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BBC Micro Elite

Loader: S% (6502SP version)

Name: S% [View in context] Type: Subroutine Category: Loader Summary: Checksum, decrypt and unscramble the main game code, and start the game
This routine reverses the three copy protection mechanisms that the Big Code File puts in place: the checksum in Checksum, the encryption in DEEOR, and the code reversal in do65c02. In the BeebAsm version here, these three protections are applied by, and the original 6502 assembly language versions of the three encryption routines can be found in the elite-checksum.asm file. It also adds in a bit of Tube-specific copy protection, by transmitting the do65c02 routine over the Tube before it is run. It's very crafty stuff!
EQUD 0 \ These bytes appear to be unused RTS \ The checksum byte goes here, at S%-1. In the original \ source this byte is set by the first call to ZP in the \ Big Code File, though in the BeebAsm version this is \ populated by .S% CLD \ Clear the D flag to make sure we are in binary mode SEC \ Set the C flag LDA #LO(G%) \ Set (1 0) = SC(1 0) = G% STA 0 STA SC LDA #HI(G%) STA 1 STA SC+1 LDA #LO(F%-1) \ Set (3 2) = F% - 1 STA 2 LDA #HI(F%-1) STA 3 LDX #LO(prtblock) \ Set (Y X) to point to the prtblock parameter block LDY #HI(prtblock) LDA #249 \ Send an OSWORD 249 command to the I/O processor, which JSR OSWORD \ copies the code of the do65c02 routine from the I/O \ processor to prtblock+2 LDX #SC \ Set X = SC, and because SC is in zero page, this means \ that X contains the whole value of SC, so jumping to \ (X), for example, would jump to the address in SC(1 0) EQUB &AD \ This is the opcode for an LDA absolute instruction, so \ it converts the two OSWORD size bytes at prtblock into \ a harmless LDA &2702 instruction .prtblock EQUB 2 \ The number of bytes to transmit with this command EQUB &27 \ The number of bytes to receive with this command JMP (SC,X) \ This block, between here and G%, is overwritten by the PHP \ code of the do65c02 routine from the I/O processor, so PHY \ this code is never run and is presumably just here to LDA #&34 \ throw the crackers off the scent - it just needs to be PHA \ at least as long as the do65c02 routine, which ends LDX #0 \ with a jump to G% below to start the game RTS BRK EQUS "ELITE - By Ian Bell & David Braben" EQUB 10 EQUB 13 BRK LDA SC ADC 2 CMP F%-1 BNE P%-2 EQUD &7547534 EQUD &452365 EQUB &8D .G% JSR DEEOR \ Decrypt the main game code between &1300 and &9FFF JSR COLD \ Copy the recursive tokens and ship blueprints to their \ correct locations JSR Checksum \ Checksum the code from &1000 to &9FFF and check it \ against S%-1 JMP BEGIN \ Jump to BEGIN to start the game NOP \ This instruction is not used