Calculating the checksum for a *.83p file.


The checksum is a total of all the data in the 83p file, from byte 56 onward (excluding the actual checksum), mod 2^16.
It is a word at the very end of the 83p file.
It is not sent to the calculator, but used by the link program on the computer.
Most computer/calculator link programs will generate an "Invalid File Format" error if the checksum is incorrect.


Example code in BASIC: 
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FOR bytenumber = 56 TO (end of data)
checksum = checksum + ASC(bytenumber) 'ASC: returns decimal value of byte (0-255) - its ASCII character code
NEXT

checksum = checksum MOD 2^16  'using modulas: checksum is divided by 2^16 and the remainder is returned
'the remainder will be between 0 and 65535

chkhi = int(checksum / 256)  'calculate the high-byte of checksum
chklo = checksum MOD 256  'and the low-byte

'write chklo followed by chkhi to the end of the 83p file (one byte each = one word).
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