+; 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.
+
+; 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
+.loop:
+ ; get the first sector of the cluster to read
+ push eax
+ sub eax, 2
+ xor ebx, ebx
+ mov bl, BPB_SecPerClus
+ mul ebx
+ add eax, [first_data_sec]
+ mov ebx, eax
+ pop eax
+
+ ; read the cluster
+ movzx cx, BPB_SecPerClus
+ call read_sectors
+
+ ; increment the load offset (and segment if needed)
+ push eax
+ xor eax, eax
+ movzx ax, BPB_SecPerClus
+ mul word BPB_BytsPerSec
+ add di, ax
+ jno .get_next_cluster
+ mov ax, es
+ add ax, 0x1000
+ mov es, ax
+
+.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
+ movzx ebx, word BPB_BytsPerSec
+ div dword 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
+ 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 ebx ; offset
+ push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
+ push eax ; desired LBA
+ xor ax, ax
+ mov es, ax
+ pop ebx ; pop LBA into EBX
+ mov di, 0x1000
+ mov cx, 1
+ call read_sectors
+
+ ; 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
+.done:
+ ; cleanup and return
+ popa
+ ret
+
+; es:di - where to load the sector(s)
+; ebx - start LBA address
+; cx - how many sectors to read
+read_sectors:
+ pusha
+ mov ah, 0x42
+ push dword 0
+ push dword ebx
+ push es
+ push di
+ push cx
+ push word 0x10
+ mov si, sp
+ mov dl, [BOOT_DRIVE]
+ int 0x13
+ jc .error
+ add sp, 16
+ popa
+ ret
+ .error:
+ mov si, read_error
+ call print
+ hlt
+ jmp $-1
+
+; no arguments
+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
+ pop ebx
+ pop eax
+ ret
+
+read_error: db "Read error", 0
+
+BOOT_DRIVE: db 0
+part_table_entry: dw 0
+first_data_sec: dd 0
projects / nameless-os.git / commitdiff