1 files changed, 39 insertions, 0 deletions
diff --git a/os/port/mul64fract.c b/os/port/mul64fract.c
new file mode 100644
index 00000000..ca627073
--- /dev/null
+++ b/os/port/mul64fract.c
@@ -0,0 +1,39 @@
+#include <u.h>
+
+/* mul64fract(uvlong*r, uvlong a, uvlong b)
+ *
+ * Multiply two 64 numbers and return the middle 64 bits of the 128 bit result.
+ *
+ * The assumption is that one of the numbers is a 
+ * fixed point number with the integer portion in the
+ * high word and the fraction in the low word.
+ *
+ * There should be an assembler version of this routine
+ * for each architecture.  This one is intended to
+ * make ports easier.
+ *
+ *	ignored		r0 = lo(a0*b0)
+ *	lsw of result	r1 = hi(a0*b0) +lo(a0*b1) +lo(a1*b0)
+ *	msw of result	r2 = 		hi(a0*b1) +hi(a1*b0) +lo(a1*b1)
+ *	ignored		r3 = hi(a1*b1)
+ */
+
+void
+mul64fract(uvlong *r, uvlong a, uvlong b)
+{
+	uvlong bh, bl;
+	uvlong ah, al;
+	uvlong res;
+
+	bl = b & 0xffffffffULL;
+	bh = b >> 32;
+	al = a & 0xffffffffULL;
+	ah = a >> 32;
+
+	res = (al*bl)>>32;
+	res += (al*bh);
+	res += (ah*bl);
+	res += (ah*bh)<<32;
+
+	*r = res;
+}