diff --git a/simulator.js b/simulator.js
index f6b0bed..b3119a1 100644
--- a/simulator.js
+++ b/simulator.js
@@ -70,9 +70,9 @@ const Instructions = {
   add_addr: (addr) => {
     CPU.currentInstruction.mnemonic = 'ADD addr';
     let sum = CPU.Acc + CPU.memory[addr];
-    if (sum > 15) {
+    if (sum > 255) {
       CPU.CF = 1;
-      CPU.Acc = (sum % 15) - 1;
+      CPU.Acc = (sum % 255) - 1;
     } else {
       CPU.CF = 0;
       CPU.Acc = sum;
@@ -85,7 +85,7 @@ const Instructions = {
     let sum = CPU.Acc - lit;
     if (sum < 0) {
       CPU.CF = 1;
-      CPU.Acc = (sum % 15) + 1; // FIXME ???
+      CPU.Acc = 255 + sum + 1;
     } else {
       CPU.CF = 0;
       CPU.Acc = sum;
@@ -93,9 +93,16 @@ const Instructions = {
     CPU.IP = CPU.IP += 2;
   },
 
-  sub_addr: (addr) => { // TODO: carry flag
-    console.log("SUB addr");
-    CPU.Acc = CPU.Acc - CPU.memory[addr];
+  sub_addr: (addr) => {
+    CPU.currentInstruction.mnemonic = 'SUB addr';
+    let sum = CPU.Acc - CPU.memory[addr];
+    if (sum < 0) {
+      CPU.CF = 1;
+      CPU.Acc = 255 + sum + 1;
+    } else {
+      CPU.CF = 0;
+      CPU.Acc = sum;
+    }
     CPU.IP = CPU.IP += 2;
   },
 
@@ -183,11 +190,11 @@ exports.runProgram = async (code, debug = false) => {
     stepCPU();
 
     console.group(`Step`);
-    if (!debug) console.clear();
-    display.printDisplay(CPU.memory);
+      if (!debug) console.clear();
+      display.printDisplay(CPU.memory);
 
-    logCPUState();
-    if (!debug) await new Promise(resolve => setTimeout(resolve, 75));
+      logCPUState();
+      if (!debug) await new Promise(resolve => setTimeout(resolve, 75));
     console.groupEnd('Step');
   };
 }