diff --git a/src/' b/src/'
new file mode 100644
index 0000000..2c69bce
--- /dev/null
+++ b/src/'
@@ -0,0 +1,11 @@
+#ifndef ZRE_COMPILER_H
+#define ZRE_COMPILER_H
+
+#include "opcodes.h"
+
+uint32_t demo_add_compile (Value *memory);
+uint32_t demo_loop_compile (Value *memory);
+uint32_t demo_function_call_compile(Value *memory);
+uint32_t compile (Value* memory, char *buffer);
+
+#endif
diff --git a/src/compiler.c b/src/compiler.c
index 15465a4..4c77639 100644
--- a/src/compiler.c
+++ b/src/compiler.c
@@ -1,57 +1,80 @@
 #include "compiler.h"
 #include "parser.h"
 
-uint32_t demo_add_compile(Value *memory) {
+uint32_t test_add_compile(Value *memory) {
   uint32_t i = 0;
   memory[i++].u = OP(OP_LOADU, 0, 0, 0);
-  memory[i++].u = 0;
-  memory[i++].u = OP(OP_LOADU, 1, 0, 0);
-  memory[i++].u = 5;
-  memory[i++].u = OP(OP_LOADU, 2, 0, 0);
   memory[i++].u = 1;
-  memory[i++].u = OP(OP_LOADF, 3, 0, 0);
-  memory[i++].f = 5.0f;
-  memory[i++].u = OP(OP_LOADF, 4, 0, 0);
-  memory[i++].f = 5.0f;
-  memory[i++].u = OP(OP_LOADU, 5, 0, 0);
-  uint32_t jmp = i + 1;
-  memory[i++].u = jmp;
-  memory[i++].u = OP(OP_ADD_REAL, 4, 4, 3);
-  memory[i++].u = OP(OP_SUB_UINT, 1, 1, 2);
-  memory[i++].u = OP(OP_JGT_UINT, 5, 1, 0);
-  memory[i++].u = OP(OP_REAL_TO_STRING, 6, 4, 0);
-  memory[i++].u = OP(OP_REAL_TO_UINT, 1, 4, 4);
-  memory[i++].u = OP(OP_UINT_TO_STRING, 7, 1, 0);
-  memory[i++].u = OP(OP_READ_STRING, 8, 0, 0);
-  memory[i++].u = OP(OP_PRINT_STRING, 0, 6, 0);
-  memory[i++].u = OP(OP_PRINT_STRING, 0, 7, 0);
-  memory[i++].u = OP(OP_PRINT_STRING, 0, 8, 0);
+  memory[i++].u = OP(OP_LOADU, 1, 0, 0);
+  memory[i++].u = 2;
+  memory[i++].u = OP(OP_ADD_UINT, 2, 1, 0); /* let sum = 1 + 2; */
+  memory[i++].u = OP(OP_UINT_TO_STRING, 3, 2, 0);
+  memory[i++].u = OP(OP_PRINT_STRING, 0, 3, 0); /* print(sum.toS()); */
   memory[i++].u = OP(OP_HALT, 0, 0, 0); /* explicit halt */
   return i;
 }
 
