reactos/boot/freeldr/bootsect/wxpfat16.asm

   1
   2 ;
   3 ; The BP register is initialized to 0x7c00, the start of
   4 ; the boot sector. The SP register is initialized to
   5 ; 0x7bf0, leaving 16 bytes of data storage space above
   6 ; the stack.
   7 ;
   8 ; The DWORD that gets stored at 0x7bfc is the logical
   9 ; sector number of the start of the data area.
  10 ;
  11 ; The DWORD that gets stored at 0x7bf8 is ????????
  12 ;
  13 ; The DWORD that gets stored at 0x7bf4 is ????????
  14 ;
  15 ; The DWORD that gets stored at 0x7bf0 is ????????
  16 ;
  17
  18
  19 org 7c00h
  20
  21 segment .text
  22
  23 bits 16
  24
  25 start:
  26         jmp short main
  27         nop
  28
  29 OEMName                         db 'MSWIN4.0'
  30 BytesPerSector          dw 512
  31 SectsPerCluster         db 1
  32 ReservedSectors         dw 1
  33 NumberOfFats            db 2
  34 MaxRootEntries          dw 0                    ;512 - Always zero for FAT32 volumes
  35 TotalSectors            dw 0                    ;2880 - Always zero for FAT32 volumes
  36 MediaDescriptor         db 0f8h
  37 SectorsPerFat           dw 0                    ;9 - Always zero for FAT32 volumes
  38 SectorsPerTrack         dw 18
  39 NumberOfHeads           dw 2
  40 HiddenSectors           dd 0
  41 TotalSectorsBig         dd 0
  42 BootDrive                       db 80h
  43 Reserved                        db 0
  44 ExtendSig                       db 29h
  45 SerialNumber            dd 00000000h
  46 VolumeLabel                     db 'NO NAME    '
  47 FileSystem                      db 'FAT16   '
  48
  49 main:
  50 00007C3E  33C9              xor cx,cx
  51 00007C40  8ED1              mov ss,cx                                   ; Setup stack
  52 00007C42  BCF07B            mov sp,0x7bf0                               ; Give us 16 bytes (4 dwords) of space above stack
  53 00007C45  8ED9              mov ds,cx
  54 00007C47  B80020            mov ax,0x2000
  55 00007C4A  8EC0              mov es,ax                                   ; Setup ES:0000 == 2000:0000
  56 00007C4C  FC                cld
  57 00007C4D  BD007C            mov bp,0x7c00
  58 00007C50  384E24            cmp [bp+BootDrive],cl               ; Compare the boot drive to zero (I think they are testing for a hard disk drive number)
  59 00007C53  7D24              jnl floppy_boot                             ; Nope, it's a floppy, skip partition table tests
  60 00007C55  8BC1              mov ax,cx                                   ; Move zero to AX
  61 00007C57  99                cwd                                                 ; DX:AX now contains zero
  62 00007C58  E83C01            call read_one_sector                ; Try to read in the MBR sector
  63 00007C5B  721C              jc floppy_boot                              ; Read failed, continue
  64 00007C5D  83EB3A            sub bx,byte +0x3a                   ; BX comes back with 512, make it equal to 454 (offset of partition table in MBR)
  65 00007C60  66A11C7C          mov eax,[HiddenSectors]             ; Put HiddenSectors in EAX
  66 find_our_partition:
  67 00007C64  26663B07          cmp eax,[es:bx]                             ; Compare partition table entry's start sector to HiddenSectors
  68 00007C68  268A57FC          mov dl,[es:bx-0x4]                  ; Get partition type byte for this entry
  69 00007C6C  7506              jnz next_partition_entry    ; If partition start sector != HiddenSectors then skip this entry
  70 00007C6E  80CA02            or dl,0x2                                   ; Set the second bit in partition type?? I guess this makes types 4 & 6 identical
  71 00007C71  885602            mov [bp+0x2],dl                             ; Save it on top of nop instruction (3rd byte of boot sector)
  72 next_partition_entry:
  73 00007C74  80C310            add bl,0x10                                 ; Add 16 to bl (offset of next entry in partition table)
  74 00007C77  73EB              jnc find_our_partition              ; Jump back until we hit the end of the partition table
  75
  76 ; We now have our partition type at 0000:7C02
  77 ; If the type was 4 or 6 then that byte is 6
  78 ; I can't imagine why the boot sector needs to store
  79 ; this information, but hopefully I will uncover it
  80 ; as I further disassemble this boot sector.
  81
  82
  83 floppy_boot:
  84 00007C79  33C9              xor cx,cx                                   ; Zero out CX
  85 00007C7B  8A4610            mov al,[bp+NumberOfFats]    ; Get the number of FATs in AL (usually 2)
  86 00007C7E  98                cbw                                                 ; Sign extend it into AX (AX == 2)
  87 00007C7F  F76616            mul word [bp+NumberOfFats]  ; Multiply it with NumberOfFats PLUS the low byte of MaxRootEntries!!??
