Page Tables
Address spaces
An address space is the range of memory addresses that a process can reference.
Memory management unit
Between CPU and memory, there is memory management unit, which translates virtual addresses from CPU into physical addresses to memory.
Typically this mapping itself is also stored in memory. So the CPU has some register (satp) that points to physical address of the page table, and MMU would read that address, get the mapping from memory, and perform the translation.
Therefore, when switching running processes, the content of satp will be replaced.
In RISC-V 64-bits registers can hold 2^64 addresses, so it’s unreasonable to store all the addresses in the page table. So you translate a page at a time, and a page on RISC-V is 4 kilobytes (4096 bytes). The is pretty common that almost all processors use page size 4k bytes, or support page size 4k bytes.
Naturely, a virtual address is splitted into two pieces: index and offset. Index is actually translated into physical memory page number, while the offset indexes into the physical page.
In fact, on the RISC-V processors that we’re using, not all the 64 bits actually used. The top 25 bits are not used at all. So that limits the size of virtual address to 2^39 bytes, which is roughly 512 GB.
For physical address space, RISC-V allows 56 bits address, which consists of 2^44 pages and 2^12 bytes in a page.