-
-uint32_t demo_function_call_compile(Value *memory) {
+uint32_t test_loop_compile(Value *memory) {
   uint32_t i = 0;
+  memory[i++].u = OP(OP_LOADF, 0, 0, 0); /* let a = 5.0 */
+  memory[i++].f = 5.0f;
+  memory[i++].u = OP(OP_LOADI, 1, 0, 0); /* do (i = 5, 0, -1) { */
+  memory[i++].i = 5;
+  memory[i++].u = OP(OP_LOADI, 2, 0, 0); /* loop check value */
+  memory[i++].i = 0;
+  memory[i++].u = OP(OP_LOADI, 3, 0, 0); /* loop incriment value */
+  memory[i++].i = -1;
+  memory[i++].u = OP(OP_LOADU, 4, 0, 0); /* loop start */
+  uint32_t jmp = i + 1;
+  memory[i++].u = jmp;
+  memory[i++].u = OP(OP_LOADF, 5, 0, 0);
+  memory[i++].f = 5.0f;
+  memory[i++].u = OP(OP_ADD_REAL, 0, 0, 5); /* a += 5.0; */
+  memory[i++].u = OP(OP_ADD_INT, 1, 1, 3);  /* (implied by loop) i = i + (-1) */
+  memory[i++].u = OP(OP_JGT_INT, 4, 1, 2);  /* } */
+  memory[i++].u = OP(OP_REAL_TO_UINT, 1, 0, 0); /* let b = a as nat; */
+  memory[i++].u = OP(OP_READ_STRING, 2, 0, 0);  /* let user_string = gets(); */
+  memory[i++].u = OP(OP_UINT_TO_STRING, 3, 1, 0);
+  memory[i++].u = OP(OP_PRINT_STRING, 0, 3, 0); /* print(a.toS()); */
+  memory[i++].u = OP(OP_REAL_TO_STRING, 3, 0, 0);
+  memory[i++].u = OP(OP_PRINT_STRING, 0, 3, 0); /* print(b.toS()); */
+  memory[i++].u = OP(OP_PRINT_STRING, 0, 2, 0); /* print(user_string); */
+  memory[i++].u = OP(OP_HALT, 0, 0, 0);         /* program done */
+  return i;
+}
+
+uint32_t test_add_function_compile(Value *memory) {
+  uint32_t i = 0;
+  memory[i++].u = OP(OP_POP_ARGS, 0, 0, 0);
+  memory[i++].u = OP(OP_ADD_INT, 2, 1, 0);
+  memory[i++].u = OP(OP_RETURN, 0, 0, 0);
+  memory[i++].u = OP(OP_HALT, 0, 0, 0);
+  return i;
+}
+
+uint32_t test_recursive_function_compile(Value *memory) {
+  uint32_t i = 0;
+  memory[i++].u = OP(OP_POP_ARGS, 0, 0, 0);
+  memory[i++].u = OP(OP_ADD_INT, 2, 1, 0);
+  memory[i++].u = OP(OP_RETURN, 0, 0, 0);
   memory[i++].u = OP(OP_HALT, 0, 0, 0);
   return i;
 }
 
 static void letDeclaration() {
-/*   uint8_t global = parseVariable("Expect variable name."); */
+  /*   uint8_t global = parseVariable("Expect variable name."); */
 
-/*   if (match(TOKEN_EQUAL)) { */
-/*     expression(); */
-/*   } else { */
-/*     emitByte(OP_NIL); */
-/*   } */
-/*   consume(TOKEN_SEMICOLON, */
-/*           "Expect ';' after variable declaration."); */
+  /*   if (match(TOKEN_EQUAL)) { */
+  /*     expression(); */
+  /*   } else { */
+  /*     emitByte(OP_NIL); */
+  /*   } */
+  /*   consume(TOKEN_SEMICOLON, */
+  /*           "Expect ';' after variable declaration."); */
 
-/*   defineVariable(global); */
+  /*   defineVariable(global); */
 }
 
-
 static void declaration(Token t) {
   if (t.type == TOKEN_LET) {
     letDeclaration();
@@ -67,7 +90,7 @@ uint32_t compile(Value *memory, char *buffer) {
   uint32_t i = 0;
   initTokenizer(buffer);
   Token t = nextToken();
-   while (t.type != TOKEN_EOF) {
+  while (t.type != TOKEN_EOF) {
     printToken(t);
     declaration(t);
     t = nextToken();
diff --git a/src/compiler.h b/src/compiler.h
index de04409..fa171a5 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -3,8 +3,10 @@
 
 #include "opcodes.h"
 
-uint32_t demo_add_compile (Value *memory);
-uint32_t demo_function_call_compile(Value *memory);
+uint32_t test_add_compile (Value *memory);
+uint32_t test_loop_compile (Value *memory);
+uint32_t test_add_function_compile(Value *memory);
+uint32_t test_recursive_function_compile(Value *memory);
 uint32_t compile (Value* memory, char *buffer);
 
 #endif
diff --git a/src/main.c b/src/main.c
index d401d2d..bad2cfb 100644
--- a/src/main.c
+++ b/src/main.c
@@ -54,7 +54,7 @@ int main(int argc, char **argv) {
   vm.frames_size = FRAMES_SIZE;
   vm.stack_size = STACK_SIZE;
   vm.memory_size = MEMORY_SIZE;
-  vm.frames[vm.fp].allocated.end = demo_add_compile(vm.memory);
+  vm.frames[vm.fp].allocated.end = test_loop_compile(vm.memory);
 
