Nightshade: Routine at 23424

The stub that the tape loader runs after loading the game, which arrives scrambled at 24576 (34816 bytes of it). It unscrambles the game in place, moves it down by 512 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 128 to 255.

The stub that the tape loader runs after loading the game, which arrives scrambled at 24576 (34816 bytes of it). It unscrambles the game in place, moves it down by 512 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 128 to 255.
Set bit 7 of the memory refresh register for the random number generator.
Decryption	23424	DI	Disable interrupts.
	23425	LD A,R	Fetch the memory refresh register into A.
	23427	OR %10000000	Set bit 7. The Z80 leaves that bit as it is written, so RandomNumber always mixes a value from 128 to 255 into the random number.
	23429	LD R,A	Write A back to the memory refresh register.
Unscramble the whole game, a pair of bytes at a time.
	23431	LD DE,24576	Set DE to 24576, the first byte of the first pair.
	23434	LD HL,24577	Set HL to 24577, the second byte of the first pair.
	23437	LD BC,17408	Set a pair counter in BC to 17408.
Decryption_Loop	23440	LD A,(DE)	Fetch the first byte of the pair.
	23441	RLD	Rotate the nibbles of the pair.
	23443	LD (DE),A	Write the unscrambled first byte back.
	23444	INC DE	Move DE on to the first byte of the next pair.
	23445	INC DE	Move DE on to the first byte of the next pair.
	23446	INC HL	Move HL on to the second byte of the next pair.
	23447	INC HL	Move HL on to the second byte of the next pair.
	23448	DEC BC	Decrease the pair counter by one.
	23449	LD A,B	Jump to Decryption_Loop until every pair has been unscrambled.
	23450	OR C
	23451	JR NZ,Decryption_Loop
Move the unscrambled game down so that it starts at GameEntry.
	23453	LD HL,24576	Move 34816 bytes from 24576 down to GameEntry.
	23456	LD DE,24064
	23459	LD BC,34816
	23462	LDIR
	23464	JP GameEntry	Jump to GameEntry to start the game.
View the equivalent code in; Gunfright Lunar Jetman