Fixes and improvements to FAT32 driver

[nameless-os.git] / boot / x86 / vbr-fat32.s

diff --git a/boot/x86/vbr-fat32.s b/boot/x86/vbr-fat32.s
index 061ca527afc3d585558c43b495c72a68536c6476..b4c71489e5bf9d421ded07e145550c13e37cb716 100644 (file)
--- a/boot/x86/vbr-fat32.s
+++ b/boot/x86/vbr-fat32.s
@@ -24,7 +24,6 @@ real_start:
 
        ; we expect the boot drive to be in DL and our partition table entry in DS:SI
        mov [BOOT_DRIVE], dl
-       mov [part_table_entry], si
 
        ; calculate the 1st sector of the data area
        call get_1st_data_sec
@@ -37,7 +36,9 @@ real_start:
        mov eax, BPB_RootClus
        call read_cluster_chain
 
+       push cx
        mov cx, 11
+       push si
 .find_stage_3:
        mov si, STAGE3_NAME
        cmp byte [es:di], 0 ; we have no more entries to look at
@@ -53,11 +54,14 @@ real_start:
        add bx, 0x1000
        mov es, bx
 .stage3_found:
-       sub di, 0xb ; "repe cmpsb" incremented this
+       pop si
+       pop cx
+       add di, 9 ; knowing that cmpsb incremented this by 11, we can also increment it by 9
+                      ; right here so we can use 1 less offset below
        ; stage 3 has been found and ES:DI points to its directory entry
-       mov ax, [es:di+dir_entry.firstclushi] ; load high half of the 1st cluster
+       mov ax, [es:di] ; load high half of the 1st cluster
        shl eax, 16
-       mov ax, [es:di+dir_entry.firstcluslo] ; load low half of the 1st cluster
+       mov ax, [es:di+(dir_entry.firstcluslo-dir_entry.firstclushi)] ; load low half of the 1st cluster
        mov bx, STAGE3_SEGMENT
        mov es, bx
        mov di, STAGE3_OFFSET
@@ -70,7 +74,6 @@ real_start:
        mov si, stage3_missing
        call print
 .halt:
-       cli
        hlt
        jmp short $-1