From f2b939c5ba779ade12e7e8711ab44092079bf936 Mon Sep 17 00:00:00 2001
From: "E. C. Masloch" <pushbx@ulukai.org>
Date: Wed, 22 Apr 2026 22:40:43 +0200
Subject: [PATCH 1/2] fatfs.c: fix earlier fix, update dir_size in map_cluster
 loop

Earlier fix was https://github.com/FDOS/kernel/pull/242 (this got the
cluster value to use wrong).

Original problem recorded in https://github.com/FDOS/kernel/issues/241

Reset disk scriptlet:

nasm -I ~/proj/lmacros/ ~/proj/bootimg/bootimg.asm \
  -D_PAYLOADFILE=::empty -o diskempt.img
cp -a ~/proj/kernel/bin/kernel.sys fdkernel.sys

Run dosemu2 with -I "floppy { device diskempt.img }"

After a test, run /sbin/dosfsck -n diskempt.img

The following Script for lDebug files are test cases:

=== testshrt.sld
e 200 "A:\test.dat" 0
f 300 l 100 26
a
 mov ah, 3C
 mov cx, 0
 mov dx, 200
 int 21
 xchg bx, ax
 mov ax, 4200
 mov cx, 1000
 mov dx, cx
 int 21
 mov dx, 300
 mov cx, 10
 mov ah, 3F
 int 21
 .
r v0 := aao
a
 mov ah, 40
 int 21
 int3
 int3
 jmp (v0)
 .

=== testfull.sld
e 200 "A:\test.dat" 0
f 400 l 400 38
a
 mov ah, 3C
 mov cx, 0
 mov dx, 200
 int 21
 xchg bx, ax
 mov ax, 4200
 mov cx, ((#1424 * #1024 - 1) >> 10)
 mov dx, ((#1424 * #1024 - 1) & FFFF)
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 1
 int 21
 mov ah, 68
 int 21
 mov ax, 4202
 mov cx, -1
 mov dx, -10
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 10
 int 21
 int3
 mov ah, 68
 int 21
 nop
 int3
 nop
 jmp 100
 .

=== testful2.sld
e 200 "A:\test.dat" 0
f 400 l 400 38
a
 mov ah, 3C
 mov cx, 0
 mov dx, 200
 int 21
 xchg bx, ax
 mov ax, 4200
 mov cx, ((#1424 * #1024 - #512 - 1) >> 10)
 mov dx, ((#1424 * #1024 - #512 - 1) & FFFF)
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 1
 int 21
 mov ah, 68
 int 21
 mov ax, 4202
 mov cx, -1
 mov dx, -10
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 10
 int 21
 int3
 mov ah, 68
 int 21
 nop
 int3
 nop
 jmp 100
 .

=== testful3.sld
e 200 "A:\test.dat" 0
f 400 l 400 38
a
 mov ah, 3C
 mov cx, 0
 mov dx, 200
 int 21
 xchg bx, ax
 mov ax, 4200
 mov cx, ((#1424 * #1024 - #512) >> 10)
 mov dx, ((#1424 * #1024 - #512) & FFFF)
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 0
 int 21
 mov ah, 68
 int 21
 mov ax, 4202
 mov cx, -1
 mov dx, -10
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 10
 int 21
 int3
 mov ah, 68
 int 21
 nop
 int3
 nop
 jmp 100
 .

===
---
 kernel/fatfs.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/fatfs.c b/kernel/fatfs.c
index ec89418f..b10b5694 100644
--- a/kernel/fatfs.c
+++ b/kernel/fatfs.c
@@ -1029,7 +1029,7 @@ COUNT map_cluster(REG f_node_ptr fnp, COUNT mode)
     fnp->f_cluster = cluster;
     fnp->f_cluster_offset++;
     fnp->f_dir.dir_size =
-      ((ULONG)fnp->f_cluster
+      (((ULONG)fnp->f_cluster_offset + 1)
       * (ULONG)fnp->f_dpb->dpb_secsize)
       <<
       fnp->f_dpb->dpb_shftcnt;

From c63f9f4caf2eb97b47fb8b9865ea26d58b711530 Mon Sep 17 00:00:00 2001
From: "E. C. Masloch" <pushbx@ulukai.org>
Date: Wed, 22 Apr 2026 23:14:14 +0200
Subject: [PATCH 2/2] fatfs.c: comment unexpected alloc of cluster past needed
 if CX=0

Reset disk scriptlet:

nasm -I ~/proj/lmacros/ ~/proj/bootimg/bootimg.asm \
  -D_PAYLOADFILE=::empty -o diskempt.img
cp -a ~/proj/kernel/bin/kernel.sys fdkernel.sys

Run dosemu2 with -I "floppy { device diskempt.img }"

After a test, run /sbin/dosfsck -n diskempt.img

The following Script for lDebug is a test case:

=== testsect.sld
e 200 "A:\test.dat" 0
f 400 l 400 38
a
 mov ah, 3C
 mov cx, 0
 mov dx, 200
 int 21
 xchg bx, ax
 mov ax, 4200
 mov cx, (#1024 >> 10)
 mov dx, (#1024 & FFFF)
 int 21
 mov ah, 40
 mov dx, 400
 mov cx, 0
 int 21
 int3
 mov ah, 68
 int 21
 nop
 int3
 nop
 jmp 100
 .

===
---
 kernel/fatfs.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/fatfs.c b/kernel/fatfs.c
index b10b5694..249ced95 100644
--- a/kernel/fatfs.c
+++ b/kernel/fatfs.c
@@ -1218,9 +1218,21 @@ long rwblock(COUNT fd, VOID FAR * buffer, UCOUNT count, int mode)
     
     if (dos_extend(fnp) != SUCCESS)
     {
+      /* ecm: control flow may end up here if CX = 0000h and
+                the extending failed to allocate a cluster
+                behind the last needed. in this case, our
+                return here of 0 happens to be correct and
+                indicates the extension succeeded. */
       fnode_to_sft(fnp);
       return 0;
     }
+    /* ecm: if CX = 0000h and seek was on a cluster boundary > size,
+                the dos_extend call will have allocated one cluster
+                too many. this is truncated later without problems.
+                but does this aid fragmentation maybe ?
+            if CX > 0000h then we want to write to the cluster
+                anyway, so extending to the cluster that starts at
+                the boundary is desired. */
   }
   
   /* Test that we are really about to do a data transfer. If the  */