diff --git a/bootstrap.fs b/bootstrap.fs
index e13d5d7..a68faa3 100644
--- a/bootstrap.fs
+++ b/bootstrap.fs
@@ -1465,6 +1465,9 @@ decimal
 s" Not implemented" exception constant NOT-IMPLEMENTED
 : not-implemented NOT-IMPLEMENTED throw ;
 
+s" Not supported" exception constant NOT-SUPPORTED
+: not-supported NOT-SUPPORTED throw ;
+
 ( 31 bytes )
 s" Not reachable here. may be a bug" exception constant NOT-REACHABLE
 : not-reachable NOT-REACHABLE throw ;
diff --git a/lib/bitscan.fs b/lib/bitscan.fs
new file mode 100644
index 0000000..e51ba64
--- /dev/null
+++ b/lib/bitscan.fs
@@ -0,0 +1,85 @@
+\ planckforth -
+\ Copyright (C) 2021 nineties
+
+( === Bit-Scan operations === )
+
+cell 16 > [if]
+    ." Bit-Scan for integers longer than 128bits is not supported"
+    not-supported
+[then]
+
+private{
+
+create bsf-modulo-131-table
+    -1 , 0 , 1 , 72 , 2 , 46 , 73 , 96 ,
+    3 , 14 , 47 , 56 , 74 , 18 , 97 , 118 ,
+    4 , 43 , 15 , 35 , 48 , 38 , 57 , 23 ,
+    75 , 92 , 19 , 86 , 98 , 51 , 119 , 29 ,
+    5 , -1 , 44 , 12 , 16 , 41 , 36 , 90 ,
+    49 , 126 , 39 , 124 , 58 , 60 , 24 , 105 ,
+    76 , 62 , 93 , 115 , 20 , 26 , 87 , 102 ,
+    99 , 107 , 52 , 82 , 120 , 78 , 30 , 110 ,
+    6 , 64 , -1 , 71 , 45 , 95 , 13 , 55 ,
+    17 , 117 , 42 , 34 , 37 , 22 , 91 , 85 ,
+    50 , 28 , 127 , 11 , 40 , 89 , 125 , 123 ,
+    59 , 104 , 61 , 114 , 25 , 101 , 106 , 81 ,
+    77 , 109 , 63 , 70 , 94 , 54 , 116 , 33 ,
+    21 , 84 , 27 , 10 , 88 , 122 , 103 , 113 ,
+    100 , 80 , 108 , 69 , 53 , 32 , 83 , 9 ,
+    121 , 112 , 79 , 68 , 31 , 8 , 111 , 67 ,
+    7 , 66 , 65 ,
+
+\ Find the index of the least significant 1 bit
+\ If u == 0, returns -1
+: bitscan-forward ( u -- u )
+    dup negate and \ LS1B isolation
+    131 mod \ perfect hashing
+    cells bsf-modulo-131-table + @
+; export
+
+
+create msb-table
+    -1 , 0 , 1 , 1 , 2 , 2 , 2 , 2 , 3 , 3 , 3 , 3 , 3 , 3 , 3 , 3 ,
+    4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 , 4 ,
+    5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 ,
+    5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 , 5 ,
+    6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 ,
+    6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 ,
+    6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 ,
+    6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 , 6 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+    7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 , 7 ,
+
+\ Find the index of the most significant 1 bit
+\ If u == 0, returns -1
+: bitscan-reverse ( u -- u )
+    dup 0xff u< if cells msb-table + @ exit then
+    8 rshift recurse 8 +
+; export
+
+}private
+
+T{ 0 bitscan-forward -> -1 }T
+T{ 1 bitscan-forward -> 0 }T
+T{ 2 bitscan-forward -> 1 }T
+T{ 3 bitscan-forward -> 0 }T
+T{ 4 bitscan-forward -> 2 }T
+T{ 5 bitscan-forward -> 0 }T
+T{ -1 bitscan-forward -> 0 }T
+T{ max-int bitscan-forward -> 0 }T
+T{ min-int bitscan-forward -> cell 8 * 1- }T
+
+T{ 0 bitscan-reverse -> -1 }T
+T{ 1 bitscan-reverse -> 0 }T
+T{ 2 bitscan-reverse -> 1 }T
+T{ 3 bitscan-reverse -> 1 }T
+T{ 4 bitscan-reverse -> 2 }T
+T{ -1 bitscan-reverse -> cell 8 * 1- }T
+T{ max-int bitscan-reverse -> cell 8 * 2 - }T
+T{ min-int bitscan-reverse -> cell 8 * 1- }T
diff --git a/runtests.fs b/runtests.fs
index 932dc87..4f78a84 100644
--- a/runtests.fs
+++ b/runtests.fs
@@ -15,6 +15,7 @@ include test/coreexttest.fs
 
 include test/export.fs
 include lib/array.fs
+include lib/bitscan.fs
 
 report-errors
 report-and-exit