\ --- Mod: Code removed for Demonstration Disc: -------> \.CHECK \ \LDX #NT%-2 \ Set X to the size of the commander data block, less \ \ 2 (to omit the checksum bytes and the save count) \ \CLC \ Clear the C flag so we can do addition without the \ \ C flag affecting the result \ \TXA \ Seed the checksum calculation by setting A to the \ \ size of the commander data block, less 2 \ \ \ We now loop through the commander data block, \ \ starting at the end and looping down to the start \ \ (so at the start of this loop, the X-th byte is the \ \ last byte of the commander data block, i.e. the save \ \ count) \ \.QUL2 \ \ADC NA%+7,X \ Add the X-1-th byte of the data block to A, plus the \ \ C flag \ \EOR NA%+8,X \ EOR A with the X-th byte of the data block \ \DEX \ Decrement the loop counter \ \BNE QUL2 \ Loop back for the next byte in the calculation, until \ \ we have added byte #0 and EOR'd with byte #1 of the \ \ data block \ \RTS \ Return from the subroutine \ --- End of removed code ----------------------------->Name: CHECK, Removed [Show more] Type: Subroutine Category: Save and load Summary: Calculate the checksum for the last saved commander data block Deep dive: Commander save filesContext: See this subroutine in context in the source code References: No direct references to this subroutine in this source file
The checksum for the last saved commander data block is saved as part of the commander file, in two places (CHK AND CHK2), to protect against file tampering. This routine calculates the checksum and returns it in A. This algorithm is also implemented in elite-checksum.py.
Returns: A The checksum for the last saved commander data block