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Elite on the BBC Micro

Save and load: CHECK [Elite-A, Parasite]

Name: CHECK [Show more] Type: Subroutine Category: Save and load Summary: Calculate the checksum for the last saved commander data block Deep dive: Commander save files
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * DFAULT calls CHECK * SVE calls CHECK

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
.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) SEC \ Set the C flag to increase the checksum value by 1, \ so the Elite-A checksum is subtly different to the \ standard version's checksum 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