FAT filesystem

run.sh

@@ -0,0 +1 @@
+qemu-system-i386 -fda build/main_floppy.img

src/bootloader/boot.asm

@@ -1,7 +1,7 @@
 org 0x7C00
 bits 16
 
-%define ENDL 0x0A
+%define ENDL 0x0D, 0x0A
 
 ;
 ; FAT12 header
@@ -36,7 +36,189 @@ ebr_system_id:              db 'FAT12   '           ; 8 bytes
 ;
 
 start:
-  jmp main
+  ; setup data segments
+  mov ax, 0           ; can't write to ds/es directly
+  mov ds, ax
+  mov es, ax
+
+  ; setup stack
+  mov ss, ax
+  mov sp, 0x7C00      ; stack grows downwards from where we are loaded in memory
+
+  ; some BIOSes might start us at 07C0:0000 instead of 0000:7C00, make sure we are in the 
+  ; expected location
+  push es
+  push word .after
+  retf
+
+.after:
+  ; read something from floppy disk
+  ; BIOS should set DL to drive number
+  mov [ebr_drive_number], dl
+
+  ; show loading message
+  mov si, msg_loading
+  call puts
+
+  ; read drive parametes (sectors per track and head count),
+  ; instead of relying on data on formatted disk
+  push es
+  mov ah, 08h
+  int 13h
+  jc floppy_error
+  pop es
+
+  and cl, 0x3F                      ; remove top 2 bits
+  xor ch, ch
+  mov [bdb_sectors_per_track], cx   ; sector count
+
+  inc dh
+  mov [bdb_heads], dh               ; head count
+
+  ; read FAT root directory
+  mov ax, [bdb_sectors_per_fat]     ; compute LBA of root directory = reserved + fats * sectors_per_fat
+  mov bl, [bdb_fat_count]
+  xor bh, bh
+  mul bx                            ; ax = (fats * sectors_per_fat)
+  add ax, [bdb_reserved_sectors]    ; ax = LBA of root directory
+  push ax
+
+  ; compute size of root directory = (32 * number_of_entries) / bytes_per_sector
+  mov ax, [bdb_dir_entries_count]
+  shl ax, 5                         ; ax *= 32
+  xor dx, dx                        ; dx = 0
+  div word [bdb_bytes_per_sector]   ; number of sectors we need to read
+
+  test dx, dx                       ; if dx != 0, add 1
+  jz .root_dir_after
+  inc ax                            ; division remainder != 0, add 1
+                                    ; this means we have a sector only partially filled with entries
+
+.root_dir_after:
+
+  ; read root directory
+  mov cl, al                        ; cl = number of sectors to read = size of root directory
+  pop ax                            ; ax = LBA of root directory
+  mov dl, [ebr_drive_number]        ; dl = drive number (we saved it previously)
+  mov bx, buffer                    ; es:bx = buffer
+  call disk_read
+
+  ; search for kernel.bin
+  xor bx, bx
+  mov di, buffer
+
+.search_kernel:
+  mov si, file_kernel_bin
+  mov cx, 11                        ; compare up to 11 characters
+  push di
+  repe cmpsb
+  pop di
+  je .found_kernel
+
+  add di, 32
+  inc bx
+  cmp bx, [bdb_dir_entries_count]
+  jl .search_kernel
+
+  ; kernel not found
+  jmp kernel_not_found_error
+
+.found_kernel:
+
+  ; di should have the address to the entry
+  mov ax, [di + 26]               ; first logical cluster field (offset 26)
+  mov [kernel_cluster], ax
+
+  ; load FAT from disk into memory
+  mov ax, [bdb_reserved_sectors]
+  mov bx, buffer
+  mov cl, [bdb_sectors_per_fat]
+  mov dl, [ebr_drive_number]
+  call disk_read
+
+  ; read kernel and process FAT chain
+  mov bx, KERNEL_LOAD_SEGMENT
+  mov es, bx
+  mov bx, KERNEL_LOAD_OFFSET
+
+.load_kernel_loop:
+
+  ; read next cluster
+  mov ax, [kernel_cluster]
+  add ax, 31                      ; first cluster = (cluster numer - 2) * sectors_per_cluster + start_sector
+                                  ; start sector = reserved + fats + root directory size = 1 + 18 + 134 == 33
+  mov cl, 1
+  mov dl, [ebr_drive_number]
+  call disk_read
+
+  add bx, [bdb_bytes_per_sector]
+
+  ; compute location of next cluster
+  mov ax, [kernel_cluster]
+  mov cx, 3
+  mul cx
+  mov cx, 2
+  div cx                          ; ax = index of entry in FAT, dx = cluster mod 2
+
+  mov si, buffer
+  add si, ax
+  mov ax, [ds:si]                 ; read entry from FAT table at index ax
+
+  or dx, dx
+  jz .even
+
+.odd:
+  shr ax, 4
+  jmp .next_cluster_after
+
+.even:
+  and ax, 0x0FFF
+
+.next_cluster_after:
+  cmp ax, 0x0FF8                  ; end of chain
+  jae .read_finish
+
+  mov [kernel_cluster], ax
+  jmp .load_kernel_loop
+
+.read_finish:
+
+  ; jump to our kernel
+  mov dl, [ebr_drive_number]      ; boot device in dl
+
+  mov ax, KERNEL_LOAD_SEGMENT     ; set segment registers
+  mov ds, ax
+  mov es, ax
+
+  jmp KERNEL_LOAD_SEGMENT:KERNEL_LOAD_OFFSET
+
+  jmp wait_key_and_reboot         ; should never happen
+
+  cli                             ; disable interrupts, this way CPU can't get out of "halt" state
+  hlt
+
+;
+; Error handlers
+;
+
+floppy_error:
+  mov si, msg_read_failed
+  call puts
+  jmp wait_key_and_reboot
+
+kernel_not_found_error:
+  mov si, msg_kernel_not_found
+  call puts
+  jmp wait_key_and_reboot
+  
+wait_key_and_reboot:
+  mov ah, 0
+  int 16h           ; wait for keypress
+  jmp 0FFFFh:0      ; jump to beginning of BIOS, should reboot
+
+.halt:
+  cli               ; disable interrupts, this way CPU can't get out of "halt" state
+  hlt
 
