Add INTERRUPT_ENABLE flag, fix bug with register preservation

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/libvm/src/flags.rs

@@ -4,5 +4,6 @@ bitflags! {
     pub struct Flags: u64 {
         const HALT    = 1;
         const COMPARE = 1 << 1;
+        const INTERRUPT_ENABLE = 1 << 2;
     }
 }

src/libvm/src/state.rs

@@ -225,11 +225,10 @@ impl State {
         self.push(Source::Imm(status))?;
 
         // push R0 .. R31
-        for reg in R00 ..= R31 {
+        for reg in R00..=R31 {
             self.push(Source::Reg(reg))?;
         }
 
-
         let sp = self.sp();
         self.set_reg_unchecked(FP, sp - Self::INTERRUPT_REG_SPACE);
         self.set_reg_unchecked(R00, index as u64);
@@ -246,7 +245,8 @@ impl State {
         self.set_reg_unchecked(SP, sp);
 
         // pop R31 .. R0
-        for reg in R31 ..= R00 {
+        for reg in R00..=R31 {
+            let reg = R31 - reg + R00;
             self.pop(Dest::Reg(reg))?;
         }
         self.pop(Dest::Reg(STATUS))?;