Nightshade: Routine at E0FB

Used by the routine at StartGame.

Builds the tables that the sprite drawing routines use to shift graphics sideways, and the table that reverses the bits in a byte.

There are three pairs of 256-byte pages for shifting a byte by two, four and six pixels (the low byte of the shifted result is in the first page of each pair, counting down from FF, and the high byte is in the second). FA to FF hold the shifts of the bytes as they are, and F2 to F7 hold the shifts of the bytes with their bits reversed. The page at ReverseBitsTable holds the bytes with their bits reversed.

Used by the routine at StartGame. Builds the tables that the sprite drawing routines use to shift graphics sideways, and the table that reverses the bits in a byte. There are three pairs of 256-byte pages for shifting a byte by two, four and six pixels (the low byte of the shifted result is in the first page of each pair, counting down from FF, and the high byte is in the second). FA to FF hold the shifts of the bytes as they are, and F2 to F7 hold the shifts of the bytes with their bits reversed. The page at ReverseBitsTable holds the bytes with their bits reversed.
Build the tables for the bytes as they are.
BuildShiftTables	E0FB	LD L,$00	Start with the byte at 00.
Start with the byte as a 16-bit number, and point HL at the first page of the tables.
BuildShiftTables_Normal	E0FD	LD D,$00	Make the word the byte.
	E0FF	LD E,L	Make the word the byte.
	E100	LD H,$FF	Start at page FF.
	E102	LD B,$03	Set a count of 03 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_NormalLoop	E104	SLA E	Shift the word left by two bits.
	E106	RL D
	E108	SLA E
	E10A	RL D
	E10C	LD (HL),E	Store the pair in the next two pages.
	E10D	DEC H
	E10E	LD (HL),D
	E10F	DEC H
	E110	DJNZ BuildShiftTables_NormalLoop	Jump to BuildShiftTables_NormalLoop if there are any left.
Move on to the next byte, and loop back until all 256 have been done.
	E112	INC L	Move on to the next byte.
	E113	JR NZ,BuildShiftTables_Normal	Jump to BuildShiftTables_Normal if there are any left.
Build the tables for the bytes with their bits reversed.
	E115	LD L,$00	Start with the byte at 00.
BuildShiftTables_Reversed	E117	LD D,$00	Clear the high byte of the word.
	E119	LD B,$08	Set a count of 08 bits, and copy the byte into A.
	E11B	LD A,L	Set a count of 08 bits, and copy the byte into A.
Turn the bits of the byte round.
BuildShiftTables_Reverse	E11C	RRCA	Move the next bit into C from the other end.
	E11D	RL C	Move the next bit into C from the other end.
	E11F	DJNZ BuildShiftTables_Reverse	Jump to BuildShiftTables_Reverse if there are any left.
Start with the reversed byte as a 16-bit number, and point HL at the first page of the reversed tables.
	E121	LD E,C	Make the low byte the reversed byte.
	E122	LD H,$F7	Start at page F7.
	E124	LD B,$03	Set a count of 03 pairs.
Shift the word left by two bits and store it.
BuildShiftTables_ReversedLoop	E126	SLA E	Shift the word left by two bits.
	E128	RL D
	E12A	SLA E
	E12C	RL D
	E12E	LD (HL),E	Store the pair in the next two pages.
	E12F	DEC H
	E130	LD (HL),D
	E131	DEC H
	E132	DJNZ BuildShiftTables_ReversedLoop	Jump to BuildShiftTables_ReversedLoop if there are any left.
Move on to the next byte, and loop back until all 256 have been done.
	E134	INC L	Move on to the next byte.
	E135	JR NZ,BuildShiftTables_Reversed	Jump to BuildShiftTables_Reversed if there are any left.
Build the table of reversed bytes.
	E137	LD HL,$F900	Point HL at ReverseBitsTable.
BuildShiftTables_ReverseTable	E13A	LD D,L	Start with the byte, and count eight bits.
	E13B	LD B,$08	Start with the byte, and count eight bits.
BuildShiftTables_ReverseBits	E13D	SRL D	Move the lowest bit of the byte into the other end of E.
	E13F	RL E	Move the lowest bit of the byte into the other end of E.
	E141	DJNZ BuildShiftTables_ReverseBits	Jump to BuildShiftTables_ReverseBits if there are any left.
	E143	LD (HL),E	Store the byte turned round.
	E144	INC L	Move on to the next byte.
	E145	JR NZ,BuildShiftTables_ReverseTable	Jump to BuildShiftTables_ReverseTable if there are any left.
	E147	RET	Return.
View the equivalent code in; Gunfright