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BBC Micro Elite

Copy protection: BEGIN%

Name: BEGIN% [View in context] Type: Subroutine Category: Copy protection Summary: Single-byte decryption and copying routine, run on the stack
This routine is copied to the stack at &01F1. It pushes BLOCK to ENDBLOCK onto the stack, and decrypts the code from TUT onwards. The 15 instructions for this routine are pushed onto the stack and executed there. The instructions are pushed onto the stack in reverse (as the stack grows downwards in memory), so first the JMP gets pushed, then the STA, and so on. This is the code that is pushed onto the stack. It gets run by a JMP call to David2, which then calls the routine on the stack with JSR &01F1. 01F1 : PLA \ Remove the return address from the stack that was 01F2 : PLA \ put here by the JSR that called this routine 01F3 : LDA BLOCK,Y \ Set A = the Y-th byte of BLOCK 01F6 : PHA \ Push A onto the stack 01F7 : EOR TUT,Y \ EOR the Y-th byte of TUT with A 01FA : STA TUT,Y 01FD : JMP (David9) \ Jump to the address in David9 The routine is called inside a loop with Y as the counter. It counts from 0 to ENDBLOCK - BLOCK, so the routine eventually pushes every byte between BLOCK and ENDBLOCK onto the stack, as well as EOR'ing each byte from TUT onwards to decrypt that section. The elite-checksums.py script reverses the order of the bytes between BLOCK and ENDBLOCK in the final file, so pushing them onto the stack (which is a descending stack) realigns them in memory as assembled below. Not only that, but the last two bytes pushed on the stack are the ones that are at the start of the block at BLOCK, and these contain the address of ENTRY2. This is why the RTS at the end of part 4 above actually jumps to ENTRY2 in part 5.
.BEGIN% EQUB HI(David9) \ JMP (David9) EQUB LO(David9) EQUB &6C EQUB HI(TUT) \ STA TUT,Y EQUB LO(TUT) EQUB &99 IF _REMOVE_CHECKSUMS EQUB HI(TUT) \ LDA TUT,Y EQUB LO(TUT) EQUB &B9 ELSE EQUB HI(TUT) \ EOR TUT,Y EQUB LO(TUT) EQUB &59 ENDIF PHA \ PHA EQUB HI(BLOCK) \ LDA BLOCK,Y EQUB LO(BLOCK) EQUB &B9 PLA \ PLA PLA \ PLA