diff --git a/check.t b/check.t
index 25a3739..c4612d5 100644
--- a/check.t
+++ b/check.t
@@ -1,4 +1,4 @@
-# $MirOS: src/bin/mksh/check.t,v 1.252 2008/12/13 17:02:11 tg Exp $
+# $MirOS: src/bin/mksh/check.t,v 1.253 2008/12/17 19:39:21 tg Exp $
 # $OpenBSD: bksl-nl.t,v 1.2 2001/01/28 23:04:56 niklas Exp $
 # $OpenBSD: history.t,v 1.5 2001/01/28 23:04:56 niklas Exp $
 # $OpenBSD: read.t,v 1.3 2003/03/10 03:48:16 david Exp $
@@ -7,7 +7,7 @@
 # http://www.research.att.com/~gsf/public/ifs.sh
 
 expected-stdout:
-	@(#)MIRBSD KSH R36 2008/12/13
+	@(#)MIRBSD KSH R36 2008/12/17
 description:
 	Check version of shell.
 stdin:
@@ -240,6 +240,43 @@ expected-stdout:
 	6
 	6,5,3
 ---
+name: arith-unsigned-1
+description:
+	Check if unsigned arithmetics work
+stdin:
+	# signed vs unsigned
+	print x1 $((-1)) $((#-1))
+	# calculating
+	typeset -i vs
+	typeset -Ui vu
+	vs=4123456789; vu=4123456789
+	print x2 $vs $vu
+	(( vs %= 2147483647 ))
+	(( vu %= 2147483647 ))
+	print x3 $vs $vu
+	vs=4123456789; vu=4123456789
+	(( # vs %= 2147483647 ))
+	(( # vu %= 2147483647 ))
+	print x4 $vs $vu
+	# make sure the calculation does not change unsigned flag
+	vs=4123456789; vu=4123456789
+	print x5 $vs $vu
+	# short form
+	print x6 $((# vs % 2147483647)) $((# vu % 2147483647))
+	# array refs
+	set -A va
+	va[1975973142]=right
+	va[4123456789]=wrong
+	print x7 ${va[#4123456789%2147483647]}
+expected-stdout:
+	x1 -1 4294967295
+	x2 -171510507 4123456789
+	x3 -171510507 4123456789
+	x4 1975973142 1975973142
+	x5 -171510507 4123456789
+	x6 1975973142 1975973142
+	x7 right
+---
 name: bksl-nl-ign-1
 description:
 	Check that \newline is not collasped after #
diff --git a/expr.c b/expr.c
index cc9dfd5..829d424 100644
--- a/expr.c
+++ b/expr.c
@@ -2,7 +2,7 @@
 
 #include "sh.h"
 
-__RCSID("$MirOS: src/bin/mksh/expr.c,v 1.21 2008/12/13 17:02:13 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/expr.c,v 1.22 2008/12/17 19:39:21 tg Exp $");
 
 /* The order of these enums is constrained by the order of opinfo[] */
 enum token {
@@ -114,8 +114,26 @@ struct expr_state {
 	struct tbl *evaling;		/* variable that is being recursively
 					 * expanded (EXPRINEVAL flag set) */
 	bool arith;			/* evaluating an $(()) expression? */
+	bool natural;			/* unsigned arithmetic calculation */
 };
 
+#define bivui(x, op, y)	(es->natural ?		\
+	    (long)((x)->val.u op (y)->val.u) :	\
+	    (long)((x)->val.i op (y)->val.i)	\
+	)
+#define chvui(x, op)	do {			\
+	if (es->natural)			\
+		(x)->val.u = op (x)->val.u;	\
+	else					\
+		(x)->val.i = op (x)->val.i;	\
+} while (/* CONSTCOND */ 0)
+#define stvui(x, n)	do {			\
+	if (es->natural)			\
+		(x)->val.u = (n);		\
+	else					\
+		(x)->val.i = (n);		\
+} while (/* CONSTCOND */ 0)
+
 enum error_type {
 	ET_UNEXPECTED, ET_BADLIT, ET_RECURSIVE,
 	ET_LVALUE, ET_RDONLY, ET_STR
@@ -163,6 +181,7 @@ v_evaluate(struct tbl *vp, const char *expr, volatile int error_ok,
 	curstate.noassign = 0;
 	curstate.arith = arith;
 	curstate.evaling = NULL;
+	curstate.natural = false;
 
 	newenv(E_ERRH);
 	i = sigsetjmp(e->jbuf, 0);
@@ -190,6 +209,8 @@ v_evaluate(struct tbl *vp, const char *expr, volatile int error_ok,
 	if (es->tok != END)
 		evalerr(es, ET_UNEXPECTED, NULL);
 