 #ifdef __EMSCRIPTEN__
   emscripten_set_main_loop(mainloop, 0, 1);
diff --git a/src/opcodes.h b/src/opcodes.h
index 197134b..4eb6fc9 100644
--- a/src/opcodes.h
+++ b/src/opcodes.h
@@ -18,12 +18,12 @@ typedef struct {
 #define MAX_REGS 256
 typedef struct {
   Value registers[MAX_REGS]; /* R0-R[MAX_REGS-1] */
-  uint32_t rp; /* register pointer (last unused) */
-  Slice allocated; /* start and end of global allocated block */
+  uint32_t rp;               /* register pointer (last unused) */
+  Slice allocated;           /* start and end of global allocated block */
 } Frame;
 
 #define MEMORY_SIZE 65536
-#define FRAMES_SIZE 32 
+#define FRAMES_SIZE 32
 #define STACK_SIZE 256
 typedef struct {
   uint32_t pc; /* Program counter */
@@ -46,8 +46,14 @@ typedef enum {
   OP_STOREI,  /* stri : next memory location = src1 as int */
   OP_STOREU,  /* stru : next memory location = src1 as uint */
   OP_STOREF,  /* strf : next memory location = src1 as float */
-  OP_PUSH_ARGS, /* */
-  OP_POP_ARGS,  /* */
+  OP_PUSHI,   /* pshi : push int from register onto the stack */
+  OP_PUSHU,   /* pshu : push uint from register onto the stack */
+  OP_PUSHF,   /* pshf : push float from register onto the stack */
+  OP_PUSHS,   /* pshs : push str ref from register onto the stack and copy str  */
+  OP_POPI,    /* popi : pop int from stack onto the register */
+  OP_POPU,    /* popu : pop uint from stack onto the register */
+  OP_POPF,    /* popf : pop float from stack onto the register */
+  OP_POPS,    /* pops : pop str ref from stack and move/copy to register */
   OP_ADD_INT, /* addi : dest = src1 + src2  */
   OP_SUB_INT, /* subs : dest = src1 - src2  */
   OP_MUL_INT, /* mulm : dest = src1 * src2  */
@@ -81,8 +87,8 @@ typedef enum {
   OP_REAL_TO_UINT,   /* rtou : dest = src1 as uint  */
   OP_MOV,            /* move : dest = src1	   */
   OP_JMP,            /* jump : jump to address src1 unconditionally */
-  OP_CALL,           /* creates a new frame */
-  OP_RETURN,         /* returns from a frame to the parent frame */
+  OP_CALL,           /* call : creates a new frame */
+  OP_RETURN,         /* retn : returns from a frame to the parent frame */
   OP_INT_TO_STRING,  /* itos : dest = src1 as str */
   OP_UINT_TO_STRING, /* utos : dest = src1 as str */
   OP_REAL_TO_STRING, /* rtos : dest = src1 as str */
diff --git a/test/func-simple.zre b/test/add-func.zre
similarity index 100%
rename from test/func-simple.zre
rename to test/add-func.zre
diff --git a/test/funcs.zre b/test/funcs.zre
deleted file mode 100644
index 0f2fd45..0000000
--- a/test/funcs.zre
+++ /dev/null
@@ -1,14 +0,0 @@
-fn first() {
-  int a = 1;
-  second(a, -1);
-  int b = 2;
-  second(a, b);
-}
-
-fn second(int c, int d) {
-  str numbers = c.toS() + " " + d.toS();
-  print(numbers);
-  ! implied return because return type is null
-}
-
-first();
diff --git a/test/loop.zre b/test/loop.zre
index 782fe71..9c08ca2 100644
--- a/test/loop.zre
+++ b/test/loop.zre
@@ -1,12 +1,9 @@
-for (let i = 0; i < 10; i = i + 1) {
- print(i);
+let a = 5.0;
+do (i = 5, 0, -1) {
+  a += 5.0;
 }
-
-let val = true;
-let j = 0;
-while (val) {
-      j = j + 1;
-      if (j > 9) val = false;
-      print(j);
-}
-print("done");
+let b = a as nat;
+let user_string = gets();
+print(a.toS());
+print(b.toS());
+print(user_string);