README.md

@@ -98,6 +98,8 @@ These keywords (among others) are used in conditional branches to specify flag t
 - `nem`, `nempty` … Not empty
 - `eof` … EOF
 - `neof` … Not EOF
+- `true`, `always`, `else` … Always true
+- `false`, `never` … Always false
 
 # Syntax
 
@@ -207,6 +209,7 @@ Conditonal branches are written like this:
   then only one branch is taken - there is no fall-through.
 - The definition order is preserved, i.e. if the `inval` flag is to be checked, it should be done 
   before checking e.g. `nz`, which is, incidentally, true by default, because most flags are cleared by instructions that affects flags.
+- `else` can be used as a final choice of branch that will always be taken.
 
 ## Routines
 

crsn/src/asm/instr/cond.rs

@@ -106,6 +106,10 @@ pub enum Cond {
     Eof,
     /// Not empty
     NotEof,
+    // Always true, for eg. (else? ...)
+    True,
+    /// Always false
+    False,
 }
 
 pub fn parse_cond(text: &str, pos: &SourcePosition) -> Result<Cond, CrsnError> {
@@ -134,6 +138,7 @@ pub fn parse_cond(text: &str, pos: &SourcePosition) -> Result<Cond, CrsnError> {
         "neof" => Cond::NotEof,
         "ov" => Cond::Overflow,
         "nov" => Cond::NotOverflow,
+        "else" => Cond::True,
         _ => {
             return Err(CrsnError::Parse(format!("Unknown cond: {}", text).into(), pos.clone()));
         }
@@ -174,6 +179,8 @@ impl Display for Cond {
             Cond::Valid => "valid",
             Cond::Eof => "eof",
             Cond::NotEof => "neof",
+            Cond::True => "else",
+            Cond::False => "never",
         })
     }
 }
@@ -214,6 +221,9 @@ impl Not for Cond {
 
             Cond::NotEof => Cond::Eof,
             Cond::Eof => Cond::NotEof,
+
+            Cond::True => Cond::False,
+            Cond::False => Cond::True,
         }
     }
 }

crsn/src/asm/instr/flatten.rs

@@ -49,6 +49,10 @@ impl Flatten for InstrWithBranches {
                     return Err(CrsnError::Asm(AsmError::ConditionalAlreadyUsed(cond), branch.pos()));
                 }
 
+                if cnt != branch_count - 1 && cond == Cond::True {
+                    warn!("\"Else\" conditional used in non-final branch at {}:{}", branch.pos().line, branch.pos().column);
+                }
+
                 let next_lbl = if cnt == branch_count - 1 {
                     end_lbl.clone()
                 } else {

crsn/src/runtime/frame/status.rs

@@ -115,6 +115,8 @@ impl StatusFlags {
             Cond::NotEmpty => !self.empty,
             Cond::Eof => self.eof,
             Cond::NotEof => !self.eof,
+            Cond::True => true,
+            Cond::False => false,
         }
     }
 

examples/test_cond.csn

@@ -0,0 +1,17 @@
+(
+  (ld r8 0)
+
+  (ld r0 1
+    (else? (add r8 1))
+  )
+
+  (sub r0 1
+    (pos? (fault))
+    (else? (add r8 1))
+  )
+
+
+  (cmp r8 2
+    (ne? (fault))
+  )
+)