space, and the top 12GiB are inaccessible. The 32-bit register values are zero-extended into 34-bit physical addresses.
While this isn't explicitly stated in the manual, it does say in the caption for Figure 4.17 in the Privileged ISA spec that “when mapping between narrower and wider addresses, RISC-V usually zero-extends a narrower address to a wider size.”
