diff --git a/lfs.c b/lfs.c
index cfb0a7dd..db6e0624 100644
--- a/lfs.c
+++ b/lfs.c
@@ -466,7 +466,7 @@ enum lfsr_tag_type {
     LFSR_TAG_CRC     = 0x0002,
     LFSR_TAG_CRC0    = 0x0002,
     LFSR_TAG_CRC1    = 0x0003,
-    LFSR_TAG_FCRC    = 0x0802,
+    LFSR_TAG_FCRC    = 0x000a,
 
     LFSR_TAG_ALT     = 0x0004,
     LFSR_TAG_ALTBLT  = 0x0004,
@@ -586,10 +586,18 @@ static inline uint16_t lfsr_tag_suptype(lfsr_tag_t tag) {
     return tag & 0x7807;
 }
 
-static inline uint16_t lfsr_tag_subtype(lfsr_tag_t tag) {
+static inline uint8_t lfsr_tag_subtype(lfsr_tag_t tag) {
     return (tag >> 3) & 0xff;
 }
 
+static inline uint16_t lfsr_tag_xsuptype(lfsr_tag_t tag) {
+    return tag & 0x007e;
+}
+
+static inline uint8_t lfsr_tag_xsubtype(lfsr_tag_t tag) {
+    return (tag >> 7) & 0xff;
+}
+
 static inline uint16_t lfsr_tag_type(lfsr_tag_t tag) {
     return tag & 0x7fff;
 }
@@ -1179,7 +1187,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
         }
 
         // not an end-of-commit crc
-        if ((lfsr_tag_suptype(tag) & ~0x1) != LFSR_TAG_CRC) {
+        if (lfsr_tag_xsuptype(tag) != LFSR_TAG_CRC) {
             // fcrc is only valid if the last tag was a crc
             hasfcrc = false;
 
@@ -1209,7 +1217,7 @@ static int lfsr_rbyd_fetch(lfs_t *lfs, lfsr_rbyd_t *rbyd,
                 // on a bad crc
                 count -= 1;
             // found an fcrc? save for later
-            } else if (lfsr_tag_suptype(tag) == LFSR_TAG_FCRC) {
+            } else if (lfsr_tag_xsuptype(tag) == LFSR_TAG_FCRC) {
                 err = lfs_bd_read(lfs,
                         NULL, &lfs->rcache, lfs->cfg->block_size,
                         block, off, &fcrc,
@@ -1963,14 +1971,14 @@ int lfsr_rbyd_commit(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     //
     // this gets a bit complicated as we have two types of crcs:
     // - 7-word crc with fcrc to check following prog (middle of block)
-    //     2-byte fcrc tag
+    //     1-byte fcrc tag
     //   + 1-byte fcrc len (worst case)
     //   + 4-byte fcrc crc
     //   + 4-byte fcrc crc-len (worst case)
     //   + 1-byte crc tag
     //   + 4-byte crc+padding len (worst case)
     //   + 4-byte crc crc
-    //   = 19 bytes
+    //   = 18 bytes
     // - 3-word crc with no following prog (end of block)
     //     1-byte crc tag
     //   + 4-byte crc+padding len (worst case)
@@ -1978,7 +1986,7 @@ int lfsr_rbyd_commit(lfs_t *lfs, lfsr_rbyd_t *rbyd,
     //   = 9 bytes
     // 
     const lfs_off_t aligned = lfs_alignup(
-            rbyd_.off + 2+1+4+4 + 1+4+4,
+            rbyd_.off + 1+1+4+4 + 1+4+4,
             lfs->cfg->prog_size);
 
     // not even space for the crc?
diff --git a/scripts/dbgrbyd.py b/scripts/dbgrbyd.py
index 193ce286..59720c13 100755
--- a/scripts/dbgrbyd.py
+++ b/scripts/dbgrbyd.py
@@ -54,6 +54,8 @@ def tagrepr(tag, size, off=None):
     type = tag & 0x7fff
     suptype = tag & 0x7807
     subtype = (tag >> 3) & 0xff
+    xsuptype = tag & 0x7e
+    xsubtype = (tag >> 7) & 0xff
     id = ((tag >> 15) & 0xffff) - 1
 
     if suptype == 0x0800:
@@ -73,14 +75,14 @@ def tagrepr(tag, size, off=None):
             subtype,
             ' id%d' % id if id != -1 else '',
             ' %d' % size if not tag & 0x1 else '')
-    elif (suptype & ~0x1) == 0x0002:
+    elif xsuptype == 0x0002:
         return 'crc%x%s %d' % (
-            suptype & 0x1,
-            ' 0x%02x' % subtype if subtype else '',
+            tag & 0x1,
+            ' 0x%02x' % xsubtype if xsubtype else '',
             size)
-    elif suptype == 0x0802:
+    elif xsuptype == 0x000a:
         return 'fcrc%s %d' % (
-            ' 0x%02x' % subtype if subtype else '',
+            ' 0x%02x' % xsubtype if xsubtype else '',
             size)
     elif suptype & 0x4:
         return 'alt%s%s 0x%x %s' % (
@@ -250,7 +252,7 @@ def show_log(block_size, data, rev, off, *,
         j_ += delta
 
         if not tag & 0x4:
-            if (tag & 0x7806) != 0x0002:
+            if (tag & 0x007e) != 0x0002:
                 crc = crc32c(data[j_:j_+size], crc)
             # found a crc?
             else:
@@ -599,7 +601,7 @@ def main(disk, block_size, block1, block2=None, *,
                 wastrunk = True
 
             if not tag & 0x4:
-                if (tag & 0x7806) != 0x0002:
+                if (tag & 0x007e) != 0x0002:
                     crc = crc32c(data[j_:j_+size], crc)
                     # keep track of id count
                     if (tag & 0x7807) == 0x0800: