Nightshade: Routine at 5B80

The stub that the tape loader runs after loading the game, which arrives scrambled at 6000 (8800 bytes of it). It unscrambles the game in place, moves it down by 0200 bytes so that it starts at GameEntry, and jumps there.

Each pair of bytes is unscrambled by a single RLD: the low nibble of the first byte of the pair is replaced by the high nibble of the second byte, the high nibble of the second byte by its low nibble, and the low nibble of the second byte by the old low nibble of the first byte.

The routine also sets bit 7 of the memory refresh register R. Nothing in the game ever clears that bit, and the game resets the computer if it is clear when the check at BuildRandomTable runs, which defeats any attempt to start the game without going through this stub. The random number generator at RandomNumber also reads R, so that bit keeps those values in the range 80 to FF.

The stub that the tape loader runs after loading the game, which arrives scrambled at 6000 (8800 bytes of it). It unscrambles the game in place, moves it down by 0200 bytes so that it starts at GameEntry, and jumps there. Each pair of bytes is unscrambled by a single RLD: the low nibble of the first byte of the pair is replaced by the high nibble of the second byte, the high nibble of the second byte by its low nibble, and the low nibble of the second byte by the old low nibble of the first byte. The routine also sets bit 7 of the memory refresh register R. Nothing in the game ever clears that bit, and the game resets the computer if it is clear when the check at BuildRandomTable runs, which defeats any attempt to start the game without going through this stub. The random number generator at RandomNumber also reads R, so that bit keeps those values in the range 80 to FF.
Set bit 7 of the memory refresh register for the random number generator.
Decryption	5B80	DI	Disable interrupts.
	5B81	LD A,R	Fetch the memory refresh register into A.
	5B83	OR %10000000	Set bit 7. The Z80 leaves that bit as it is written, so RandomNumber always mixes a value from 80 to FF into the random number.
	5B85	LD R,A	Write A back to the memory refresh register.
Unscramble the whole game, a pair of bytes at a time.
	5B87	LD DE,$6000	Set DE to 6000, the first byte of the first pair.
	5B8A	LD HL,$6001	Set HL to 6001, the second byte of the first pair.
	5B8D	LD BC,$4400	Set a pair counter in BC to 4400.
Decryption_Loop	5B90	LD A,(DE)	Fetch the first byte of the pair.
	5B91	RLD	Rotate the nibbles of the pair.
	5B93	LD (DE),A	Write the unscrambled first byte back.
	5B94	INC DE	Move DE on to the first byte of the next pair.
	5B95	INC DE	Move DE on to the first byte of the next pair.
	5B96	INC HL	Move HL on to the second byte of the next pair.
	5B97	INC HL	Move HL on to the second byte of the next pair.
	5B98	DEC BC	Decrease the pair counter by one.
	5B99	LD A,B	Jump to Decryption_Loop until every pair has been unscrambled.
	5B9A	OR C
	5B9B	JR NZ,Decryption_Loop
Move the unscrambled game down so that it starts at GameEntry.
	5B9D	LD HL,$6000	Move 8800 bytes from 6000 down to GameEntry.
	5BA0	LD DE,$5E00
	5BA3	LD BC,$8800
	5BA6	LDIR
	5BA8	JP GameEntry	Jump to GameEntry to start the game.
View the equivalent code in; Gunfright Lunar Jetman