+	if (es->arith && es->natural)
+		vp->flag |= INT_U;
 	if (vp->flag & INTEGER)
 		setint_v(vp, v, es->arith);
 	else
@@ -272,11 +293,11 @@ evalexpr(Expr_state *es, int prec)
 			exprtoken(es);
 			vl = intvar(es, evalexpr(es, P_PRIMARY));
 			if (op == O_BNOT)
-				vl->val.i = ~vl->val.i;
+				chvui(vl, ~);
 			else if (op == O_LNOT)
-				vl->val.i = !vl->val.i;
+				chvui(vl, !);
 			else if (op == O_MINUS)
-				vl->val.i = -vl->val.i;
+				chvui(vl, -);
 			/* op == O_PLUS is a no-op */
 		} else if (op == OPEN_PAREN) {
 			exprtoken(es);
@@ -302,7 +323,7 @@ evalexpr(Expr_state *es, int prec)
 		return (vl);
 	}
 	vl = evalexpr(es, prec - 1);
-	for (op = es->tok; IS_BINOP(op) && opinfo[(int) op].prec == prec;
+	for (op = es->tok; IS_BINOP(op) && opinfo[(int)op].prec == prec;
 	    op = es->tok) {
 		exprtoken(es);
 		vasn = vl;
@@ -320,70 +341,70 @@ evalexpr(Expr_state *es, int prec)
 			else
 				evalerr(es, ET_STR, "zero divisor");
 		}
-		switch ((int) op) {
+		switch ((int)op) {
 		case O_TIMES:
 		case O_TIMESASN:
-			res = vl->val.i * vr->val.i;
+			res = bivui(vl, *, vr);
 			break;
 		case O_DIV:
 		case O_DIVASN:
-			res = vl->val.i / vr->val.i;
+			res = bivui(vl, /, vr);
 			break;
 		case O_MOD:
 		case O_MODASN:
-			res = vl->val.i % vr->val.i;
+			res = bivui(vl, %, vr);
 			break;
 		case O_PLUS:
 		case O_PLUSASN:
-			res = vl->val.i + vr->val.i;
+			res = bivui(vl, +, vr);
 			break;
 		case O_MINUS:
 		case O_MINUSASN:
-			res = vl->val.i - vr->val.i;
+			res = bivui(vl, -, vr);
 			break;
 		case O_LSHIFT:
 		case O_LSHIFTASN:
-			res = vl->val.i << vr->val.i;
+			res = bivui(vl, <<, vr);
 			break;
 		case O_RSHIFT:
 		case O_RSHIFTASN:
-			res = vl->val.i >> vr->val.i;
+			res = bivui(vl, >>, vr);
 			break;
 		case O_LT:
-			res = vl->val.i < vr->val.i;
+			res = bivui(vl, <, vr);
 			break;
 		case O_LE:
-			res = vl->val.i <= vr->val.i;
+			res = bivui(vl, <=, vr);
 			break;
 		case O_GT:
-			res = vl->val.i > vr->val.i;
+			res = bivui(vl, >, vr);
 			break;
 		case O_GE:
-			res = vl->val.i >= vr->val.i;
+			res = bivui(vl, >=, vr);
 			break;
 		case O_EQ:
-			res = vl->val.i == vr->val.i;
+			res = bivui(vl, ==, vr);
 			break;
 		case O_NE:
-			res = vl->val.i != vr->val.i;
+			res = bivui(vl, !=, vr);
 			break;
 		case O_BAND:
 		case O_BANDASN:
-			res = vl->val.i & vr->val.i;
+			res = bivui(vl, &, vr);
 			break;
 		case O_BXOR:
 		case O_BXORASN:
-			res = vl->val.i ^ vr->val.i;
+			res = bivui(vl, ^, vr);
 			break;
 		case O_BOR:
 		case O_BORASN:
-			res = vl->val.i | vr->val.i;
+			res = bivui(vl, |, vr);
 			break;
 		case O_LAND:
 			if (!vl->val.i)
 				es->noassign++;
 			vr = intvar(es, evalexpr(es, prec - 1));
-			res = vl->val.i && vr->val.i;
+			res = bivui(vl, &&, vr);
 			if (!vl->val.i)
 				es->noassign--;
 			break;
@@ -391,13 +412,13 @@ evalexpr(Expr_state *es, int prec)
 			if (vl->val.i)
 				es->noassign++;
 			vr = intvar(es, evalexpr(es, prec - 1));
-			res = vl->val.i || vr->val.i;
+			res = bivui(vl, ||, vr);
 			if (vl->val.i)
 				es->noassign--;
 			break;
 		case O_TERN:
 			{
-				int ev = vl->val.i != 0;
+				bool ev = vl->val.i != 0;
 
 				if (!ev)
 					es->noassign++;
@@ -423,14 +444,14 @@ evalexpr(Expr_state *es, int prec)
 			break;
 		}
 		if (IS_ASSIGNOP(op)) {
-			vr->val.i = res;
+			stvui(vr, res);
 			if (vasn->flag & INTEGER)
 				setint_v(vasn, vr, es->arith);
 			else
 				setint(vasn, res);
 			vl = vr;
 		} else if (op != O_TERN)
-			vl->val.i = res;
+			stvui(vl, res);
 	}
 	return (vl);
 }
@@ -438,13 +459,20 @@ evalexpr(Expr_state *es, int prec)
 static void
 exprtoken(Expr_state *es)
 {
-	const char *cp;
+	const char *cp = es->tokp;
 	int c;
 	char *tvar;
 
