diff --git a/assembler.js b/assembler.js
index 37d31f9..bf82b0f 100644
--- a/assembler.js
+++ b/assembler.js
@@ -13,7 +13,7 @@ exports.assemble = (str, debug = false) => {
   return decodeInstructions(str);
 }
 
-const mnemonicsWithOptionalArgs = ['end', 'cfc', 'chp'];
+const mnemonicsWithOptionalArgs = ['end', 'cfc', 'chp', 'nop'];
 const mnemonics2opcodes = {
   end: { direct: 0, indirect: 0 },
   sto: { direct: 1, indirect: 2 },
@@ -24,6 +24,7 @@ const mnemonics2opcodes = {
   jmp: { direct: 11, indirect: 12 },
   cfc: { direct: 13, indirect: 13 },
   chp: { direct: 14, indirect: 14 },
+  nop: { direct: 15, indirect: 15 },
 };
 
 function decodeInstructions(str) {
diff --git a/readme.md b/readme.md
index 2c3de8f..e84bd81 100644
--- a/readme.md
+++ b/readme.md
@@ -38,22 +38,22 @@ Assemble and run, with debug output:
 
 ## Instruction set
 
-    0   END
-    1   STO lit#   ; store ... mem[lit#] <- A
-    2   STO addr   ; store ... mem[mem[addr]] <- A
-    3   LDA lit#   ; load  ... A <- lit#
-    4   LDA addr   ; load  ... A <- mem[addr]
-    5   ADD lit#   ; add   ... A <- A + lit#      ... and un/set carry flag
-    6   ADD addr   ; add   ... A <- A + mem[addr] ... and un/set carry flag
-    7   SUB lit#   ; sub   ... A <- A - lit#      ... and un/set carry flag
-    8   SUB addr   ; sub   ... A <- A - mem[addr] ... and un/set carry flag
-    9   HOP lit#   ; hop   ... skip next instruction if A == lit# ... when true: IP <- PC + 2
-    A   HOP addr   ; hop   ... skip next instruction if A == addr ... when true: IP <- PC + 2
-    B   JMP lit#   ; jump  ... IP <- lit#
-    C   JMP addr   ; jump  ... IP <- addr
-    D   CCF ————   ; clear Carry Flag ... CF = 0
-    E   CHP ————   ; carry hop ... skip next instruction if Carry Flag is set ... when true: IP <- PC + 2
-    F
+    00   END
+    01   STO lit#   ; store ... mem[lit#] <- A
+    02   STO addr   ; store ... mem[mem[addr]] <- A
+    03   LDA lit#   ; load  ... A <- lit#
+    04   LDA addr   ; load  ... A <- mem[addr]
+    05   ADD lit#   ; add   ... A <- A + lit#      ... and un/set carry flag
+    06   ADD addr   ; add   ... A <- A + mem[addr] ... and un/set carry flag
+    07   SUB lit#   ; sub   ... A <- A - lit#      ... and un/set carry flag
+    08   SUB addr   ; sub   ... A <- A - mem[addr] ... and un/set carry flag
+    09   HOP lit#   ; hop   ... skip next instruction if A == lit# ... when true: IP <- PC + 2
+    0A   HOP addr   ; hop   ... skip next instruction if A == addr ... when true: IP <- PC + 2
+    0B   JMP lit#   ; jump  ... IP <- lit#
+    0C   JMP addr   ; jump  ... IP <- addr
+    0D   CCF ————   ; clear Carry Flag ... CF = 0
+    0E   CHP ————   ; carry hop ... skip next instruction if Carry Flag is set ... when true: IP <- PC + 2
+    0F   NOP ————   ; no operation
 
 - Instructions are two bytes long:
   one byte for the opcode, one for the argument    
diff --git a/simulator.js b/simulator.js
index b7a194d..cc41257 100644
--- a/simulator.js
+++ b/simulator.js
@@ -147,6 +147,11 @@ const Instructions = {
       CPU.IP += 2;
     }
   },
+
+  no_op: () => {
+    CPU.currentInstruction.mnemonic = `NOP`;
+    CPU.IP += 2;
+  },
 }
 
 const opcodes2mnemonics = {
@@ -165,6 +170,7 @@ const opcodes2mnemonics = {
   12: (arg) => Instructions.jump_addr(arg),
   13: (arg) => Instructions.carry_clear(arg),
   14: (arg) => Instructions.carry_hop(arg),
+  15: (arg) => Instructions.no_op(arg),
 };
 
 function startCPU(code) {
diff --git a/test-programs/nop.asm b/test-programs/nop.asm
new file mode 100644
index 0000000..81f8793
--- /dev/null
+++ b/test-programs/nop.asm
@@ -0,0 +1,6 @@
+;; test NOP
+ADD $01
+NOP
+NOP
+NOP
+END
\ No newline at end of file