diff --git a/src/cali.c b/src/cali.c
index 260b4c8..9ba096e 100644
--- a/src/cali.c
+++ b/src/cali.c
@@ -18,11 +18,6 @@
  * along with this program; if not, write to the Free Software
  * Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  *
- *
- *          0.00    97/11/27 初版
- *          0.01    97/12/06 計算式の評価順序がおかしかった
- * @version 0.01-01 97/12/06 メッセージから日本語を削除
- *
 */
 /* $Id: cali.c,v 1.16 2002/12/31 04:11:19 chikama Exp $ */
 
@@ -34,19 +29,25 @@
 #include "nact.h"
 #include "xsystem35.h"
 
-#define CALI_TRUE       (1)            /* comparison is ture  */
-#define CALI_FALSE      (0)            /* comparison is false */
-#define CALI_MAX_VAL    (65535)        /* maxinum of value    */
-#define CALI_MIN_VAL    (0)            /* mininum of value    */
-#define CALI_NaN        (CALI_MIN_VAL) /* devied by 0         */
-#define CALI_OF         (CALI_MAX_VAL) /* calculate over flow */
-#define CALI_SubNG      (CALI_MIN_VAL) /* nagative result     */
-#define CALI_TERMINATER (0x7f)         /* terminater          */
+#define OP_AND 0x74
+#define OP_OR 0x75
+#define OP_XOR 0x76
+#define OP_MUL 0x77
+#define OP_DIV 0x78
+#define OP_ADD 0x79
+#define OP_SUB 0x7a
+#define OP_EQUAL 0x7b
+#define OP_LT 0x7c
+#define OP_GT 0x7d
+#define OP_NEQUAL 0x7e
+#define OP_END 0x7f
 
-#define CALI_DEPTH_MAX  (256)
+#define OP_C0_INDEX 1
+#define OP_C0_MOD 2
+#define OP_C0_LE 3
+#define OP_C0_GE 4
 
-static int buf[CALI_DEPTH_MAX]; /* 計算式バッファ */
-static int *cali = buf;         /* インデックス */
+#define CALI_DEPTH_MAX 256
 
 static int *getVar(int c0, struct VarRef *ref) {
 	int addr = sl_getIndex();
@@ -55,9 +56,9 @@ static int *getVar(int c0, struct VarRef *ref) {
 		var = c0 & 0x3f;  // 0 - 0x3f
 	} else {
 		int c1 = sl_getc();
-		if (c0 != 0xc0)
+		if (c0 != 0xc0) {
 			var = (c0 & 0x3f) * 256 + c1;  // 0x100 - 0x3fff
-		else if (c1 == 1) {
+		} else if (c1 == 1) {
 			c0 = sl_getc();
 			c1 = sl_getc();
 			var = c0 << 8 | c1;
@@ -79,178 +80,140 @@ static int *getVar(int c0, struct VarRef *ref) {
 	return store;
 }
 
-/* 変数番号が返る */
+// Returns a pointer to the variable
 int *getCaliVariable(void) {
 	int *c0 = getVar(sl_getc(), NULL);
-	if (sl_getc() != 0x7f) {
-		SYSERROR("Something is Wrong @ %03d:%05x", sl_getPage(), sl_getIndex());
+	if (sl_getc() != OP_END) {
+		SYSERROR("Invalid variable expression at %03d:%05x", sl_getPage(), sl_getIndex());
 	}
 	return c0;
 }
 
 bool getCaliArray(struct VarRef *ref) {
 	bool ok = getVar(sl_getc(), ref) != NULL;
-	if (sl_getc() != 0x7f) {
-		SYSERROR("Something is Wrong @ %03d:%05x", sl_getPage(), sl_getIndex());
+	if (sl_getc() != OP_END) {
+		SYSERROR("Invalid variable expression at %03d:%05x", sl_getPage(), sl_getIndex());
 	}
 	return ok;
 }
 
-/* 変数代入コマンド用 */
+// For variable assignment commands
 int *getVariable(void) {
 	return getVar(sl_getc(), NULL);
 }
 