  88 00007C82  03461C            add ax,[bp+HiddenSectors]   ; Result is in DX:AX
  89 00007C85  13561E            adc dx,[bp+HiddenSectors+2] ; Add HiddenSectors to DX:AX
  90 00007C88  03460E            add ax,[bp+ReservedSectors] ; Add ReservedSectors to DX:AX
  91 00007C8B  13D1              adc dx,cx                                   ; CX still contains zero
  92 00007C8D  8B7611            mov si,[bp+MaxRootEntries]  ; Get MaxRootEntries in SI
  93 00007C90  60                pusha                                               ; Save all registers (right now DX:AX has starting sector of root dir)
  94 00007C91  8946FC            mov [bp-0x4],ax                             ; Save the starting sector of the root directory
  95 00007C94  8956FE            mov [bp-0x2],dx                             ; Save it in the first 4 bytes before the boot sector
  96 00007C97  B82000            mov ax,0x20                                 ; AX == 32 (size of a directory entry)
  97 00007C9A  F7E6              mul si                                              ; Multiply it with MaxRootEntries (DX:AX == length in bytes of root directory)
  98 00007C9C  8B5E0B            mov bx,[bp+BytesPerSector]  ; Get the BytesPerSector in BX
  99 00007C9F  03C3              add ax,bx                                   ; Add it to AX (what if this addition carries? MS should 'adc dx,0' shouldn't they?)
 100 00007CA1  48                dec ax                                              ; Subtract one (basically rounding up)
 101 00007CA2  F7F3              div bx                                              ; Divide DX:AX (length of root dir in bytes) by the size of a sector
 102 00007CA4  0146FC            add [bp-0x4],ax                             ; Add the number of sectors of the root directory to our other value
 103 00007CA7  114EFE            adc [bp-0x2],cx                             ; Now the first 4 bytes before the boot sector contain the starting sector of the data area
 104 00007CAA  61                popa                                                ; Restore all registers (DX:AX has start sector of root dir)
 105 load_root_dir_sector:
 106 00007CAB  BF0000            mov di,0x0                                  ; Zero out di
 107 00007CAE  E8E600            call read_one_sector                ; Read the first sector of the root directory
 108 00007CB1  7239              jc print_disk_error_message ; Read failed, print disk error and reboot
 109 search_directory:
 110 00007CB3  26382D            cmp [es:di],ch                              ; If the first byte of the directory entry is zero then we have reached the end
 111 00007CB6  7417              jz print_ntldr_error_message; of the directory and NTLDR is not here so reboot
 112 00007CB8  60                pusha                                               ; Save all registers
 113 00007CB9  B10B              mov cl,0xb                                  ; Put 11 in cl (length of filename in directory entry)
 114 00007CBB  BEA17D            mov si,NTLDR                                ; Put offset of filename string in DS:SI
 115 00007CBE  F3A6              repe cmpsb                                  ; Compare this directory entry against 'NTLDR      '
 116 00007CC0  61                popa                                                ; Restore all the registers
 117 00007CC1  7432              jz found_ntldr                              ; If we found NTLDR then jump
 118 00007CC3  4E                dec si                                              ; SI holds MaxRootEntries, subtract one
 119 00007CC4  7409              jz print_ntldr_error_message; If we are out of root dir entries then reboot
 120 00007CC6  83C720            add di,byte +0x20                   ; Increment DI by the size of a directory entry
 121 00007CC9  3BFB              cmp di,bx                                   ; Compare DI to BX (DI has offset to next dir entry, BX has address of end of directory sector in memory)
 122 00007CCB  72E6              jc search_directory                 ; If DI is less than BX loop again
 123 00007CCD  EBDC              jmp short load_root_dir_sector      ; Didn't find NTLDR in this directory sector, try again
 124 print_ntldr_error_message:
 125 00007CCF  A0FB7D            mov al,[NTLDR_ERR_offset_from_0x7d00]
 126 putchars:
 127 00007CD2  B47D              mov ah,0x7d
 128 00007CD4  8BF0              mov si,ax
 129 get_another_char:
 130 00007CD6  AC                lodsb
 131 00007CD7  98                cbw
 132 00007CD8  40                inc ax
 133 00007CD9  740C              jz print_reboot_message
 134 00007CDB  48                dec ax
 135 00007CDC  7413              jz reboot
 136 00007CDE  B40E              mov ah,0xe
 137 00007CE0  BB0700            mov bx,0x7
 138 00007CE3  CD10              int 0x10
 139 00007CE5  EBEF              jmp short get_another_char
 140 print_reboot_message:
 141 00007CE7  A0FD7D            mov al,[RESTART_ERR_offset_from_0x7d00]
 142 00007CEA  EBE6              jmp short putchars
 143 print_disk_error_message:
 144 00007CEC  A0FC7D            mov al,[DISK_ERR_offset_from_0x7d00]
 145 00007CEF  EBE1              jmp short putchars
 146 reboot:
 147 00007CF1  CD16              int 0x16
 148 00007CF3  CD19              int 0x19
 149 found_ntldr:
 150 00007CF5  268B551A          mov dx,[es:di+0x1a]                 ; Get NTLDR start cluster in DX
 151 00007CF9  52                push dx                                             ; Save it on the stack
 152 00007CFA  B001              mov al,0x1                                  ; Read 1 cluster? Or is this one sector?
