Implemented file system 3/3

anurag-os-dev/src/bootloader/boot.asm

@@ -20,7 +20,7 @@ bdb_total_sectors:                    dw 2880                   ; 2880*512 = 1.4
 bdb_media_descriptor_type:            db 0F0h                   ; f0 =m 3.5" floppy disk 
 bdb_sectors_per_fat:                  dw 9                      ; 9 sectos/fat 
 bdb_sectors_per_track:                dw 18 
-bdb_heads:                            dw 2
+bdb_heads:                            dw 2 
 bdb_hidden_sectors:                   dd 0
 bdb_large_sector_count:               dd 0 
 
@@ -33,6 +33,10 @@ ebr_volume_label:                     db 'ANURAG OS'            ; 11 bytes, padd
 ebr_system_id:                        db 'FAT12 '               ; 8 bytes                      
 
 
+
+;
+;code here
+;
 start:
     jmp main 
 
@@ -77,17 +81,161 @@ main:
     mov ss, ax
     mov sp, 0x7C00  ; stack grows downwards from where loaded into memory
 
-; print message 
+    ; 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 message 
     mov si,  msg_hello
     call puts  
 
+    
+    cli
     hlt
 
+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:
-    jmp .halt
+    cli                                 ; disable interrupts, this way we can't get out of of "halt"  state 
+    hlt 
+
+
+
+;
+;Disk routines
+;
+; Converts an LBA address to a CHS address
+; Parameters:
+; - ax: LBA address
+; Returns:
+; - cx [bits 0-5]: sector nu mber
+; - cx [bits 6-15]: cylinder
+; dh: head
+
+
+
+
+lba_to_chs:
+
+    push ax
+    push dx    
+
+    xor dx, dx                                  ; dx = 0
+    div word [bdb_sectors_per_track]            ; ax = LBA / Sectors PerTrack
+                                                ; dx = LBA % SectorsPerTrack
+
+    inc dx                                      ; dx = (LBA % SectorsPerTrack + 1) = sector
+    mov cx, dx                                  ; cx = sector
+
+    xor dx, dx                                  ; dx = 0
+    div word [bdb_heads]                        ; ax = (LBA / SectorsPerTrack) / Heads = cylinder
+                                                ; dx = (LBA / SectorsPerTrack) % Heads = head
+
+    mov dh, dl                                  ; dh = head
+    mov ch, al                                  ; ch = cylinder (lower 8 bits)  
+    shl ah, 6 
+    or cl, ah                                   ; put upper 2 bits of cylinder in CL
+
+    pop ax 
+    mov dl, al                                  ; restore DL 
+    pop ax
+    ret 
+
+
+
+;
+;
+;Reads sectors from a disk 
+;Parameters:
+;       - ax: LBA address
+;       - cl: number of sectors to read(upto 128) 
+;       - dl: drive number 
+;       - es:bs: memory address where to store  read data.
+;
+
+disk_read:
+
+    push ax                     ; save registers will modify  
+    push bx 
+    push cx
+    push dx 
+    push di 
+
+
+
+    push cx                     ; temporarily save CL (number of sectors to read) 
+    call lba_to_chs             ; compute CHS
+    pop  ax                     ; AL = number of sectors to read    
+
+    mov  ah, 02h                   
+    mov  di, 3                  ; retry count
 
 
-msg_hello: db  'Hello world!', ENDL, 0
+.retry:
+    pusha                      ; save all registers, we don't know what bios modifies 
+    stc                         ; set carry flag, some BIOS'es don't set it
+    int  13h                    ; carry flag cleared = succes 
+    jnc .done                   ; jump if carry not set 
+
+   ;read failed 
+    popa                     ;  
+    call disk_reset
+
+
+    dec  di 
+    test di, di 
+    jnz .retry
+
+.fail:
+    jmp floppy_error
+
+.done:
+    popa
+
+
+    pop  di 
+    pop  dx 
+    pop  cx 
+    pop  bx 
+    pop  ax                         ; restore registers modified 
+    ret 
+
+;
+;Resets disk controller 
+;Parameters:
+;   dl: drive number
+;
+;
+disk_reset:
+    pusha
+    mov ah, 0 
+    stc
+    int 13h
+    jc floppy_error 
+    popa
+    ret
+
+msg_hello:          db  'Hello world!', ENDL, 0
+msg_read_failed:    db  'Read from disk failed',  ENDL, 0 
 
 
 times 510-($-$$) db 0

bochs_config

@@ -0,0 +1,6 @@
+megs: 128 
+romimage: file=C:\Program Files\Bochs-2.7\BIOS-bochs-latest, address=0xfffe0000
+vgaromimage: file=C:\Program Files\Bochs-2.7\VGABIOS-lgpl-latest
+floppya: 1_44=\\wsl.localhost\Debian\home\anurag\os-project\anurag-os-dev\build\main_floppy.img, status=inserted
+boot: floppy
+