[SLOF] [PATCH] fat-files: Fix bug with root-entries = 0 on certain FAT32 file systems

[Thomas Huth]

The maximum number of root directory entries can be zero on
certain FAT32 file systems. In this case, we've got to read
the cluster number of the root directory from the extended BPB
instead. See also the following URL for details:
https://en.wikipedia.org/wiki/Design_of_the_FAT_file_system#BPB20_OFS_06h

Signed-off-by: Thomas Huth <thuth at redhat.com>
---
 slof/fs/packages/fat-files.fs | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/slof/fs/packages/fat-files.fs b/slof/fs/packages/fat-files.fs
index 0cec366..d919452 100644
--- a/slof/fs/packages/fat-files.fs
+++ b/slof/fs/packages/fat-files.fs
@@ -18,6 +18,7 @@ INSTANCE VARIABLE sectors/cluster
 INSTANCE VARIABLE #reserved-sectors
 INSTANCE VARIABLE #fats
 INSTANCE VARIABLE #root-entries
+INSTANCE VARIABLE fat32-root-cluster
 INSTANCE VARIABLE total-#sectors
 INSTANCE VARIABLE media-descriptor
 INSTANCE VARIABLE sectors/fat
@@ -59,9 +60,18 @@ INSTANCE VARIABLE next-cluster
 : read-cluster ( cluster# -- )
   dup bytes/cluster @ * cluster-offset @ + bytes/cluster @ read-data
   read-fat dup #clusters @ >= IF drop 0 THEN next-cluster ! ;
+
 : read-dir ( cluster# -- )
-  ?dup 0= IF root-offset @ #root-entries @ 20 * read-data 0 next-cluster !
-  ELSE read-cluster THEN ;
+    ?dup 0= IF
+        #root-entries @ 0= IF
+            fat32-root-cluster @ read-cluster
+        ELSE
+            root-offset @ #root-entries @ 20 * read-data 0 next-cluster !
+        THEN
+    ELSE
+        read-cluster
+    THEN
+;
 
 : .time ( x -- )
   base @ >r decimal
@@ -137,6 +147,7 @@ CREATE dos-name b allot
 
   \ For FAT32:
   sectors/fat @ 0= IF data @ 24 + 4c@ bljoin sectors/fat ! THEN
+  #root-entries @ 0= IF data @ 2c + 4c@ bljoin ELSE 0 THEN fat32-root-cluster !
 
   \ XXX add other FAT32 stuff (offsets 28, 2c, 30)
 
-- 
1.8.3.1