lab17: part 5

lab17/compiler.js

@@ -134,10 +134,9 @@ class Compiler {
       this.writeByte(ast.value);
       return;
     } else if (ast.type === BOOL) {
-      //
-      // ***YOUR CODE HERE***
-      //
       // Booleans will be stored as either 1 for true, or as a 0 for false.
+      this.writeOp('PUSH1');
+      this.writeByte(ast.value ? 1 : 0);
       return;
     } else if (ast.type === VAR) {
       // We look up the offset for a variable and push the offset
@@ -190,12 +189,35 @@ class Compiler {
         break;
 
       case "if":
-        //
-        // ***YOUR CODE HERE***
-        //
         // EXTRA CREDIT!
         // Add support for if expressions.
         // The cond.scm file gives you some good examples.
+        this.writeBytecode(second);
+
+        // get the offset program counter to change it later
+        this.writeOp('PUSH1');
+        const counterOffset = this.offset;
+        this.writeByte(0x00);
+        this.writeOp('JUMPI');
+
+        // else
+        this.writeBytecode(rest[1]);
+        // change this to after the then
+        this.writeOp('PUSH1');
+        const elseOffset = this.offset;
+        this.writeByte(0x00);
+        this.writeOp('JUMP');
+
+        // then
+        const thenOffset = this.offset;
+        this.writeOp('JUMPDEST');
+        this.writeBytecode(rest[0]);
+        const endOffset = this.offset;
+        this.writeOp('JUMPDEST');
+
+        // update the offsets
+        this.bytecode[counterOffset] = thenOffset;
+        this.bytecode[elseOffset] = endOffset;
         break;
 
       case "+":

lab17/cond.scm

@@ -2,3 +2,15 @@
 (if #t (println 3) (println 4))
 (if #f (println 3) (println 4))
 
+; PUSH1 1
+; PUSH1 offset_to_then
+; JUMPI
+;
+; #else
+; PUSH1 offset_to_end
+; JUMP
+;
+; JUMPDEST @offset_to_then
+; #then
+;
+; JUMPDEST @offset_to_end

lab17/op-codes.js

@@ -27,6 +27,17 @@ let opcodes = {
     let value = vm.stack.pop();
     vm.memory[offset] = value;
   }},
+  0x56: { mnemonic: 'JUMP', evaluate: (vm) => {
+    let offset = vm.stack.pop();
+    vm.pc = offset;
+  }},
+  0x57: { mnemonic: 'JUMPI', evaluate: (vm) => {
+    let offset = vm.stack.pop();
+    let condition = vm.stack.pop();
+    if (condition) {
+      vm.pc = offset;
+    }
+  }},
   0x5B: { mnemonic: 'JUMPDEST', evaluate: (vm) => {
     // Does nothing.  We could check to make sure that jumps
     // always land at JUMPDEST opcodes, but it is not totally