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def calculate_checksum(block):
    """
    Calculate the checksum for a 256 bytes block
    """
    # Constants
    SEED_CRC = 0xFFFF    # Seed for CRC
    MASK_CRC = 0x1021    # Mask of CRC-16-CCITT (ISO 3309 / x^16 + x^12 + x^5 + 1)

    crc_tmp = SEED_CRC   # Initialize CRC
    for i in block:
        crc_tmp = crc_tmp ^ (ord(i) << 8)
        for j in range(8):
            if (crc_tmp & 0x8000):
                crc_tmp = (crc_tmp << 1) ^ MASK_CRC
            else:
                crc_tmp = crc_tmp << 1
    crc_tmp = (crc_tmp & 0xFFFF) ^ 0xFFFF  # Make the CRC ones' complement
   
    return chr(crc_tmp // 256) + chr(crc_tmp % 256)

Code: Alles auswählen

#CRC16POLY = $1021

Procedure CRC16(*MEMStart.BYTE, Length)
	Protected P, k, MyCRC = $FFFF
	;
	While Length
		P = *MEMStart\b & $FF
		For k = 1 To 8
			If (P ! MyCRC) & 1 = 1
				MyCRC >> 1
				MyCRC ! #CRC16POLY
				MyCRC & $FFFF
			Else
				MyCRC >> 1
			EndIf
			P >> 1
		Next
		*MEMStart + 1
		Length - 1
	Wend

	ProcedureReturn MyCRC
EndProcedure