implementing filesystem 2/3

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

@@ -0,0 +1,94 @@
+org 0x7C00
+bits 16
+
+%define     ENDL   0x0D, 0x0A
+
+;
+;FAT12 header 
+;
+
+jmp short start
+nop 
+
+bdb_oem:                              db 'MSWIN4.1'             ; 8 bytes 
+bdb_bytes_per_sector:                 dw 512                    ;
+bdb_sectors_per_clusters:             db 1                      ;
+bdb_reserved_sectors:                 dw 1
+bdb_fat_count:                        db 2 
+bdb_dir_entries_count:                dw 0E0h
+bdb_total_sectors:                    dw 2880                   ; 2880*512 = 1.44megabytes
+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_hidden_sectors:                   dd 0
+bdb_large_sector_count:               dd 0 
+
+;extended Boot record 
+ebr_drive_number:                     db 0                      ; 0x00 floppy, 0x80 hdd, useless
+                                      db 0                      ; reserved
+ebr_signature:                        db 29h
+ebr_volume_id:                        db 12h, 34h, 56h, 78h     ; serial does not mean to our purpose.
+ebr_volume_label:                     db 'ANURAG OS'            ; 11 bytes, padded with space 
+ebr_system_id:                        db 'FAT12 '               ; 8 bytes                      
+
+
+start:
+    jmp main 
+
+;
+;
+;Prints a String to the screen.
+;Params:
+; - ds:si points to strings
+;
+;
+puts:
+    push si 
+    push ax 
+    push bx 
+
+.loop:
+    lodsb           ;loads the next character in al 
+    or al, al       ;verify if next character is null?
+    jz .done
+    
+
+    mov ah, 0x0E    ;call bios interrupt routine
+    mov bh, 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 loaded into memory
+
+; print message 
+    mov si,  msg_hello
+    call puts  
+
+    hlt
+
+.halt:
+    jmp .halt
+
+
+msg_hello: db  'Hello world!', ENDL, 0
+
+
+times 510-($-$$) db 0
+dw 0AA55h