Make number of physical address bits configurable #1367
Description
Currently all bits are always implemented (64 bits on RV64, 34 bits on RV32... although I'm not 100% sure that all 34 bits actually work).
We should add a config setting for physical address bits, where higher bits are effectively read-only zero. The config validation function should check it isn't more than 34 on RV32.
This affects:
- PPN bits in
satp. pmpaddrN
Actually I think that's it, so it should be very easy. Something like:
let pa_bits = config ...
...
function pmpWriteAddr(locked: bool, tor_locked: bool, reg: xlenbits, v: xlenbits) -> xlenbits =
if (locked | tor_locked) then reg
else {
// Address bits higher then PA_BITS-1 are read-only zero, but
// pmpaddr does not contain the lowest 2 bits of the addresses
// so we actually use PA_BITS-3.
assert(pa_bits <= xlen + 2);
zero_extend(v[pa_bits - 3 .. 0])
}
...
function legalize_satp(
...
let s = [
s with
// If full 16-bit ASID is not supported then the high bits will be read only zero.
Asid = zero_extend(s[Asid][asid_bits - 1 .. 0]),
// Bits above the physically addressable memory are read only zero.
PPN = zero_extend(s[PPN][pa_bits - pagesize_bits - 1 .. 0]),
];
(We need asid_bits configuration too!)