Gunfright: Routine at D4FD

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 FE has the part which stays in the byte and page FF has the part which moves into the next byte. Pages FC and FD are the same for four pixels, and pages FA and FB for two. Pages F6 and F7 F4 and F5 and F2 and F3 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 FE has the part which stays in the byte and page FF has the part which moves into the next byte. Pages FC and FD are the same for four pixels, and pages FA and FB for two. Pages F6 and F7 F4 and F5 and F2 and F3 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	D4FD	LD L,$00	Start with the byte at 00.
BuildShiftTables_Normal	D4FF	LD D,$00	Make the word the byte.
	D501	LD E,L	Make the word the byte.
	D502	LD H,$FF	Start at page FF.
	D504	LD B,$03	Set a count of 03 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_NormalLoop	D506	SLA E	Shift the word left by two bits.
	D508	RL D
	D50A	SLA E
	D50C	RL D
	D50E	LD (HL),E	Store the pair in the next two pages.
	D50F	DEC H
	D510	LD (HL),D
	D511	DEC H
	D512	DJNZ BuildShiftTables_NormalLoop	Jump to BuildShiftTables_NormalLoop if there are any left.
	D514	INC L	Move on to the next byte.
	D515	JR NZ,BuildShiftTables_Normal	Jump to BuildShiftTables_Normal if there are any left.
Build ReversedShiftTables.
	D517	LD L,$00	Start with the byte at 00.
BuildShiftTables_Reversed	D519	LD D,$00	Clear the high byte of the word.
	D51B	LD B,$08	Set a count of 08 bits, and copy the byte into A.
	D51D	LD A,L	Set a count of 08 bits, and copy the byte into A.
Turn the bits of the byte round.
BuildShiftTables_Reverse	D51E	RRCA	Move the next bit into C from the other end.
	D51F	RL C	Move the next bit into C from the other end.
	D521	DJNZ BuildShiftTables_Reverse	Jump to BuildShiftTables_Reverse if there are any left.
	D523	LD E,C	Make the low byte the reversed byte.
	D524	LD H,$F7	Start at page F7.
	D526	LD B,$03	Set a count of 03 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_ReversedLoop	D528	SLA E	Shift the word left by two bits.
	D52A	RL D
	D52C	SLA E
	D52E	RL D
	D530	LD (HL),E	Store the pair in the next two pages.
	D531	DEC H
	D532	LD (HL),D
	D533	DEC H
	D534	DJNZ BuildShiftTables_ReversedLoop	Jump to BuildShiftTables_ReversedLoop if there are any left.
	D536	INC L	Move on to the next byte.
	D537	JR NZ,BuildShiftTables_Reversed	Jump to BuildShiftTables_Reversed if there are any left.
Build ReverseTable.
	D539	LD HL,$F900	Point HL at ReverseTable.
BuildShiftTables_ReverseTable	D53C	LD D,L	Copy the byte into D, and set a count of 08 bits.
	D53D	LD B,$08	Copy the byte into D, and set a count of 08 bits.
BuildShiftTables_ReverseBits	D53F	SRL D	Move the lowest bit of the byte into the other end of E.
	D541	RL E	Move the lowest bit of the byte into the other end of E.
	D543	DJNZ BuildShiftTables_ReverseBits	Jump to BuildShiftTables_ReverseBits if there are any left.
	D545	LD (HL),E	Store the byte turned round.
	D546	INC L	Move on to the next byte.
	D547	JR NZ,BuildShiftTables_ReverseTable	Jump to BuildShiftTables_ReverseTable if there are any left.
	D549	RET	Return.
View the equivalent code in; Nightshade