Support writes to misa.C in riscv.

2 files changed, 15 insertions, 7 deletions

diff --git a/riscv/riscv.sail b/riscv/riscv.sail
index 0b651f04..7ba02a77 100644
--- a/riscv/riscv.sail
+++ b/riscv/riscv.sail
@@ -1159,7 +1159,7 @@ function readCSR csr : csreg -> xlenbits =
     0x305 => mtvec.bits(),
     0x306 => EXTZ(mcounteren.bits()),
     0x340 => mscratch,
-    0x341 => mepc,
+    0x341 => mepc & pc_alignment_mask(),
     0x342 => mcause.bits(),
     0x343 => mtval,
     0x344 => mip.bits(),
@@ -1175,7 +1175,7 @@ function readCSR csr : csreg -> xlenbits =
     0x105 => stvec.bits(),
     0x106 => EXTZ(scounteren.bits()),
     0x140 => sscratch,
-    0x141 => sepc,
+    0x141 => sepc & pc_alignment_mask(),
     0x142 => scause.bits(),
     0x143 => stval,
     0x144 => lower_mip(mip, mideleg).bits(),
diff --git a/riscv/riscv_sys.sail b/riscv/riscv_sys.sail
index 3152b95c..36942eed 100644
--- a/riscv/riscv_sys.sail
+++ b/riscv/riscv_sys.sail
@@ -40,9 +40,14 @@ bitfield Misa : bits(64) = {
 }
 register misa : Misa
 
-function legalize_misa(m : Misa, v : xlenbits) -> Misa =
-  /* Ignore all writes for now. */
-  m
+function legalize_misa(m : Misa, v : xlenbits) -> Misa = {
+  /* Allow modifications to C. */
+  let v = Mk_Misa(v);
+  // Suppress changing C if nextPC would become misaligned.
+  if v.C() == false & nextPC[1] == true
+  then m
+  else update_C(m, v.C())
+}
 
 bitfield Mstatus : bits(64) = {
   SD   : 63,
@@ -112,6 +117,9 @@ function haveRVC()     -> bool = { misa.C() == true }
 function haveMulDiv()  -> bool = { misa.M() == true }
 function haveFP()      -> bool = { misa.F() == true | misa.D() == true }
 
+function pc_alignment_mask() -> xlenbits =
+  ~(EXTZ(if misa.C() == true then 0b00 else 0b10))
+
 /* interrupt registers */
 
 bitfield Minterrupts : bits(64) = {
@@ -866,7 +874,7 @@ function handle_exception(cur_priv : Privilege, ctl : ctl_result,
       print("ret-ing from " ^ prev_priv ^ " to " ^ cur_privilege);
 
       cancel_reservation();
-      mepc
+      mepc & pc_alignment_mask()
     },
     (_, CTL_SRET())  => {
       let prev_priv = cur_privilege;
@@ -879,7 +887,7 @@ function handle_exception(cur_priv : Privilege, ctl : ctl_result,
       print("ret-ing from " ^ prev_priv ^ " to " ^ cur_privilege);
 
       cancel_reservation();
-      sepc
+      sepc & pc_alignment_mask()
     }
   }
 }