 	/* skip white space */
-	for (cp = es->tokp; (c = *cp), ksh_isspace(c); cp++)
-		;
+ skip_spaces:
+	while ((c = *cp), ksh_isspace(c))
+		++cp;
+	if (es->tokp == es->expression && c == '#') {
+		/* expression begins with # */
+		es->natural = true;	/* switch to unsigned */
+		++cp;
+		goto skip_spaces;
+	}
 	es->tokp = cp;
 
 	if (c == '\0')
@@ -499,7 +527,7 @@ exprtoken(Expr_state *es)
 		for (i = 0; (n0 = opinfo[i].name[0]); i++)
 			if (c == n0 &&
 			    strncmp(cp, opinfo[i].name, opinfo[i].len) == 0) {
-				es->tok = (enum token) i;
+				es->tok = (enum token)i;
 				cp += opinfo[i].len;
 				break;
 			}
@@ -514,12 +542,23 @@ static struct tbl *
 do_ppmm(Expr_state *es, enum token op, struct tbl *vasn, bool is_prefix)
 {
 	struct tbl *vl;
-	int oval;
+	long oval;
 
 	assign_check(es, op, vasn);
 
 	vl = intvar(es, vasn);
-	oval = op == O_PLUSPLUS ? vl->val.i++ : vl->val.i--;
+	oval = vl->val.i;
+	if (op == O_PLUSPLUS) {
+		if (es->natural)
+			++vl->val.u;
+		else
+			++vl->val.i;
+	} else {
+		if (es->natural)
+			--vl->val.u;
+		else
+			--vl->val.i;
+	}
 	if (vasn->flag & INTEGER)
 		setint_v(vasn, vl, es->arith);
 	else
@@ -534,9 +573,9 @@ static void
 assign_check(Expr_state *es, enum token op, struct tbl *vasn)
 {
 	if (vasn->name[0] == '\0' && !(vasn->flag & EXPRLVALUE))
-		evalerr(es, ET_LVALUE, opinfo[(int) op].name);
+		evalerr(es, ET_LVALUE, opinfo[(int)op].name);
 	else if (vasn->flag & RDONLY)
-		evalerr(es, ET_RDONLY, opinfo[(int) op].name);
+		evalerr(es, ET_RDONLY, opinfo[(int)op].name);
 }
 
 static struct tbl *
diff --git a/mksh.1 b/mksh.1
index f413708..02d9ae6 100644
--- a/mksh.1
+++ b/mksh.1
@@ -1,4 +1,4 @@
-.\" $MirOS: src/bin/mksh/mksh.1,v 1.147 2008/12/17 19:32:47 tg Exp $
+.\" $MirOS: src/bin/mksh/mksh.1,v 1.148 2008/12/17 19:39:22 tg Exp $
 .\" $OpenBSD: ksh.1,v 1.122 2008/05/17 23:31:52 sobrado Exp $
 .\"-
 .\" Try to make GNU groff and AT&T nroff more compatible
@@ -2096,6 +2096,13 @@ as numeric arguments to the
 .Ic test
 command, and as the value of an assignment to an integer parameter.
 .Pp
+Expressions are calculated using signed arithmetic and the
+.Vt long
+host type, unless they begin with a sole
+.Sq #
+character, in which case they use
+.Vt unsigned long .
+.Pp
 Expressions may contain alpha-numeric parameter identifiers, array references,
 and integer constants and may be combined with the following C operators
 (listed and grouped in increasing order of precedence):
diff --git a/sh.h b/sh.h
index c7f3ba2..2268ec7 100644
--- a/sh.h
+++ b/sh.h
@@ -103,9 +103,9 @@
 #define __SCCSID(x)	__IDSTRING(sccsid,x)
 
 #ifdef EXTERN
-__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.267 2008/12/13 18:32:27 tg Exp $");
+__RCSID("$MirOS: src/bin/mksh/sh.h,v 1.268 2008/12/17 19:39:23 tg Exp $");
 #endif
-#define MKSH_VERSION "R36 2008/12/13"
+#define MKSH_VERSION "R36 2008/12/17"
 
 #ifndef MKSH_INCLUDES_ONLY
 
@@ -775,6 +775,7 @@ struct tbl {			/* table item */
 	union {
 		char *s;		/* string */
 		long i;			/* integer */
+		unsigned long u;	/* unsigned integer */
 		int (*f)(const char **);/* int function */
 		struct op *t;		/* "function" tree */
 	} val;			/* value */