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BBC Micro Elite

Copy protection: DOMOVE

Name: DOMOVE [View in context] Type: Subroutine Category: Copy protection Summary: Multi-byte decryption and copying routine, run on the stack
This routine is copied to the stack at &01DF. It moves and decrypts a block of memory. The original source refers to the stack routine as MVDL. The 18 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 RTS gets pushed, then the BNE, and so on. This is the code that is pushed onto the stack. It gets run by a JMP call to crunchit, which then calls the routine on the stack at MVDL, or &01DF. The label MVDL comes from a comment in the original source file ELITES. 01DF : .MVDL 01DF : LDA (ZP),Y \ Set A = the Y-th byte from the block whose address \ is in ZP(1 0) 01E1 : EOR OSB,Y \ EOR A with the Y-th byte on from OSB 01E4 : STA (P),Y \ Store A in the Y-th byte of the block whose \ address is in P(1 0) 01E6 : DEY \ Decrement the loop counter 01E7 : BNE MVDL \ Loop back to copy and EOR the next byte until we \ have copied an entire page (256 bytes) 01E9 : INC P+1 \ Increment the high byte of P(1 0) so it points to \ the next page of 256 bytes 01EB : INC ZP+1 \ Increment ZP(1 0) so it points to the next page of \ 256 bytes 01ED : DEX \ Decrement X 01EE : BNE MVDL \ Loop back to copy the next page 01F0 : RTS \ Return from the subroutine, which takes us back \ to the caller of the crunchit routine using a \ tail call, as we called this with JMP crunchit We call MVDL with the following arguments: (X Y) The number of bytes to copy ZP(1 0) The source address P(1 0) The destination address The routine moves and decrypts a block of memory, and is used in part 3 to move blocks of code and images that are embedded within the loader binary, either into low memory locations below PAGE (for the recursive token table and page at UU%), or into screen memory (for the loading screen and dashboard images). If checksums are disabled in the build, we don't do the EOR instruction, so the routine just moves and doesn't decrypt.
.DOMOVE RTS \ RTS EQUW &D0EF \ BNE MVDL DEX \ DEX EQUB ZP+1 \ INC ZP+1 INC P+1 \ INC P+1 EQUB &E6 EQUW &D0F6 \ BNE MVDL DEY \ DEY EQUB P \ STA(P),Y EQUB &91 IF _REMOVE_CHECKSUMS NOP \ Skip the EOR if checksums are disabled NOP NOP ELSE EQUB HI(OSB) \ EOR OSB,Y EQUB LO(OSB) EQUB &59 ENDIF EQUB ZP \ LDA(ZP),Y EQUB &B1