Everyone's A Wally: Routine at FEDE

FEDE: Main loader (2)


FEDE	LD L,$01
FEE0	CALL $FEA5
FEE3	JP NC,$0000
FEE6	LD A,$C6
FEE8	CP B
FEE9	RL L
FEEB	LD B,$B0
FEED	JP NC,$FEE0
FEF0	LD A,H
FEF1	XOR L
FEF2	LD H,A
FEF3	LD A,D
FEF4	OR E
FEF5	JR NZ,$FEC7
FEF7	LD HL,$4000
FEFA	LD BC,$BF24
FEFD	LD A,$00
FEFF	ADD A,(HL)
FF00	LD E,A
FF01	INC HL
FF02	DEC BC
FF03	LD A,B
FF04	OR C
FF05	LD A,E
FF06	JR NZ,$FEFF
FF08	CP (HL)
FF09	JP NZ,$0000
The rest of the code is not present in the BASIC loader and only loaded after the end of the main tape block.
FF0C	LD DE,$5B80	Point DE at the start of the block to decrypt. (5B80)
FF0F	LD HL,$53FE	Set the intial key in HL to 53FE.
FF12	LD A,(DE)	Get the next byte to decrypt.
FF13	XOR (HL)	Invert the bits against the key.
FF14	RES 4,H	Reset bit 4 in the key.
FF16	XOR (HL)	Invert the bits again.
FF17	SET 4,H	Set it back again ready for the next pass.
FF19	LD (DE),A	Store the decrypted byte.
FF1A	DEC HL	Decrease the key value.
FF1B	INC DE	Increase the address to look at.
FF1C	LD A,E	Has the last address (5B9A) been reached yet?
FF1D	CP $9A	Has the last address (5B9A) been reached yet?
FF1F	JR NZ,$FF12	Jump back if not.
FF21	JP $5B94	Otherwise run the main decrypter.