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stevia/include/fat32/FAT32_SYS.inc

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; Copyright (C) 2025 Elaina Claus
;
; This program is free software: you can redistribute it and/or modify
; it under the terms of the GNU General Public License as published by
; the Free Software Foundation, either version 3 of the License, or
; (at your option) any later version.
;
; This program is distributed in the hope that it will be useful,
; but WITHOUT ANY WARRANTY; without even the implied warranty of
; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
; GNU General Public License for more details.
;
; You should have received a copy of the GNU General Public License
; along with this program. If not, see <https://www.gnu.org/licenses/>.
%ifndef __INC_FAT32_SYS
%include "partition_table.inc"
%include "fat32/bpb_offset_bx.inc"
%include "fat32/fat32_structures.inc"
; Clobbers: eax, edx
; returns: none
ALIGN 4, db 0x90
InitFATDriver:
__CDECL16_ENTRY
.func:
mov ax, FAT32_State_t_size
push ax ; length of fat32_state structure
xor ax, ax
push ax ; init fat32_state with zero
mov ax, fat32_state
push ax ; address of structure
call kmemset
sub sp, 0x6
.calc_active_part:
mov ax, word [partition_offset]
mov di, fat32_state
mov dx, partition_table
add dx, ax ; dx points to the partition that was booted from
mov bx, dx ; set bx, should point at our partition
mov eax, dword [bx + PartEntry_t.lba_start]
mov dword [di + FAT32_State_t.curr_drive_lba_32], eax
mov si, fat32_bpb
mov bx, fat32_ebpb
.calc_first_fat:
movzx eax, word [si + FAT32_bpb_t.reserved_sectors_word] ; first fat from start of partition
add eax, dword [di + FAT32_State_t.curr_drive_lba_32] ; calculate offset from start of drive
jc InitFATDriver.error
mov dword [di + FAT32_State_t.first_fat_sector_32], eax
.calc_total_fat:
mov edx, dword [bx + FAT32_ebpb_t.FATSz_dword]
movzx eax, byte [si + FAT32_bpb_t.fat_count_byte]
mul edx ; result in EDX:EAX, CF set on > 32bit return value
jc InitFATDriver.error ; as a catch for unhandled overflow, just error if value is greater than 32bits
mov dword [di + FAT32_State_t.fat_size_32], eax
.calc_first_data:
mov edx, dword [di + FAT32_State_t.first_fat_sector_32]
add eax, edx
jc InitFATDriver.error
mov dword [di + FAT32_State_t.first_data_sector_32], eax
.set_first_dir:
mov eax, dword [bx + FAT32_ebpb_t.root_clus_dword]
mov dword [di + FAT32_State_t.curr_dir_cluster_32], eax
.endp:
__CDECL16_EXIT
ret
.error:
ERROR STAGE2_FAT32_INIT_CF
ALIGN 4, db 0x90
FSInfoPrinter:
__CDECL16_ENTRY
.func:
;info we want to print to validate we are loading stuff from the disk correctly
; boot_drive # (i.e 0x80)
; active/bootable partition info (lba_start, lba_length, part_type)
; BPB Info: BS_OEMName = ident_8, BPB_BytsPerSec = bytes_per_sector_16, BPB_SecPerClus = sectors_per_cluster_8
; eBPB info: FATSz_dword, volid, vol label
;
; print entire FAT32 state
;
.endp:
__CDECL16_EXIT
ret
.error:
ERROR STAGE2_ERROR_INFOPRINTER
; this involves using the low memory buffer for the bios call and moving the file sector by sector to high memory
;
; SFN is a 8.3 file name, all uppercase, and padded with spaces
; do not add a null byte to the end of the string
; eg. kernel.bin == "KERNEL BIN"
;
; returns first cluster of file if found
; halts/errors if file is not found
; uint32_t SearchFATDIR(uint8_t* SFN);
ALIGN 4, db 0x90
SearchFATDIR:
__CDECL16_ENTRY
.file_lookup:
print_string SearchFATDIR_info
mov bx, fat32_state
.load_first_dir:
mov eax, dword [bx + FAT32_State_t.curr_dir_cluster_32]
push dword eax ; cluster
mov ax, dir_buffer
push ax ; offset
xor ax, ax
push ax ; segment
call ReadFATCluster ; uint8_t ReadFATCluster(uint16_t seg, uint16_t offset, uint32_t cluster)
add sp, 0x8
mov si, dir_buffer
jmp SearchFATDIR.empty_dir_entry
.load_next_dir:
; if eax >= 0x0FFFFFF8 then there are no more clusters (end of chain)
; if eax == 0x0FFFFFF7 then this is a cluster that is marked as bad
mov eax, dword [bx + FAT32_State_t.curr_dir_cluster_32]
push dword eax
call NextCluster ; uint32_t NextCluster(uint32_t active_cluster);
add sp, 0x4
cmp eax, 0x0fff_fff7
;je SearchFATDIR.bad_cluster ; TODO: Implement Bad cluster checks
jb SearchFATDIR.load_next_dir_next_OK
ERROR STAGE2_FAT32_END_OF_CHAIN
.load_next_dir_next_OK:
; load 512 bytes of directory entries from data sector
mov eax, [bx + FAT32_State_t.curr_dir_cluster_32]
push dword eax ; cluster
mov ax, dir_buffer
push ax ; offset
xor ax, ax
push ax ; segment
call ReadFATCluster ; uint8_t ReadFATCluster(uint16_t seg, uint16_t offset, uint32_t cluster)
sub sp, 0x8
.empty_dir_entry:
; check for 0x0 in first byte, if true then there are no more files
; if true we did not find the file, we should error here
cmp byte [si], 0
jne SearchFATDIR.unused_dir_entry
ERROR STAGE2_FAT32_NO_FILE
.unused_dir_entry:
; check for 0xe5 and 0x05 in first byte, if true then this entry is unused, but it is not the last entry.
