用x86 16位汇编编写的调试校验和算法

Question

crc_cal proc    far

var_4= word ptr -4
arg_0= word ptr  6
arg_2= dword ptr  8

mov ax, seg dseg37
inc bp
push    bp
mov bp, sp
push    ds
mov ds, ax
sub sp, 2
push    si
push    di
xor cx, cx
mov dx, 0FFFFh
mov [bp+var_4], 8408h

loc_42646:
les bx, [bp+arg_2]
add bx, cx
mov al, es:[bx]
xor al, dl
mov dl, al
inc cx
xor di, di
jmp short loc_42672

loc_42657:
mov si, dx
dec si
mov ax, si
shr ax, 1
mov si, ax
mov ax, dx
shr ax, 1
mov dx, ax
cmp si, dx
jnz short loc_42671
mov ax, dx
xor ax, [bp+var_4]
mov dx, ax

loc_42671:
inc di

loc_42672:
cmp di, 8
jb  short loc_42657
cmp cx, [bp+arg_0]
jb  short loc_42646
mov ax, dx
not ax
mov dx, ax
les bx, [bp+arg_2]
add bx, cx
mov es:[bx], dl
inc cx
mov ax, dx
shr ax, 8
mov dx, ax
les bx, [bp+arg_2]
add bx, cx
mov es:[bx], dl
inc cx
pop di
pop si
pop cx
pop ds
pop bp
dec bp
retf
crc_cal endp

|                           DATA                           |Inc|CRC|
|----------------------------------------------------------|---|---|
00 00 00 00 00 00 01 ef f7 fe ef ff fd ef fb fa fd a2 aa 21 01 f4 e0
00 00 00 00 00 00 01 ef f7 fd ef ff fd fe fb fa fd a2 aa 21 02 f4 d1
00 00 00 00 00 00 01 f7 fe fd fd ff fd df ff fb fd a2 aa 21 03 f4 cd
00 00 00 00 00 00 01 f7 fe fe fd ff f7 ef ff fa fd a2 aa 21 04 f4 c2
00 00 00 00 00 00 01 ef f7 fe ef ff fe ef fb fa fd a2 aa 21 05 f4 db
00 00 00 00 00 00 01 ef f7 fe ef ff fd ef fb fa fd a2 aa 21 06 f4 db

Answer 1

所示数据与《儿童权利公约》不符，如本答复所述：

查找使用的CRC-16算法

该代码是一个过于复杂的右移CRC实现（以dx为单位），poly=0x8408，初始值=0xFFFF，xor out=0xFFFF。检查每行后面的2个字节，看看这是否是追加CRC的地方。

crc_cal proc    far

var_4   =       word ptr -4     ; used to store poly
arg_0   =       word ptr  6     ; number of bytes of data
arg_2   =       dword ptr 8     ; far pointer to data

        mov     ax, seg dseg37  ; for ds that is never used
        inc     bp              ; bp += 1, (bp&1 == far call indicator)
        push    bp              ; save bp+1
        mov     bp, sp          ; bp = sp, base for the equated offsets
        push    ds              ; save ds
        mov     ds, ax          ; ds = dseg37  (never used)
        sub     sp, 2           ; allocate space for poly (var_4)
        push    si              ; save si, di
        push    di
        xor     cx, cx          ; cx = offset to data
        mov     dx, 0FFFFh      ; dx = initial crc
        mov     [bp+var_4], 8408h ;store poly

loc_42646:
        les     bx, [bp+arg_2]  ; al = next byte of data
        add     bx, cx
        mov     al, es:[bx]
        xor     al, dl          ; crclo ^= data
        mov     dl, al
        inc     cx              ; increment offset to data
        xor     di, di          ; di = bit counter (0 to 7)
        jmp     short loc_42672

loc_42657:
        mov     si, dx          ; si = (crc-1)>>1
        dec     si              ;  if lsb was 0, then
        mov     ax, si          ;  si != dx later on
        shr     ax, 1
        mov     si, ax
        mov     ax, dx          ; dx = (crc)>>1
        shr     ax, 1
        mov     dx, ax
        cmp     si, dx          ; br if prior lsb of crc was 0
        jnz     short loc_42671
        mov     ax, dx          ; crc ^= 0x8408
        xor     ax, [bp+var_4]
        mov     dx, ax

loc_42671:
        inc     di              ; increment bit counter

loc_42672:
        cmp     di, 8           ; loop till byte done
        jb      short loc_42657
        cmp     cx, [bp+arg_0]  ; loop till all bytes done
        jb      short loc_42646
        mov     ax, dx          ; dx = ~ crc
        not     ax
        mov     dx, ax
        les     bx, [bp+arg_2]  ; append crc to data, lsbyte first
        add     bx, cx
        mov     es:[bx], dl
        inc     cx
        mov     ax, dx
        shr     ax, 8
        mov     dx, ax
        les     bx, [bp+arg_2]
        add     bx, cx
        mov     es:[bx], dl
        inc     cx              ; useless, cx gets overwritten below
        pop     di              ; restore di, si
        pop     si
        pop     cx              ; cx = poly
        pop     ds              ; restore ds, bp
        pop     bp
        dec     bp              ; bp -= 1 (undo inc bp from above)
        retf
crc_cal endp