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Elite on the BBC Micro and NES

Copy protection: crunchit

[Acorn Electron version, Loader]

Name: crunchit [Show more] Type: Subroutine Category: Copy protection Summary: Multi-byte decryption and copying routine
Context: See this subroutine in context in the source code References: This subroutine is called as follows: * Elite loader (Part 2 of 5) calls crunchit * Elite loader (Part 3 of 5) calls crunchit

In the unprotected version of the loader on this site, this routine just moves data from one location to another. In the protected version, it also decrypts the data as it is moved, but that part is disabled in the following. Arguments: (X Y) The number of bytes to copy ZP(1 0) The source address P(1 0) The destination address
.crunchit LDA (ZP),Y \ Copy the Y-th byte of ZP(1 0) to the Y-th byte of NOP \ P(1 0), without any decryption (hence the NOPs) NOP NOP STA (P),Y DEY \ Decrement the byte counter BNE crunchit \ Loop back to crunchit to copy the next byte until we \ have done a whole page INC P+1 \ Increment the high bytes of the source and destination INC ZP+1 \ addresses so we can copy the next page DEX \ Decrement the page counter BNE crunchit \ Loop back to crunchit to copy the next page until we \ have done X pages RTS \ Return from the subroutine