Gunfright: Routine at 54525

Used by the routine at InitialiseGame.

Builds ShiftTables, ReversedShiftTables and ReverseTable, which the routines for drawing sprites use. Each table is a page, which has an entry for each value of a byte at the low byte of the address. For a byte which is moved to the right along a row by six pixels, page 254 has the part which stays in the byte and page 255 has the part which moves into the next byte. Pages 252 and 253 are the same for four pixels, and pages 250 and 251 for two. Pages 246 and 247 244 and 245 and 242 and 243 are the same for the byte with its bits turned round, which is used for sprites which are reflected. ReverseTable has each byte with its bits turned round.

Used by the routine at InitialiseGame. Builds ShiftTables, ReversedShiftTables and ReverseTable, which the routines for drawing sprites use. Each table is a page, which has an entry for each value of a byte at the low byte of the address. For a byte which is moved to the right along a row by six pixels, page 254 has the part which stays in the byte and page 255 has the part which moves into the next byte. Pages 252 and 253 are the same for four pixels, and pages 250 and 251 for two. Pages 246 and 247 244 and 245 and 242 and 243 are the same for the byte with its bits turned round, which is used for sprites which are reflected. ReverseTable has each byte with its bits turned round.
Build ShiftTables.
BuildShiftTables	54525	LD L,0	Start with the byte at 0.
BuildShiftTables_Normal	54527	LD D,0	Make the word the byte.
	54529	LD E,L	Make the word the byte.
	54530	LD H,255	Start at page 255.
	54532	LD B,3	Set a count of 3 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_NormalLoop	54534	SLA E	Shift the word left by two bits.
	54536	RL D
	54538	SLA E
	54540	RL D
	54542	LD (HL),E	Store the pair in the next two pages.
	54543	DEC H
	54544	LD (HL),D
	54545	DEC H
	54546	DJNZ BuildShiftTables_NormalLoop	Jump to BuildShiftTables_NormalLoop if there are any left.
	54548	INC L	Move on to the next byte.
	54549	JR NZ,BuildShiftTables_Normal	Jump to BuildShiftTables_Normal if there are any left.
Build ReversedShiftTables.
	54551	LD L,0	Start with the byte at 0.
BuildShiftTables_Reversed	54553	LD D,0	Clear the high byte of the word.
	54555	LD B,8	Set a count of 8 bits, and copy the byte into A.
	54557	LD A,L	Set a count of 8 bits, and copy the byte into A.
Turn the bits of the byte round.
BuildShiftTables_Reverse	54558	RRCA	Move the next bit into C from the other end.
	54559	RL C	Move the next bit into C from the other end.
	54561	DJNZ BuildShiftTables_Reverse	Jump to BuildShiftTables_Reverse if there are any left.
	54563	LD E,C	Make the low byte the reversed byte.
	54564	LD H,247	Start at page 247.
	54566	LD B,3	Set a count of 3 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_ReversedLoop	54568	SLA E	Shift the word left by two bits.
	54570	RL D
	54572	SLA E
	54574	RL D
	54576	LD (HL),E	Store the pair in the next two pages.
	54577	DEC H
	54578	LD (HL),D
	54579	DEC H
	54580	DJNZ BuildShiftTables_ReversedLoop	Jump to BuildShiftTables_ReversedLoop if there are any left.
	54582	INC L	Move on to the next byte.
	54583	JR NZ,BuildShiftTables_Reversed	Jump to BuildShiftTables_Reversed if there are any left.
Build ReverseTable.
	54585	LD HL,63744	Point HL at ReverseTable.
BuildShiftTables_ReverseTable	54588	LD D,L	Copy the byte into D, and set a count of 8 bits.
	54589	LD B,8	Copy the byte into D, and set a count of 8 bits.
BuildShiftTables_ReverseBits	54591	SRL D	Move the lowest bit of the byte into the other end of E.
	54593	RL E	Move the lowest bit of the byte into the other end of E.
	54595	DJNZ BuildShiftTables_ReverseBits	Jump to BuildShiftTables_ReverseBits if there are any left.
	54597	LD (HL),E	Store the byte turned round.
	54598	INC L	Move on to the next byte.
	54599	JR NZ,BuildShiftTables_ReverseTable	Jump to BuildShiftTables_ReverseTable if there are any left.
	54601	RET	Return.
View the equivalent code in; Nightshade