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Routines |
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The stub which the tape loader runs once the game has been loaded at 24576. It decrypts the whole game, 32768 bytes, two bytes at a time, and then moves the game down by 512 bytes to its real home at GameStart before starting it.
The tape loader sets up this stub, and the game checks for the loader's signature in the frame counter (see InitialiseGame).
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Set up the first pair and the number of pairs to decrypt.
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| DecryptGame | 23424 | LD DE,24576 | Set DE to 24576, the address of the first byte of the pair to decrypt. | |
| 23427 | LD HL,24577 | Set HL to 24577, the address of the second byte of the pair. | ||
| 23430 | LD BC,16384 | Set BC to 16384, the number of byte pairs to decrypt. | ||
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Start rotating each byte through A to decrypt it.
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| DecryptGame_Loop | 23433 | LD A,(DE) | Fetch the first byte of the pair. | |
| 23434 | RLD | Rotate the nibbles of the two bytes through A. | ||
| 23436 | LD (DE),A | Write the decrypted byte back. | ||
| 23437 | INC DE | Move on to the first byte of the next pair. | ||
| 23438 | INC DE | |||
| 23439 | INC HL | Move HL onto the second byte of the next pair. | ||
| 23440 | INC HL | |||
| 23441 | DEC BC | Decrease the pair counter by one. | ||
| 23442 | LD A,B | Loop back to DecryptGame_Loop until every pair has been decrypted. | ||
| 23443 | OR C | |||
| 23444 | JR NZ,DecryptGame_Loop | |||
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Move the decrypted game down to its real address and start it.
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| 23446 | LD HL,24576 | Copy 32768 bytes of the game from 24576 to GameStart. | ||
| 23449 | LD DE,24064 | |||
| 23452 | LD BC,32768 | |||
| 23455 | LDIR | |||
| 23457 | JP GameStart | Jump to GameStart to start the game. | ||
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View the equivalent code in;
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