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Annotation of truecrypt/boot/windows/bootsector.asm, revision 1.1.1.2

1.1       root        1: ;
                      2: ; Copyright (c) 2008 TrueCrypt Foundation. All rights reserved.
                      3: ;
                      4: ; Governed by the TrueCrypt License 2.4 the full text of which is contained
                      5: ; in the file License.txt included in TrueCrypt binary and source code
                      6: ; distribution packages.
                      7: ;
                      8: 
                      9: .MODEL tiny
                     10: .386
                     11: _TEXT SEGMENT
                     12: 
                     13: INCLUDE BootDefs.i
                     14: 
                     15: ORG 7C00h ; Boot sector offset
                     16: 
                     17: start:
                     18:        ; BIOS executes boot sector from 0:7C00 or 7C0:0000 (default CD boot loader address).
                     19:        ; Far jump to the next instruction ensures the offset matches the ORG directive.
                     20:        db 0EAh                         ; jmp 0:main
                     21:        dw main, 0
                     22: 
                     23:        db 'TrueCrypt'
                     24:        
                     25: main:  
1.1.1.2 ! root       26:        xor ax, ax
1.1       root       27:        mov ds, ax
                     28:        
                     29:        lea si, intro_msg
                     30:        call print
                     31:        
                     32:        mov ax, TC_BOOT_LOADER_SEGMENT
                     33:        mov es, ax ; Default load segment
                     34: 
                     35:        ; Check available memory
1.1.1.2 ! root       36:        cmp word ptr [ds:413h], TC_BOOT_LOADER_SEGMENT / 1024 * 16 + TC_BOOT_MEMORY_REQUIRED
1.1       root       37:        jge clear_memory
                     38:        
                     39:        ; Insufficient memory
                     40:        mov ax, TC_BOOT_LOADER_LOWMEM_SEGMENT
                     41:        mov es, ax
                     42: 
                     43:        ; Ensure clear BSS section
                     44: clear_memory:
                     45:        xor al, al
                     46:        mov di, TC_BOOT_LOADER_OFFSET
                     47:        mov cx, TC_BOOT_MEMORY_REQUIRED * 1024 - 1
1.1.1.2 ! root       48:        cld
1.1       root       49:        rep stosb
                     50:        
                     51:        ; Read boot loader
                     52:        mov bx, TC_BOOT_LOADER_OFFSET
                     53:        mov ch, 0           ; Cylinder
                     54:        mov dh, 0           ; Head
                     55:        mov cl, 2           ; Sector
                     56:        mov al, TC_BOOT_LOADER_AREA_SECTOR_COUNT - 2
                     57:                                                ; DL = drive number passed from BIOS
                     58:        mov ah, 2
                     59:        int 13h
                     60:        jnc exec_loader
                     61:        
                     62:        lea si, read_error_msg
                     63:        call print
                     64:        jmp $
                     65:        
                     66:        ; Execute boot loader
                     67: exec_loader:
                     68: 
                     69:        ; DH = boot sector flags
                     70:        mov dh, byte ptr [start + TC_BOOT_SECTOR_CONFIG_OFFSET]
                     71:        
                     72:        mov ax, es
                     73:        mov ds, ax
                     74:        cli
                     75:        mov ss, ax
1.1.1.2 ! root       76:        mov sp, TC_BOOT_MEMORY_REQUIRED * 1024 - 4
1.1       root       77:        sti
                     78:        
                     79:        mov word ptr [cs:jump_seg], es
                     80:        
                     81:        db 0EAh                          ; jmp TC_BOOT_LOADER_SEGMENT:TC_BOOT_LOADER_OFFSET
                     82:        dw TC_BOOT_LOADER_OFFSET
                     83: jump_seg:
                     84:        dw TC_BOOT_LOADER_SEGMENT
                     85: 
                     86:        ; Print string
                     87: print:
                     88:        xor bx, bx
                     89:        mov ah, 0eh
                     90:        
                     91: @@:    lodsb
                     92:        test al, al
                     93:        jz print_end
                     94:        
                     95:        int 10h
                     96:        jmp @B
                     97: 
                     98: print_end:
                     99:        ret
                    100:        
                    101: intro_msg db 13, 10, "TrueCrypt Boot Loader", 13, 10, 0
                    102: read_error_msg db "Disk read error", 13, 10, 7, 0
                    103: 
                    104:        db 508 - ($ - start) dup (0)
                    105:        dw 0, 0AA55h            ; Boot sector signature
                    106: 
                    107: _TEXT ENDS
                    108: END start

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