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FFD4: Decrypt a data block using bit shifting
Used by the routine at FF70.
Input
HL Start address
BC Length
FFD4 LD A,(HL) Get next byte
FFD5 RRD Shift bits and store it
FFD7 INC HL Point to next byte
FFD8 DEC BC Decrease length
FFD9 LD A,B Is length 0?
FFDA OR C
FFDB JR NZ,$FFD4 No, loop back for more
FFDD RET All done
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