-/* 計算式の評価後の値が返る */
+// Returns the result of the calculation
 int getCaliValue(void) {
-	register int ingVal,edVal,rstVal;
+	uint16_t stack[CALI_DEPTH_MAX];
+	uint16_t *sp = stack;
+	int rhs, lhs, result;
 	int c0,c1;
-	int *bufc = cali;
-	
-	while((c0 = sl_getc()) != CALI_TERMINATER) {
-		if ((c0 & 0x80) != 0) { /* variable */
+
+	while ((c0 = sl_getc()) != OP_END) {
+		if (c0 & 0x80) { // variable
 			int c1 = sl_getcAt(sl_getIndex());
 			int *t;
 			if (c0 == 0xc0) {
-				if (c1 == 1 || c1 >= 0x34) goto l_var;
+				if (c1 == OP_C0_INDEX || c1 >= 0x34) goto l_var;
 				c1 = sl_getc();
-				if (c1 == 2) { /* % */
-					ingVal = *--cali;
-					edVal  = *--cali;
-					if (ingVal == 0) {
-						*cali = CALI_NaN;
-					} else {
-						rstVal = edVal % ingVal;
-						*cali = rstVal;
-					}
-					cali++;
+				if (c1 == OP_C0_MOD) {
+					rhs = *--sp;
+					lhs = *--sp;
+					*sp++ = rhs ? lhs % rhs : 0;
 					continue;
-				} else if (c1 == 3) { /* <= */
-					ingVal = *--cali;
-					edVal  = *--cali;
-					if (edVal <= ingVal) { *cali = CALI_TRUE;  }
-					else                 { *cali = CALI_FALSE; }
-					cali++;
+				} else if (c1 == OP_C0_LE) {
+					rhs = *--sp;
+					lhs = *--sp;
+					*sp++ = (lhs <= rhs);
 					continue;
-				} else if (c1 == 4) { /* >= */
-					ingVal = *--cali;
-					edVal  = *--cali;
-					if (edVal >= ingVal) { *cali = CALI_TRUE;  }
-					else                 { *cali = CALI_FALSE; }
-					cali++;
+				} else if (c1 == OP_C0_GE) {
+					rhs = *--sp;
+					lhs = *--sp;
+					*sp++ = (lhs >= rhs);
 					continue;
 				} else if (c1 < 0x34) {
-					SYSERROR("Unknow Parameter @ %03d:%05x", sl_getPage(), sl_getIndex());
+					SYSERROR("Unknown operator at %03d:%05x", sl_getPage(), sl_getIndex());
 				}
 			}
 		l_var:
 			t = getVar(c0, NULL);
-			*cali++ = t ? *t : 0;
+			*sp++ = t ? *t : 0;
 		} else {
 			switch(c0) {
-			case 0x7e: /* != */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				if (edVal != ingVal) { *cali = CALI_TRUE;  }
-				else                 { *cali = CALI_FALSE; }
-				cali++;
+			case OP_NEQUAL:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs != rhs);
 				break;
-			case 0x7d: /* > */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				if (edVal > ingVal) { *cali = CALI_TRUE;  }
-				else                { *cali = CALI_FALSE; }
-				cali++;
+			case OP_GT:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs > rhs);
 				break;
-			case 0x7c: /* < */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				if (edVal < ingVal) { *cali = CALI_TRUE;  }
-				else                { *cali = CALI_FALSE; }
-				cali++;
+			case OP_LT:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs < rhs);
 				break;
-			case 0x7b: /* == */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				if (edVal == ingVal) { *cali = CALI_TRUE;  }
-				else                 { *cali = CALI_FALSE; }
-				cali++;
+			case OP_EQUAL:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs == rhs);
 				break;
-			case 0x7a: /* - */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				rstVal = edVal - ingVal;
-				if (rstVal < CALI_MIN_VAL) { *cali = CALI_SubNG; }
-				else                       { *cali = rstVal;     }
-				cali++;
+			case OP_SUB:
+				rhs = *--sp;
+				lhs = *--sp;
+				result = lhs - rhs;
+				*sp++ = result < 0 ? 0 : result;
 				break;
-			case 0x79: /* + */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				rstVal = edVal + ingVal;
-				if (rstVal > CALI_MAX_VAL) { *cali = CALI_OF; }
-				else                       { *cali = rstVal;  }
-				cali++;
+			case OP_ADD:
+				rhs = *--sp;
+				lhs = *--sp;
+				result = lhs + rhs;
+				*sp++ = result > 0xffff ? 0xffff : result;
 				break;
-			case 0x78: /* / */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				if (ingVal == 0) {
-					*cali = CALI_NaN;
-				} else {
-					rstVal = edVal / ingVal;
-					*cali = rstVal;
-				}
-				cali++;
+			case OP_DIV:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = rhs ? lhs / rhs : 0;
 				break;
-			case 0x77: /* * */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				rstVal = edVal * ingVal;
-				if (rstVal > CALI_MAX_VAL) { *cali = CALI_OF; }
-				else                       { *cali = rstVal;  }
-				cali++;
+			case OP_MUL:
+				rhs = *--sp;
+				lhs = *--sp;
+				result = lhs * rhs;
+				*sp++ = result > 0xffff ? 0xffff : result;
 				break;
-			case 0x76: /* XOR */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				*cali = (edVal ^ ingVal); cali++;
+			case OP_XOR:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs ^ rhs);
 				break;
-			case 0x75:  /* OR */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				*cali = (edVal | ingVal); cali++;
+			case OP_OR:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs | rhs);
 				break;
-			case 0x74: /* AND */
-				ingVal = *--cali;
-				edVal  = *--cali;
-				*cali = (edVal & ingVal); cali++;
+			case OP_AND:
+				rhs = *--sp;
+				lhs = *--sp;
+				*sp++ = (lhs & rhs);
 				break;
-			default:
-				if ((c0 & 0x40) == 0) { /* WORD const */
+			default:  // immediate value
+				if (c0 & 0x40) {  // 0x0 - 0x33
+					*sp++ = (c0 & 0x3f);
+				} else {  // 0x34 - 0x3fff
 					c1 = sl_getc();
-					if (c0 == 0) { /* 34h-ffh */
-						if (c1 <= 0x33) {
-							SYSERROR("Unknown Parameter @ %03d:%05x", sl_getPage(), sl_getIndex());
-						}
-					} else { /* 100h- 3fff */
-						c1 += ((c0 & 0x3f) * 256);
-					}
-				} else { /* byte const 0-33h */
-					c1 = (c0 & 0x3f);
+					if (c0 == 0 && c1 <= 0x33)
+						SYSERROR("Invalid cali at %03d:%05x", sl_getPage(), sl_getIndex());
+					*sp++ = (c0 & 0x3f) << 8 | c1;
 				}
-				// printf("c1 = %d ",c1);
-				*cali = c1; cali++;
 			}
 		}
 	}
-	c0 = *--cali;
 
