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Elite on the BBC Micro and NES

Loader: Elite loader (Part 5 of 6)

[BBC Micro cassette version, Loader]

Name: Elite loader (Part 5 of 6) [Show more] Type: Subroutine Category: Loader Summary: Load main game code, decrypt it, move it to the correct location
Context: See this subroutine in context in the source code References: No direct references to this subroutine in this source file

This part loads the main game code, decrypts it and moves it to the correct location for it to run. The code in this part is encrypted by and is decrypted in part 4 by the same routine that moves part 6 onto the stack.
.ENTRY2 \ We start this part of the loader by setting the \ following: \ \ OSPRNT(1 0) = WRCHV \ WRCHV(1 0) = TT26 \ (Y X) = MESS1(1 0) \ \ so any character printing will use the TT26 routine LDA &020E \ Copy the low byte of WRCHV to the low byte of OSPRNT STA OSPRNT LDA #LO(TT26) \ Set the low byte of WRCHV to the low byte of TT26 STA &020E LDX #LO(MESS1) \ Set X to the low byte of MESS1 LDA &020F \ Copy the high byte of WRCHV to the high byte of OSPRNT STA OSPRNT+1 LDA #HI(TT26) \ Set the high byte of WRCHV to the high byte of TT26 LDY #HI(MESS1) \ and set Y to the high byte of MESS1 STA &020F JSR AFOOL \ This calls AFOOL, which jumps to the address in FOOLV, \ which contains the address of FOOL, which contains an \ RTS instruction... so overall this does nothing, but \ in a rather roundabout fashion JSR command \ Call command to execute the OSCLI command pointed to \ by (Y X) in MESS1, which starts loading the main game \ code JSR 512-LEN+CHECKER-ENDBLOCK \ Call the CHECKER routine in its new location on \ the stack, to run a number of checksums on the \ code (this routine, along with the whole of part \ 6, was pushed onto the stack in part 4) JSR AFOOL \ Another call to the round-the-houses routine to try \ and distract the crackers, presumably IF DISC LDA #140 \ Call OSBYTE with A = 140 and X = 12 to select the LDX #12 \ tape filing system (i.e. do a *TAPE command) JSR OSBYTE ENDIF LDA #0 \ Set SVN to 0, as the main game code checks the value STA SVN \ of this location in its IRQ1 routine, so it needs to \ be set to 0 so it can work properly once it takes over \ when the game itself runs \ We now decrypt and move the main game code from &1128 \ to &0F40 LDX #HI(LC%) \ Set X = high byte of LC%, the maximum size of the main \ game code, so if we move this number of pages, we will \ have definitely moved all the game code down LDA #LO(L%) \ Set ZP(1 0) = L% (the start of the game code) STA ZP LDA #HI(L%) STA ZP+1 LDA #LO(C%) \ Set P(1 0) = C% = &0F40 STA P LDA #HI(C%) STA P+1 LDY #0 \ Set Y as a counter for working our way through every \ byte of the game code. We EOR the counter with the \ current byte to decrypt it .ML1 TYA \ Copy the counter into A IF _REMOVE_CHECKSUMS LDA (ZP),Y \ If we have disabled checksums, just fetch the byte to \ copy from the Y-th block pointed to by ZP(1 0) ELSE EOR (ZP),Y \ Fetch the byte and EOR it with the counter ENDIF STA (P),Y \ Store the copied (and decrypted) byte in the Y-th byte \ of the block pointed to by P(1 0) INY \ Increment the loop counter BNE ML1 \ Loop back for the next byte until we have finished the \ first 256 bytes INC ZP+1 \ Increment the high bytes of both ZP(1 0) and P(1 0) to INC P+1 \ point to the next 256 bytes DEX \ Decrement the number of pages we need to copy in X BPL ML1 \ Loop back to copy and decrypt the next page of bytes \ until we have done them all \ S% points to the entry point for the main game code, \ so the following copies the addresses from the start \ of the main code (see the S% label in the main game \ code for the vector values) LDA S%+6 \ Set BRKV to point to the BR1 routine in the main game STA &0202 \ code LDA S%+7 STA &0203 LDA S%+2 \ Set WRCHV to point to the TT26 routine in the main STA &020E \ game code LDA S%+3 STA &020F RTS \ This RTS actually does a jump to the first instruction \ in BLOCK, after the two EQUW operatives, which is now \ on the stack. This takes us to the next and final \ step of the loader in part 6. See the documentation \ for the stack routine at BEGIN% for more details .AFOOL JMP (FOOLV) \ This jumps to the address in FOOLV as part of the \ JSR AFOOL instruction above, which does nothing except \ take us on wild goose chase