OSDev.org

Code: Select all

;;; seek the directory for the stage 2 file
        mov bx, dir_buffer
        mov cx, Root_Entries
    .dir_entry_test:
        mov di, bx
        mov si, snd_stage_file
        push cx
        mov cx, filename_length
        repe cmpsb              ; is the directory entry == the stg2 file?
        pop cx
        je .entry_found
        add bx, dir_entry_size
        loop .dir_entry_test
        jmp .no_file

Code: Select all

unsigned char FAT_table[sector_size];
unsigned int fat_offset = active_cluster + (active_cluster / 2);// multiply by 1.5
unsigned int fat_sector = first_fat_sector + (fat_offset / section_size);
unsigned int ent_offset = fat_offset % section_size;

Code: Select all

;;; extract_next_fat12_entry - read a FAT entry to
;;;               see where the next FAT entry is,
;;;               if any
;;; Inputs:
;;;       DI - FAT entry buffer 
;;;       BX - current FAT entry's value
;;; Outputs:
;;;       AX - next FAT entry's value
extract_next_fat12_entry:
        mov ax, bx
        shr ax, 1               ; integer ax / 2
        add ax, bx              ; FAT offset
        xor dx, dx
        mov cx, 0x200           ; == Bytes Per Sector
        div cx
        add di, dx              ; index into FAT buffer
        mov ax, [di]
        and bx, 1               ; check if the existing entry is odd
        jz .even
        shr ax, 4               ; extract the high bits
        ret    
    .even:
        and ax, 0x0FFF          ; extract the low bits
        ret

Code: Select all

address_of_FAT_entry = address_of_FAT + (current_cluster + currect_cluster / 2)

Code: Select all

extract_next_fat12_entry:
;; address_of_FAT_entry = fat_buffer + (current_cluster + current_cluster / 2)
        mov ax, bx              ; BX == current cluster
        shr ax, 1               ;  current_cluster / 2
        add ax, bx              ; (current_cluster + current_cluster / 2) == FAT entry offset
        add di, ax              ; index fat buffer by offset
        mov ax, [di]            ; get the indexed entry
        test bx, 1              ; check if the existing entry is odd
        jz .even
    .odd:
        shr ax, lo_nibble_shift ; extract the high bits
        ret
    .even:
        and ax, hi_nibble_mask  ; extract the low bits
        ret

Code: Select all

        mov bx, 0
    .test_loop:
        mov di, fat_mockup
        call extract_next_fat12_entry
        call print_hex_word
        write nl
        inc bx
        cmp bx, 5
        jl .test_loop

Code: Select all

fat_mockup          db 0xF0, 0xFF, 0xFF, 0x00, 0xF0, 0xFF, 0x00, 0x00, 0x00

Code: Select all

0FF0    - the FAT ID
0FFF    - end of chain, in this case the reserved sectors
0000    - unused entry; this should be an end of chain for the STAGE2.SYS file, instead
0FFF    - end of chain
0FFF    - end of chain, should be an unused entry

Code: Select all

0FF0   - the  FAT ID, always this value
0FFF   - the reserved sector, always this value
0F00   - end of chain, for the single sector file
0FF0   - end of used FAT entries
0000   - unused entry

Code: Select all

0xFF0 - media type (cluster 0 doesn't exist)
0xFFF - end of chain (cluster 1 doesn't exist)
0x000 - free cluster
0xFFF - end of chain (probably stage2.sys)
0x000 - free cluster
0x000 - free cluster

Code: Select all

sudo mount -t vfat boot.img temp

mount: /home/schol-r-lea/Documents/Programming/Projects/Verbum/src/PC-x86/nasm/fat12/tests/fat_to_file_test/temp: wrong fs type, bad option, bad superblock on /dev/loop0, missing codepage or helper program, or other error.

Code: Select all

[661360.179954] loop0: detected capacity change from 2880 to 0
[661360.194688] FAT-fs (loop0): invalid media value (0x8e)
[661360.194692] FAT-fs (loop0): Can't find a valid FAT filesystem

Code: Select all

ASM = nasm -w+all -f bin
COPY = dd
FORMAT = mkfs.msdos -F 12 -n "VERBUM"
REIMAGE=qemu-img
SYS_INSTALL = ~/Deployments/ms-sys-2.5.3/bin/ms-sys --fat12
BOOT = fat_to_file_test
STAGE_TWO = stagetwo
DISKTARGET = boot.img
DISKSIZE = 1440

install: boot stage2
	$(COPY) if=/dev/zero of=$(DISKTARGET) count=$(DISKSIZE) bs=1k
	$(FORMAT) $(DISKTARGET)
	$(COPY) if=$(BOOT).bin of=$(DISKTARGET) count=1 conv=notrunc
	mkdir temp
	sudo mount -t vfat $(DISKTARGET) temp
	sudo cp $(STAGE_TWO).bin temp/STAGE2.BIN
	sudo umount temp
	rmdir temp
	$(REIMAGE) convert -f raw -O qcow2 boot.img boot.qcow2

boot:
	$(ASM) $(BOOT).asm -o $(BOOT).bin -l $(BOOT).lst

stage2:
	$(ASM) $(STAGE_TWO).asm -o $(STAGE_TWO).bin -l $(STAGE_TWO).lst

Code: Select all

$(COPY) if=$(BOOT).bin of=$(DISKTARGET) count=1 conv=notrunc

Code: Select all

$(COPY) if=$(BOOT).bin of=$(DISKTARGET) conv=notrunc bs=1 count=3
$(COPY) if=$(BOOT).bin of=$(DISKTARGET) conv=notrunc bs=1 count=450 skip=62 seek=62

Code: Select all

;;; seek_directory_entry - seek the directory for the stage 2 file
;;; Inputs:
;;;       SI - address of the filename to match against
;;;       DI - directory buffer
;;;       CX - max. number of entries
;;;       BX - size of an entry
;;; Outputs:
;;;       DI - location of the entry
seek_directory_entry:
        mov dx, bx
    .dir_entry_test:
        push di
        push si
        push cx
        mov cx, filename_length
        repe cmpsb              ; does the directory entry match?
        je .entry_found
        pop cx
        pop si
        pop di
        add di, dx
        add dx, bx 
        loop .dir_entry_test
        mov di, 0               ; if not found, return 0
    .entry_found:
        ret