RS/6000 nvram checksum algorithm

rs6000_checksum.py

def rol(r,n):
    return (r << n) | (r >> (32 - n))

def crcgen(r3,r4):
    r6 = rol(r3,8) & 0xff00
    r0 = rol(r4,0) & 0xff
    r3 = rol(r3,24) & 0xffffff
    r4 = r3 ^ r0
    r0 = rol(r4,8) & 0xffffff00
    r0 = rol(r0,20) & 0xfffff
    r7 = rol(r4,5) & 0x1fe0
    r8 = rol(r4,12) & 0xfffff000
    r0 = r0 | r8
    r3 = rol(r4,1) & 0x1e0
    r0 = r0 & 0xf00f
    r5 = rol(r4,8) & 0xf000
    r4 = r4 | r6
    r0 = r0 ^ r4
    r0 = r7 ^ r0
    r0 = r5 ^ r0
    r3 = r3 ^ r0
    return r3

def checksum(location,data):
    checksum = 0xffff
    for i,b in enumerate(data):
        offset = location + i
        if offset >= 4 and offset < 8:
            continue
        checksum = crcgen(checksum, b)
    return checksum

if __name__ == "__main__":
    import sys

    file_data = open(sys.argv[1], 'rb').read()
    print(hex(checksum(0x1fd1, file_data[0x1fd1:0x2000])))
    print(hex(checksum(0, file_data[:0x1a00])))