amd64/bareMetalOS-0.5.2/pure64.boot0/bootsector/fat16mbr.asm

   1 USE16
   2 org 0x7C00
   3
   4 entry:
   5         cli
   6         mov [DriveNumber], dl   ; BIOS passes drive number in DL
   7         xor ax, ax
   8         mov ss, ax
   9         mov sp, 0x7C00
  10         mov si, sp
  11         push ax
  12         pop es
  13         push ax
  14         pop ds
  15         sti
  16
  17 ; Copy MBR sector to 0x0600 and jump there
  18         cld
  19         mov di, 0x0600
  20         mov cx, 0x0100
  21         repne movsw
  22         jmp 0x0000:0x0621
  23 ; 0x0621
  24
  25 ; Print message
  26         mov si, msg_Load
  27         call print_string_16
  28
  29         mov si, 0x07BE
  30         cmp byte [si], 0x80
  31         jne NoActivePartition
  32
  33         mov ecx, [0x07C6]
  34         mov bx, 0x7C00
  35         xor esi, esi
  36         xor edi, edi
  37         mov si, bx
  38         mov di, bx
  39         add eax, ecx
  40         call readsector
  41         jmp 0x0000:0x7C00
  42
  43 NoActivePartition:
  44         mov si, msg_NoPartition
  45         call print_string_16
  46         jmp $
  47
  48
  49 ;------------------------------------------------------------------------------
  50 ; Read a sector from a disk, using LBA
  51 ; input:        EAX - 32-bit DOS sector number
  52 ;               ES:BX - destination buffer
  53 ; output:       ES:BX points one byte after the last byte read
  54 ;               EAX - next sector
  55 readsector:
  56         push dx
  57         push si
  58         push di
  59
  60 read_it:
  61         push eax        ; Save the sector number
  62         mov di, sp      ; remember parameter block end
  63
  64         push byte 0     ; other half of the 32 bits at [C]
  65         push byte 0     ; [C] sector number high 32bit
  66         push eax        ; [8] sector number low 32bit
  67         push es         ; [6] buffer segment
  68         push bx         ; [4] buffer offset
  69         push byte 1     ; [2] 1 sector (word)
  70         push byte 16    ; [0] size of parameter block (word)
  71
  72         mov si, sp
  73         mov dl, [DriveNumber]
  74         mov ah, 42h     ; EXTENDED READ
  75         int 0x13        ; http://hdebruijn.soo.dto.tudelft.nl/newpage/interupt/out-0700.htm#0651
  76
  77         mov sp, di      ; remove parameter block from stack
  78         pop eax         ; Restore the sector number
  79
  80         jnc read_ok     ; jump if no error
  81
  82         push ax
  83         xor ah, ah      ; else, reset and retry
  84         int 0x13
  85         pop ax
  86         jmp read_it
  87
  88 read_ok:
  89         inc eax                 ; next sector
  90         add bx, 512             ; Add bytes per sector
  91         jnc no_incr_es          ; if overflow...
  92
  93 incr_es:
  94         mov dx, es
  95         add dh, 0x10            ; ...add 1000h to ES
  96         mov es, dx
  97
  98 no_incr_es:
  99         pop di
 100         pop si
 101         pop dx
 102         ret
 103 ;------------------------------------------------------------------------------
 104
 105
 106 ;------------------------------------------------------------------------------
 107 ; 16-bit Function to print a sting to the screen
 108 ; input:        SI - Address of start of string
 109 print_string_16:                        ; Output string in SI to screen
 110         pusha
 111         mov ah, 0x0E                    ; int 0x10 teletype function
 112 .repeat:
 113         lodsb                           ; Get char from string
 114         cmp al, 0
 115         je .done                        ; If char is zero, end of string
 116         int 0x10                        ; Otherwise, print it
 117         jmp short .repeat
 118 .done:
 119         popa
 120         ret
 121 ;------------------------------------------------------------------------------
 122
 123 msg_Load db "Loading... ", 0
 124 msg_NoPartition db "No active partition found"
 125 DriveNumber db 0x00
 126
 127 times 446-$+$$ db 0
 128
 129 tables db "XXXXXXXXXXXXXXXX  DO NOT OVERWRITE THIS AREA!!! XXXXXXXXXXXXXXXX" ; 64 bytes in length
 130
 131 sign dw 0xAA55