 ;
 ; Prints a string to the screen.
@@ -63,51 +245,6 @@ puts:
   pop si
   ret
 
-main:
-
-  ; setup data segments
-  mov ax, 0           ; can't write to ds/es directly
-  mov ds, ax
-  mov es, ax
-
-  ; setup stack
-  mov ss, ax
-  mov sp, 0x7C00      ; stack grows downwards from where we are loaded in memory
-
-  ; read something from floppy disk
-  ; BIOS should set DL to drive number
-  mov [ebr_drive_number], dl
-
-  mov ax, 1           ; LBA=1, second sector from disk
-  mov cl, 1           ; 1 sector to read
-  mov bx, 0x7E00      ; data should be after the bootloader
-  call disk_read
-
-  ; print hello wrold message
-  mov si, msg_hello
-  call puts
-
-  cli                 ; disable interrupts, this way CPU can't get out of "halt" state
-  hlt
-
-;
-; Error handlers
-;
-
-floppy_error:
-  mov si, msg_read_failed
-  call puts
-  jmp wait_key_and_reboot
-  
-wait_key_and_reboot:
-  mov ah, 0
-  int 16h           ; wait for keypress
-  jmp 0FFFFh:0      ; jump to beginning of BIOS, should reboot
-
-.halt:
-  cli               ; disable interrupts, this way CPU can't get out of "halt" state
-  hlt
-
 ;
 ; Disk routines
 ;
@@ -214,9 +351,16 @@ disk_reset:
   popa
   ret
 
-msg_hello:          db 'Hello World!', ENDL, 0
-msg_read_failed:    db 'Read from disk failed!', ENDL, 0
+msg_loading:            db 'Loading...', ENDL, 0
+msg_read_failed:        db 'Read from disk failed!', ENDL, 0
+msg_kernel_not_found:   db 'KERNEL.BIN file not found!', ENDL, 0 
+file_kernel_bin:        db 'KERNEL  BIN'
+kernel_cluster:         dw 0
 
+KERNEL_LOAD_SEGMENT     equ 0x2000
+KERNEL_LOAD_OFFSET      equ 0
 
 times 510-($-$$) db 0
 dw 0AA55h
+
+buffer:

src/kernel/main.asm

@@ -1,10 +1,17 @@
-org 0x7C00
+org 0x0
 bits 16
 
-%define ENDL 0x0A
+%define ENDL 0x0D, 0x0A
 
 start:
-  jmp main
+
+  ; print message
+  mov si, msg_hello
+  call puts
+
+.halt:
+  cli
+  hlt
 
 ;
 ; Prints a string to the screen.
@@ -15,43 +22,23 @@ puts:
   ; save registers we will modify
   push si
   push ax
+  push bx
 
 .loop:
   lodsb               ; loads next character in al
   or al, al           ; verify if next character is null?
   jz .done
 
-  mov ah, 0x0e        ; call bios interrupt
+  mov ah, 0x0E        ; call bios interrupt
+  mov bh, 0x10        ; set page number to 0
   int 0x10
 
   jmp .loop
 
 .done:
+  pop bx
   pop ax
   pop si
   ret
 
-main:
-
-  ; setup data segments
-  mov ax, 0           ; can't write to ds/es directly
-  mov ds, ax
-  mov es, ax
-
-  ; setup stack
-  mov ss, ax
-  mov sp, 0x7C00      ; stack grows downwards from where we are loaded in memory
-
-  ; print message
-  mov si, ms_hello
-  call puts
-
-  hlt
-
-.halt:
-  jmp .halt
-
-ms_hello: db 'Hello World!', ENDL, 0
-  
-times 510-($-$$) db 0
-dw 0AA55h
+msg_hello: db 'Hello World!', ENDL, 0