Cyberun: Routine at 5B80

The loader copies this routine here and runs it once the encrypted game has been loaded to 5E80. The helper at 6000 copies this stub here and the move routine to FC00. It first moves the game down to its home at 5C80 (see CopyGameDown) and then decrypts it, one byte at a time and from the top down, by running RRD over 9E00 bytes starting at FA00. 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 5BFF is reused, for the records of the segments of the four tiles that DrawTerrain makes (starting at 5B80, 5BA5, 5BCA and 5BEF). That routine also sets the stack pointer to 5B7A.

The loader copies this routine here and runs it once the encrypted game has been loaded to 5E80. The helper at 6000 copies this stub here and the move routine to FC00. It first moves the game down to its home at 5C80 (see CopyGameDown) and then decrypts it, one byte at a time and from the top down, by running RRD over 9E00 bytes starting at FA00. 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 5BFF is reused, for the records of the segments of the four tiles that DrawTerrain makes (starting at 5B80, 5BA5, 5BCA and 5BEF). That routine also sets the stack pointer to 5B7A.
Set up the stack and move the game down into place.
GameEntry	5B80	LD SP,$FE00	Set the stack pointer to FE00.
	5B83	DI	Disable interrupts.
	5B84	CALL CopyGameDown	Call CopyGameDown to move the game down to 5C80.
Decrypt every byte from FA00 down to 5C01.
	5B87	LD HL,$FA00	Set the start address in HL to FA00.
	5B8A	XOR A	Clear the nibble carried between bytes in A.
	5B8B	LD BC,$9E00	Set the byte count in BC to 9E00.
Decryption_Loop	5B8E	RRD	Rotate the nibbles of the current byte through A.
	5B90	DEC HL	Move down to the previous byte.
	5B91	DEC C	Decrease the low byte of the counter by one and loop back to Decryption_Loop until none remain.
	5B92	JR NZ,Decryption_Loop
	5B94	DJNZ Decryption_Loop	Decrease the high byte of the counter by one and loop back to Decryption_Loop until every byte has been decrypted.
	5B96	JP StartGame	Jump to StartGame.