Fixes and improvements to FAT32 driver

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

diff --git a/boot/x86/fat32.s b/boot/x86/fat32.s
index 31288cb92cad9433592f3a2a3e090895d26b187d..caed0361a311af54737594d17b47ae644f517707 100644 (file)
--- a/boot/x86/fat32.s
+++ b/boot/x86/fat32.s
@@ -1,12 +1,20 @@
 ; FAT32 driver
-; BOOT_DRIVE, part_table_entry and bp must be set up by the code using this driver.
-; first_data_sec is to be set up by calling get_1st_data_sec.
+; BOOT_DRIVE, si and bp must be set up by the code using this driver.
+; (Specifically, si must be a pointer to the partition's partition
+; table entry and bp must be a pointer to the BPB.)
+; Before reading any cluster chains, callers must call get_1st_data_sec
+; and must be careful not to trash ecx after calling it.
 
 ; eax - start cluster number
 ; es:di - where to load the cluster chain
 ; NOTE: Cluster chain might be the root directory or a file.
 read_cluster_chain:
-       pusha
+       push eax
+       push ebx
+       push ecx
+       push edx
+       push es
+       push di
 .loop:
        ; get the first sector of the cluster to read
        push eax
@@ -14,13 +22,15 @@ read_cluster_chain:
        xor ebx, ebx
        mov bl, BPB_SecPerClus
        mul ebx
-       add eax, [first_data_sec]
+       add eax, ecx
        mov ebx, eax
        pop eax
 
        ; read the cluster
+       push cx
        movzx cx, BPB_SecPerClus
        call read_sectors
+       pop cx
 
        ; increment the load offset (and segment if needed)
        push eax
@@ -36,22 +46,21 @@ read_cluster_chain:
 .get_next_cluster:
        pop eax
        ; get FAT sector number and offset containing the next cluster number
-       xor edx, edx
-       mov ebx, 4
-       mul dword ebx
+       xor ebx, ebx
+       mov bl, 4 ; put 4 in EBX, doing this instead of "mov ebx, 4" saves us an entire byte
+       mul ebx
        movzx ebx, word BPB_BytsPerSec
-       div dword ebx
+       div ebx
        movzx ebx, word BPB_RsvdSecCnt
        add eax, ebx
-       push bp
-       mov bp, [part_table_entry]
-       add eax, [bp+part_entry.lba_start]
-       pop bp
+       add eax, [si]
        mov ebx, edx
        ; reminder for myself: EAX is FAT sector number, (E)BX is offset into FAT sector
        
        ; load the FAT sector we're looking for
 
+       push cx
+       push di
        push ebx ; offset
        push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
        push eax ; desired LBA
@@ -65,15 +74,20 @@ read_cluster_chain:
        ; find the cluster we're looking for
        pop es ; restore STAGE3_SEGMENT
        pop ebx ; pop FAT offset back into EBX for cmp
-       cmp dword [di+bx], 0xffffff7
-       jge .done ; if cluster number is greater than or equal to the defective cluster value, we're done
-                 ; TODO: should this perhaps be changed so that equal makes it error out?
-       ; otherwise, load the next sector number in eax and read again
        mov eax, [di+bx]
-       jmp .loop
+       pop di
+       pop cx
+       cmp eax, 0xffffff7
+       jl .loop ; if cluster number is lower than the defective cluster value, we're not done
+                ; TODO: perhaps check is it equal defective and error out in that case?
 .done:
        ; cleanup and return
-       popa
+       pop di
+       pop es
+       pop edx
+       pop ecx
+       pop ebx
+       pop eax
        ret
 
 ; es:di - where to load the sector(s)
@@ -101,18 +115,17 @@ read_sectors:
                hlt
                jmp $-1
 
-; no arguments
+; ds:si - pointer to partition table entry
+; RETURN: ecx - first data sector
 get_1st_data_sec:
        push eax
        push ebx
-       xor eax, eax
        movzx ax, BPB_NumFats 
        mul dword BPB_FatSz32 ; space occupied by all FATs
        movzx ebx, word BPB_RsvdSecCnt
        add eax, ebx ; space occupied by the reserved area
-       mov bx, [part_table_entry]
-       add eax, [bx+part_entry.lba_start] ; space before the partition
-       mov [first_data_sec], eax
+       add eax, [si] ; space before the partition
+       mov ecx, eax
        pop ebx
        pop eax
        ret
@@ -120,5 +133,3 @@ get_1st_data_sec:
 read_error: db "Read error", 0
 
 BOOT_DRIVE: db 0
-part_table_entry: dw 0
-first_data_sec: dd 0