Routines
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FF70: Main decrypter
Used by the routine at FE45.
FF70 LD A,$00 Set the border to black
FF72 OUT ($FE),A
FF74 CALL $FE1F Get hash from the screen
FF77 LD HL,$FE43 Does it equal the value in FE43?
FF7A CP (HL)
FF7B JP Z,$FF89 Yes, so move on
FF7E LD HL,$EE48 Hash is wrong. There's a loading error. Wipe the memory, which will lead to the code at 0000 to be executed, causing a reset.
FF81 LD BC,$FFFF
FF84 LD DE,$EE49
FF87 LDIR
FF89 LD HL,$FFA3 Start of block to decrypt
FF8C LD BC,$0045 Length of block
FF8F LD A,D D is 12, set up by the code at FE6F.
FF90 XOR (HL) Flip bits
FF91 LD (HL),A Store decrypted value
FF92 INC HL Point to next byte
FF93 DEC BC Decrease length
FF94 LD A,B Is length 0?
FF95 OR C
FF96 JP NZ,$FF8F Loop back if not
FF99 CALL $FE31 Get hash from memory
FF9C LD HL,$FE44 Does it equal the value in FE44?
FF9F CP (HL)
FFA0 JP NZ,$FF7E If not, jump back to routine that resets the machine
After the decrypter has run, the next block becomes executable code
FFA3 LD HL,$5DC0 Start of block
FFA6 LD BC,$7530 Length
FFA9 CALL $FFD4 Decrypt it
FFAC LD HL,$5DC0 Start of block
FFAF LD BC,$7530 Length
FFB2 CALL $FFDE Decrypt it
FFB5 LD HL,$FA1C Move the game into the right place
FFB8 LD DE,$FF1C
FFBB LD BC,$9F1D
FFBE LDDR
FFC0 LD HL,$A710 Set value to calculate start position
FFC3 LD ($5C36),HL Set CHARS to this value - pretty meaningless if using custom character set
FFC6 LD BC,$DF10 Calculate offset to change HL by - -20F0
FFC9 XOR A Reset carry flag
FFCA SBC HL,BC Subtract negative offset (ie: adding it) to give C800
FFCC LD SP,$FFFF Put the stack pointer somewhere safe
FFCF IM 1 Turn interrupts back on
FFD1 JP $006F 006F contains JP (HL) (part of the ROM's NMI handler)
Control passes to the game's main entry point at C800
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