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Elite on the BBC Micro

Copy protection: Checksum [6502SP version]

Name: Checksum [Show more] Type: Subroutine Category: Copy protection Summary: Checksum the code from &1000 to &9FFF and check against S%-1
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * S% calls Checksum

In the original source, the checksum byte at S%-1 is set by the first call to ZP in the Big Code File, though in the BeebAsm version this is populated by elite-checksum.py. The original 6502 assembly language version of the ZP routine can be found in the elite-checksum.asm file.
.Checksum SEC \ Set the C flag, so it gets included in the checksum LDY #0 \ Set Y = 0, to act as a byte counter STY V \ Set V = 0 LDX #&10 \ Set X = &10, so we start with (X Y) = &1000 LDA (SC) \ This has no effect, as A is overwritten by the next \ instruction TXA \ Set A = &10 .CHKLoop STX V+1 \ Set V(1 0) = (X 0) STY T \ Set T = Y ADC (V),Y \ Set A = A + C + contents of (V(1 0) + Y) \ = A + C + contents of ((X 0) + Y) \ = A + C + contents of (X Y) EOR T \ Set A = A EOR Y SBC V+1 \ Set A = A - (1 - C) - X DEY \ Decrement the loop counter to process the next byte BNE CHKLoop \ Loop back until we have done the whole page INX \ Increment the page counter to point to the next page CPX #&A0 \ Loop back to do the next page until X = &A0, when BCC CHKLoop \ (X Y) = &A000 CMP S%-1 \ Compare the calculated checksum in A with the checksum \ stored in S%-1 IF _REMOVE_CHECKSUMS NOP \ If we have disabled checksums, then ignore the result NOP \ of the comparison and return from the subroutine ELSE BNE Checksum \ If the checksum we just calculated does not match \ the value in location S%-1, jump to Checksum to enter \ an infinite loop, which crashes the game ENDIF RTS \ Return from the subroutine