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Annotation of truecrypt/boot/windows/bootsector.asm, revision 1.1.1.1

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: TC_PC_MEMORY_SEGMENT_SIZE = 64
                     18: TC_PC_BASE_MEMORY_SIZE = 640
                     19: 
                     20: start:
                     21:        ; BIOS executes boot sector from 0:7C00 or 7C0:0000 (default CD boot loader address).
                     22:        ; Far jump to the next instruction ensures the offset matches the ORG directive.
                     23:        db 0EAh                         ; jmp 0:main
                     24:        dw main, 0
                     25: 
                     26:        db 'TrueCrypt'
                     27:        
                     28: main:  
                     29:        mov ax, cs
                     30:        mov ds, ax
                     31:        
                     32:        lea si, intro_msg
                     33:        call print
                     34:        
                     35:        mov ax, TC_BOOT_LOADER_SEGMENT
                     36:        mov es, ax ; Default load segment
                     37:        xor si, si ; Default SP
                     38: 
                     39:        ; Check available memory
                     40:        mov ax, word ptr [ds:413h]
                     41:        
                     42:        ; >= 640 KB?
                     43:        mov bx, TC_PC_BASE_MEMORY_SIZE
                     44:        cmp ax, bx
                     45:        jge clear_memory
                     46: 
                     47:        ; < required memory?
                     48:        cmp ax, TC_PC_BASE_MEMORY_SIZE - (TC_PC_MEMORY_SEGMENT_SIZE - TC_BOOT_MEMORY_REQUIRED)
                     49:        jge s0
                     50:        
                     51:        ; Insufficient memory
                     52:        mov ax, TC_BOOT_LOADER_LOWMEM_SEGMENT
                     53:        mov es, ax
                     54:        jmp clear_memory
                     55:        
                     56: s0:
                     57:        ; SP = 0 - ((640 - *0:0413) * 1024)
                     58:        sub bx, ax
                     59:        shl bx, 10
                     60:        sub si, bx
                     61: 
                     62:        ; Ensure clear BSS section
                     63: clear_memory:
                     64:        xor al, al
                     65:        mov di, TC_BOOT_LOADER_OFFSET
                     66:        mov cx, TC_BOOT_MEMORY_REQUIRED * 1024 - 1
                     67:        rep stosb
                     68:        
                     69:        ; Read boot loader
                     70:        mov bx, TC_BOOT_LOADER_OFFSET
                     71:        mov ch, 0           ; Cylinder
                     72:        mov dh, 0           ; Head
                     73:        mov cl, 2           ; Sector
                     74:        mov al, TC_BOOT_LOADER_AREA_SECTOR_COUNT - 2
                     75:                                                ; DL = drive number passed from BIOS
                     76:        mov ah, 2
                     77:        int 13h
                     78:        jnc exec_loader
                     79:        
                     80:        lea si, read_error_msg
                     81:        call print
                     82:        jmp $
                     83:        
                     84:        ; Execute boot loader
                     85: exec_loader:
                     86: 
                     87:        ; DH = boot sector flags
                     88:        mov dh, byte ptr [start + TC_BOOT_SECTOR_CONFIG_OFFSET]
                     89:        
                     90:        mov ax, es
                     91:        mov ds, ax
                     92:        cli
                     93:        mov ss, ax
                     94:        mov sp, si
                     95:        sti
                     96:        
                     97:        mov word ptr [cs:jump_seg], es
                     98:        
                     99:        db 0EAh                          ; jmp TC_BOOT_LOADER_SEGMENT:TC_BOOT_LOADER_OFFSET
                    100:        dw TC_BOOT_LOADER_OFFSET
                    101: jump_seg:
                    102:        dw TC_BOOT_LOADER_SEGMENT
                    103: 
                    104:        ; Print string
                    105: print:
                    106:        xor bx, bx
                    107:        mov ah, 0eh
                    108:        
                    109: @@:    lodsb
                    110:        test al, al
                    111:        jz print_end
                    112:        
                    113:        int 10h
                    114:        jmp @B
                    115: 
                    116: print_end:
                    117:        ret
                    118:        
                    119: intro_msg db 13, 10, "TrueCrypt Boot Loader", 13, 10, 0
                    120: read_error_msg db "Disk read error", 13, 10, 7, 0
                    121: 
                    122:        db 508 - ($ - start) dup (0)
                    123:        dw 0, 0AA55h            ; Boot sector signature
                    124: 
                    125: _TEXT ENDS
                    126: END start

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