-	if (cali != bufc) {
-		WARNING("Something is wrong @ %03d:%05x", sl_getPage(), sl_getIndex());
-		cali = bufc;
+	if (sp != stack + 1) {
+		WARNING("Unexpected end of expression at %03d:%05x", sl_getPage(), sl_getIndex());
 		return 0;
 	}
-
-	return c0;
+	return *--sp;
 }
diff --git a/test/System39.ain b/test/System39.ain
index ff670da..e415a19 100644
Binary files a/test/System39.ain and b/test/System39.ain differ
diff --git a/test/arith.adv b/test/arith.adv
new file mode 100644
index 0000000..b91f9a4
--- /dev/null
+++ b/test/arith.adv
@@ -0,0 +1,35 @@
+	MS FILE, "arith.adv":
+	~AssertEquals 3+4, 7, __LINE__:
+	~AssertEquals 50000+50000, 65535, __LINE__:
+	~AssertEquals 10-3, 7, __LINE__:
+	~AssertEquals 3-10, 0, __LINE__:
+	~AssertEquals 3*4, 12, __LINE__:
+	~AssertEquals 1000*1000, 65535, __LINE__:
+	~AssertEquals 1000*1000-1, 65534, __LINE__:
+	~AssertEquals 10/2, 5, __LINE__:
+	~AssertEquals 10/3, 3, __LINE__:
+	~AssertEquals 10/0, 0, __LINE__:
+	~AssertEquals 10%3, 1, __LINE__:
+	~AssertEquals 10%0, 0, __LINE__:
+
+	~AssertEquals 5&3, 1, __LINE__:
+	~AssertEquals 5|3, 7, __LINE__:
+	~AssertEquals 5^3, 6, __LINE__:
+
+	~AssertEquals 3<2, 0, __LINE__:
+	~AssertEquals 3<3, 0, __LINE__:
+	~AssertEquals 3<4, 1, __LINE__:
+	~AssertEquals 3<=2, 0, __LINE__:
+	~AssertEquals 3<=3, 1, __LINE__:
+	~AssertEquals 3<=4, 1, __LINE__:
+	~AssertEquals 3>2, 1, __LINE__:
+	~AssertEquals 3>3, 0, __LINE__:
+	~AssertEquals 3>4, 0, __LINE__:
+	~AssertEquals 3>=2, 1, __LINE__:
+	~AssertEquals 3>=3, 1, __LINE__:
+	~AssertEquals 3>=4, 0, __LINE__:
+	~AssertEquals 3=2, 0, __LINE__:
+	~AssertEquals 3=3, 1, __LINE__:
+	~AssertEquals 3\2, 1, __LINE__:
+	~AssertEquals 3\3, 0, __LINE__:
+	%0:
diff --git a/test/test.adv b/test/test.adv
index 08e7ff5..36405a5 100644
--- a/test/test.adv
+++ b/test/test.adv
@@ -9,6 +9,7 @@
 	!tests_passed:0!
 	!tests_failed:0!
 
+	%#arith.adv:
 	%#array.adv:
 	%#strvar.adv:
 	%#stack.adv:
diff --git a/test/test.hed b/test/test.hed
index 296e8ed..8da7b91 100644
--- a/test/test.hed
+++ b/test/test.hed
@@ -1,6 +1,7 @@
 #SYSTEM35
 test.adv
 util.adv
+arith.adv
 array.adv
 strvar.adv
 stack.adv
diff --git a/test/testSA.ALD b/test/testSA.ALD
index 557633f..1f066ac 100644
Binary files a/test/testSA.ALD and b/test/testSA.ALD differ