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一开始我必须说我是 mips 汇编语言以及 Stackoverflow 的新手。

我有 mips 汇编语言的工作代码，它使用给定文本文件的查找表计算 crc 32，我想更改它以计算 crc 16 和 crc 8。

我几乎可以肯定，对于所有情况都正确生成了查找表：crc 32、crc 16 和 crc 8。我知道我应该更改例如 crc 16 的初始化值0xffff，但这还不够。我认为问题在于更改此值后，该算法从查找表中获取了错误的索引，对吗？

提前感谢您的帮助。

##### This subroutine first generates 256 words crc32 table for ASCII codes and then computes the actual crc32 checksum of the file
input:
$a1 = block
$v0 = length
output:
$v0 = crc32 checksum

crc32_checksum:
    li      $t0, 0xedb88320         # CRC32 code generator
    #li     $t0, 0xa001     # CRC16 code generator
    #li     $t0, 0x8c       # CRC8 code generator

    la      $t1, crc_tab        # address of the table to fill in

    li      $t2, 0      # load 0 into $t2

tab_gen:
    move    $t3, $t2        # move counter of 8 digit hex values packed into table to $t3

    li  $t4, 8      # digit/bit counter equals 8

tab_byte:
    and     $t5, $t3, 1     # AND data with 1
    beqz    $t5, shift      # branch to 'shift' if equal 0

    srl     $t3, $t3, 1     # shift right data
    xor     $t3, $t3, $t0   # XOR both values (shifted data with CRC polynomial)
    b       next        # branch to next 

shift:
    srl     $t3, $t3, 1     # shift right data

next:
    sub     $t4, $t4, 1     # decrese digit/bit counter
    bnez    $t4, tab_byte   # branch if byte/bit counter is not equal to zero

    sw      $t3, 0($t1)     # store 8 digit hex value
    add     $t1, $t1, 4     # move to the next address to be fill


    add     $t2, $t2, 1     # increase counter of 8 digit hex values packed into table
    bltu    $t2, 256, tab_gen   # branch until 256 8 digit hex values are packed into table


#### # Calculate the actual CRC32

    li      $t0, 0xffffffff # initialize crc value for CRC32 code
    #li     $t0, 0x0000     # initialize crc value for CRC16 code
    #li     $t0, 0xffff     # initialize crc value for CRC16 code
    #li     $t0, 0xff       # initialize crc value for CRC8 code

    la      $t1, crc_tab        # point to crc_tab

crc32:
    lbu     $t2, 0($a1)     # load byte of data
    add     $a1, $a1, 1     # advance the data pointer
    xor     $t2, $t2, $t0   # byte of data XOR with crc
    and     $t2, $t2, 0xff  # (byte of data XOR with crc) AND with 0xff (to produce a table index)
    sll     $t2, $t2, 2         # scale (*4) the index because of addressing 32-bit words
    add     $t2, $t2, $t1   # form the final address in the table

    lw      $t2, 0($t2)     # load a value from the table
    srl     $t3, $t0, 8     # crc shifted 8 bits right
    xor     $t0, $t2, $t3   # XOR both values (i.e. shifted crc and the value read from the table)


    sub     $v0, $v0, 1     # decrement the byte counter
    bnez    $v0, crc32      # repeat untill all bytes of data are processed

    not     $v0, $t0        # invert all bits of final crc

    move    $t7, $v0

    jr      $ra     # jump to return address
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1 回答 1

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First, I suggest that you better define your goals. There is no single "CRC 16" nor single "CRC 8". Take a look at this table, and then decide which flavor of CRC16 and CRC8 you wish to implement. (Without additional information, I would guess that you are interested in the CCITT versions. They are most prevelant in my experience.)

Once you know where you are going, try googling "ccitt 16 pseudocode" for some tips on the 16-bit algorithm. The Dr. Dobbs link is good, as well as this one and this one (although it is geared more toward hardware implementations). The 8-bit algorithm can be researched similarly. Try "ccitt 8 algorithm". This link is reasonable.

Once you gain a grasp of the pseudocode, then you're in a better position to implement it in assembler.

于 2014-01-07T02:10:02.687 回答