cmp byte [si], 0xe5
je SearchFATDIR.next_entry
cmp byte [si], 0x05
je SearchFATDIR.next_entry
jmp SearchFATDIR.parse_dir
.next_entry:
; increment offset by 32 bytes to read the next entry in this set of dir entries
; if we are at the end of the buffer, then load the next buffer
add si, 0x20 ; 32 bytes
mov ax, dir_buffer
add ax, 0x1FF ; 512 - 1 bytes
cmp si, ax
jae SearchFATDIR.load_next_dir
jmp SearchFATDIR.empty_dir_entry
; TODO: move this to a seperate string search function
.parse_dir:
print_string MaybeFound_Boot_info
.lfn_check:
; check for ATTR_READ_ONLY | ATTR_HIDDEN | ATTR_SYSTEM | ATTR_VOLUME_ID (0x0F) in offset 11
; TODO: going to skip LFN for now, since all valid volumes will have SFN's
cmp byte [si+11], 0x0F
je SearchFATDIR.next_entry
.sfn_file_name_check:
push si
push di
mov cx, 0xA ; max of 11 filename length of 11 characters
; si points to the start of the current directory entry
mov di, BootTarget ; current memory location (8.3 name is at offset 0)
repe cmpsb ; compare the strings
pop di
pop si
jne SearchFATDIR.next_entry
.sfn_entry_found:
mov ax, [si + FAT32_SFN_t.cluster_16_high]
shl eax, 16
mov ax, [si + FAT32_SFN_t.cluster_16_low]
; eax == first cluster of file
.endp:
__CDECL16_EXIT
ret
; BUG: this function needs review
; uint32_t NextCluster(uint32_t active_cluster);
; if eax >= 0x0FFFFFF8 then there are no more clusters (end of chain)
; if eax == 0x0FFFFFF7 then this is a cluster that is marked as bad
ALIGN 4, db 0x90
NextCluster:
__CDECL16_ENTRY
.func:
print_string NextFATCluster_info
mov ax, FAT32_NextClusterData_t_size
push ax ; length
xor ax, ax
push ax ; init with zero
mov ax, fat32_nc_data
push ax ; address of structure
call kmemset
sub sp, 0x6
mov edx, dword [bp + 4]
mov si, fat32_nc_data ; instead of push/pop and moving the data back
mov di, fat32_bpb ; load si & di then use xchg
mov bx, fat32_state
.calc_offset:
; fat_offset = active_cluster * 4
mov eax, 4
mul edx
jc NextCluster.error_cfdivz
mov dword [si + FAT32_NextClusterData_t.fat_offset], eax ; move lower 32 bits to fat offset
.calc_fat_sector:
; fat_sector = first_fat_sector + (fat_offset / sector_size)
; entry_offset = fat_offset % sector_size
xor edx, edx
movzx ecx, word [di + FAT32_bpb_t.bytes_per_sector_word]
cmp eax, 0
je NextCluster.error_cfdivz
div ecx ; eDX:eAX / eCX = fat_sector - first_fat_sector in eAX
; eDX = remainder (fat_offset mod sector_size)
mov dword [si + FAT32_NextClusterData_t.entry_offset], edx
mov ecx, dword [bx + FAT32_State_t.first_fat_sector_32]
add eax, ecx ; fat_sector + first_fat_sector
mov dword [si + FAT32_NextClusterData_t.fat_sector], eax
.load_fat_table:
xor ax, ax
mov al, byte [boot_drive]
push ax
mov ax, 0x1
push ax
; load correct fat
mov eax, dword [si + FAT32_NextClusterData_t.fat_sector]
push dword eax
mov ax, fat_buffer
push ax
xor ax, ax
push ax
call read_disk_raw
add sp, 0xC
; uint8_t read_stage2_raw(uint16_t buf_segment, uint16_t buf_offset,
; uint32_t lba,
; uint16_t count, uint16_t drive_num)
.read_cluster:
; next_cluster = fat_buffer[entry_offset]
mov ebx, dword [si + FAT32_NextClusterData_t.entry_offset]
mov si, fat_buffer
mov eax, dword [bx+si+0]
.endp:
__CDECL16_EXIT
ret
.error_cfdivz:
ERROR STAGE2_FAT32_NCLUS_CFDIVZ
; uint8_t ReadFATCluster(uint16_t seg, uint16_t offset, uint32_t cluster)
ALIGN 4, db 0x90
ReadFATCluster:
__CDECL16_ENTRY
.func:
print_string ReadFATCluster_info
mov bx, fat32_bpb
mov si, fat32_ebpb
xor ax, ax
mov al, byte [boot_drive]
push ax
mov ax, 0x1 ; count = 1
push ax
mov eax, dword [bp + 8] ; cluster
sub eax, 2
movzx edx, byte [bx + FAT32_bpb_t.sectors_per_cluster_byte]
mul edx ; result in eax, error on carry
jc ReadFATCluster.error
add eax, dword [si + FAT32_State_t.first_data_sector_32]
jc ReadFATCluster.error
; eax contains the LBA now
push dword eax ; lba = ClusterToLBA(..)
mov ax, word [bp + 6] ; offset
push ax
mov ax, word [bp + 4]
push ax
; uint8_t read_disk_raw(uint16_t buf_segment, uint16_t buf_offset,
; uint32_t lba,
; uint16_t count, uint16_t drive_num)
call read_disk_raw
add sp, 0xC
.endp:
__CDECL16_EXIT
ret
.error:
ERROR STAGE2_FAT32_CLS2LBA_CF
%endif
%define __INC_FAT32_SYS
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