diff --git a/src/cpu.js b/src/cpu.js
index 54df653..c66fa69 100644
--- a/src/cpu.js
+++ b/src/cpu.js
@@ -54,8 +54,8 @@ module.exports = class CPU {
     this._cycleStartCallbacks.forEach((fn) => fn());
 
     if (this.IP >= this.memory.length) {
-      console.error('HALTING - IP greater than memory size'); // FIXME - halting the CPU should throw an error instead
       this.running = false;
+      throw new Error('HALTING - IP greater than memory size');
     } else {
       this.instruction.opcode = this.memory[this.IP];
       this.instruction.operand = this.memory[this.IP+1];
@@ -68,8 +68,8 @@ module.exports = class CPU {
 
     // Temporary limit as a lazy way to halt infinite loops
     if ((this._cycleLimit > 0) && this.dbg.cycleCounter >= this._cycleLimit) {
-      console.warn('  HALTING - reached cycle limit'); // FIXME - throw error instead
       this.running = false;
+      throw new Error('  HALTING - reached cycle limit');
     }
 
     this._cycleEndCallbacks.forEach((fn) => fn());
@@ -292,8 +292,8 @@ module.exports = class CPU {
 
     flag_toggle: (flagNum) => {
       if (flagNum === null) {
-        console.error('Invalid flag number');
-        process.exit(); // FIXME -- throw error instead
+        let info = this.dbg.sourceInfo[this.IP];
+        throw new Error(`Invalid flag number: '${flagNum}' on line ${info.lineNumber}: ${info.source}`);
       }
       const flagName = this.flagNums[flagNum];
       this.dbg.currentMnemonic = `FTG ${flagName}`;