 153 00007CFC  BB0000            mov bx,0x0                                  ; ES:BX is the load address (2000:0000)
 154 00007CFF  E83B00            call read_cluster                   ; Do the read
 155 00007D02  72E8              jc print_disk_error_message ; If it failed then reboot
 156 00007D04  5B                pop bx                                              ; Get the start cluster of NTLDR in BX
 157 00007D05  8A5624            mov dl,[bp+BootDrive]               ; Get boot drive in DL
 158 00007D08  BE0B7C            mov si,0x7c0b
 159 00007D0B  8BFC              mov di,sp
 160 00007D0D  C746F03D7D        mov word [bp-0x10],read_cluster
 161 00007D12  C746F4297D        mov word [bp-0xc],0x7d29
 162 00007D17  8CD9              mov cx,ds
 163 00007D19  894EF2            mov [bp-0xe],cx
 164 00007D1C  894EF6            mov [bp-0xa],cx
 165 00007D1F  C606967DCB        mov byte [0x7d96],0xcb
 166 00007D24  EA03000020        jmp 0x2000:0x3
 167 00007D29  0FB6C8            movzx cx,al
 168 00007D2C  668B46F8          mov eax,[bp-0x8]
 169 00007D30  6603461C          add eax,[bp+HiddenSectors]
 170 00007D34  668BD0            mov edx,eax
 171 00007D37  66C1EA10          shr edx,0x10
 172 00007D3B  EB5E              jmp short 0x7d9b
 173 read_cluster:
 174 00007D3D  0FB6C8            movzx cx,al
 175 00007D40  4A                dec dx
 176 00007D41  4A                dec dx
 177 00007D42  8A460D            mov al,[bp+SectsPerCluster]
 178 00007D45  32E4              xor ah,ah
 179 00007D47  F7E2              mul dx
 180 00007D49  0346FC            add ax,[bp-0x4]
 181 00007D4C  1356FE            adc dx,[bp-0x2]
 182 00007D4F  EB4A              jmp short 0x7d9b
 183
 184 read_sectors:
 185 00007D51  52                push dx
 186 00007D52  50                push ax
 187 00007D53  06                push es
 188 00007D54  53                push bx
 189 00007D55  6A01              push byte +0x1
 190 00007D57  6A10              push byte +0x10
 191 00007D59  91                xchg ax,cx
 192 00007D5A  8B4618            mov ax,[bp+SectorsPerTrack]
 193 00007D5D  96                xchg ax,si
 194 00007D5E  92                xchg ax,dx
 195 00007D5F  33D2              xor dx,dx
 196 00007D61  F7F6              div si
 197 00007D63  91                xchg ax,cx
 198 00007D64  F7F6              div si
 199 00007D66  42                inc dx
 200 00007D67  87CA              xchg cx,dx
 201 00007D69  F7761A            div word [bp+NumberOfHeads]
 202 00007D6C  8AF2              mov dh,dl
 203 00007D6E  8AE8              mov ch,al
 204 00007D70  C0CC02            ror ah,0x2
 205 00007D73  0ACC              or cl,ah
 206 00007D75  B80102            mov ax,0x201
 207 00007D78  807E020E          cmp byte [bp+0x2],0xe
 208 00007D7C  7504              jnz 0x7d82
 209 00007D7E  B442              mov ah,0x42
 210 00007D80  8BF4              mov si,sp
 211 00007D82  8A5624            mov dl,[bp+BootDrive]
 212 00007D85  CD13              int 0x13
 213 00007D87  61                popa
 214 00007D88  61                popa
 215 00007D89  720B              jc 0x7d96
 216 00007D8B  40                inc ax
 217 00007D8C  7501              jnz 0x7d8f
 218 00007D8E  42                inc dx
 219 00007D8F  035E0B            add bx,[bp+BytesPerSector]
 220 00007D92  49                dec cx
 221 00007D93  7506              jnz 0x7d9b
 222 00007D95  F8                clc
 223 00007D96  C3                ret
 224
 225 read_one_sector:
 226 00007D97  41                inc cx
 227 00007D98  BB0000            mov bx,0x0
 228 00007D9B  60                pusha
 229 00007D9C  666A00            o32 push byte +0x0
 230 00007D9F  EBB0              jmp short 0x7d51
 231
 232
 233 NTLDR           db 'NTLDR      '
 234
 235
 236 NTLDR_ERR       db 0dh,0ah,'NTLDR is missing',0ffh
 237 DISK_ERR        db 0dh,0ah,'Disk error',0ffh
 238 RESTART_ERR     db 0dh,0ah,'Press any key to restart',0dh,0ah
 239
 240 filler          times 18        db 0
 241
 242
 243 NTLDR_offset_from_0x7d00                                                db 0
 244 NTLDR_ERR_offset_from_0x7d00                                    db 0ach
 245 DISK_ERR_offset_from_0x7d00                                             db 0bfh
 246 RESTART_ERR_offset_from_0x7d00                                  db 0cch
 247
 248                                                 dw 0
 249                                                 dw 0aa55h