diff --git a/sys/man/2/mp b/sys/man/2/mp
index 5223c8a10..678f0eb7f 100644
--- a/sys/man/2/mp
+++ b/sys/man/2/mp
@@ -127,6 +127,9 @@ void	mptrunc(mpint *b, int n, mpint *res)
 void	mpxtend(mpint *b, int n, mpint *res)
 .PP
 .B
+void	mpasr(mpint *b, int n, mpint *res)
+.PP
+.B
 void	mpmul(mpint *b1, mpint *b2, mpint *prod)
 .PP
 .B
@@ -584,6 +587,12 @@ Logical operations (treating negative numbers using two's complement):
 .I mpnot
 .BR "res = ~b1" .
 .TP
+.I mpasr
+.BR "res = b>>shift"
+(\fImpasr\fR, unlike
+.IR mpright ,
+uses two's complement).
+.TP
 .I mptrunc
 truncates
 .I b
diff --git a/sys/src/libmp/port/mplogic.c b/sys/src/libmp/port/mplogic.c
index 3dd9be2ab..16de7571b 100644
--- a/sys/src/libmp/port/mplogic.c
+++ b/sys/src/libmp/port/mplogic.c
@@ -85,7 +85,8 @@ void
 mpnot(mpint *b, mpint *r)
 {
 	mpadd(b, mpone, r);
-	r->sign ^= -2;
+	if(r->top != 0)
+		r->sign ^= -2;
 }
 
 void
@@ -193,3 +194,15 @@ mpxtend(mpint *b, int n, mpint *r)
 	}
 	mpnorm(r);
 }
+
+void
+mpasr(mpint *b, int n, mpint *r)
+{
+	if(b->sign > 0 || n <= 0){
+		mpright(b, n, r);
+		return;
+	}
+	mpadd(b, mpone, r);
+	mpright(r, n, r);
+	mpsub(r, mpone, r);
+}