Cyberun: Routine at 23424

The loader copies this routine here and runs it once the encrypted game has been loaded to 24192. The helper at 24576 copies this stub here and the move routine to 64512. It first moves the game down to its home at 23680 (see CopyGameDown) and then decrypts it, one byte at a time and from the top down, by running RRD over 40448 bytes starting at 64000. Each RRD passes a nibble along from one byte to the next, so the whole game is stored shifted by half a byte. Control then goes to StartGame. Once the game is running the space up to 23551 is reused, for the records of the segments of the four tiles that DrawTerrain makes (starting at 23424, 23461, 23498 and 23535). That routine also sets the stack pointer to 23418.

The loader copies this routine here and runs it once the encrypted game has been loaded to 24192. The helper at 24576 copies this stub here and the move routine to 64512. It first moves the game down to its home at 23680 (see CopyGameDown) and then decrypts it, one byte at a time and from the top down, by running RRD over 40448 bytes starting at 64000. Each RRD passes a nibble along from one byte to the next, so the whole game is stored shifted by half a byte. Control then goes to StartGame. Once the game is running the space up to 23551 is reused, for the records of the segments of the four tiles that DrawTerrain makes (starting at 23424, 23461, 23498 and 23535). That routine also sets the stack pointer to 23418.
Set up the stack and move the game down into place.
GameEntry	23424	LD SP,65024	Set the stack pointer to 65024.
	23427	DI	Disable interrupts.
	23428	CALL CopyGameDown	Call CopyGameDown to move the game down to 23680.
Decrypt every byte from 64000 down to 23553.
	23431	LD HL,64000	Set the start address in HL to 64000.
	23434	XOR A	Clear the nibble carried between bytes in A.
	23435	LD BC,40448	Set the byte count in BC to 40448.
Decryption_Loop	23438	RRD	Rotate the nibbles of the current byte through A.
	23440	DEC HL	Move down to the previous byte.
	23441	DEC C	Decrease the low byte of the counter by one and loop back to Decryption_Loop until none remain.
	23442	JR NZ,Decryption_Loop
	23444	DJNZ Decryption_Loop	Decrease the high byte of the counter by one and loop back to Decryption_Loop until every byte has been decrypted.
	23446	JP StartGame	Jump to StartGame.