Martianoids: Routine at B294

Used by the routine at InitialiseGame.

Builds the lookup tables used to shift sprite bytes right by any number of pixels, and the table used to reverse the bits of a byte. The shift tables occupy E000 to EFFF. For each byte value 00 to FF and each shift of 00 to 07 bits to the left, the byte is stored as a 10 bit number. The low byte of the number is stored in page EF for a shift of 00, page ED for a shift of 01, and so on in steps of two pages. The high byte is stored in the page below it. Both are stored at the offset given by the original value. The table at F000 gives the value of each byte with its bits reversed, and is used to flip sprites.

Used by the routine at InitialiseGame. Builds the lookup tables used to shift sprite bytes right by any number of pixels, and the table used to reverse the bits of a byte. The shift tables occupy E000 to EFFF. For each byte value 00 to FF and each shift of 00 to 07 bits to the left, the byte is stored as a 10 bit number. The low byte of the number is stored in page EF for a shift of 00, page ED for a shift of 01, and so on in steps of two pages. The high byte is stored in the page below it. Both are stored at the offset given by the original value. The table at F000 gives the value of each byte with its bits reversed, and is used to flip sprites.
BuildShiftTables	B294	LD L,$00	Start with the byte value 00.
BuildShiftTables_NextValue	B296	LD D,$00	Clear the high byte of the shifted value.
	B298	LD E,L	Copy the byte value into the low byte of the shifted value.
	B299	LD H,$EF	Point H at the first table page, EF.
	B29B	LD B,$08	There are 08 shifts to tabulate.
BuildShiftTables_Shift	B29D	LD A,E	Store the low byte of the shifted value.
	B29E	LD (HL),A	Store the low byte of the shifted value.
	B29F	DEC H	Move to the page below.
	B2A0	LD A,D	Store the high byte of the shifted value.
	B2A1	LD (HL),A	Store the high byte of the shifted value.
	B2A2	DEC H	Move to the next page below.
	B2A3	SLA E	Shift the 10 bit value left by one bit.
	B2A5	RL D	Shift the 10 bit value left by one bit.
	B2A7	DJNZ BuildShiftTables_Shift	Loop back until all shifts have been stored.
	B2A9	INC L	Move on to the next byte value, and loop back until all 0100 have been done.
	B2AA	JR NZ,BuildShiftTables_NextValue
	B2AC	LD HL,$F000	Point HL at ReversedBytes.
BuildShiftTables_ReverseByte	B2AF	LD A,L	Take the byte value to reverse.
	B2B0	LD B,$08	There are 08 bits to move.
BuildShiftTables_ReverseBit	B2B2	RRA	Move the lowest bit out into the carry flag.
	B2B3	RL E	Move the carry flag in at the bottom of E, so the bits end up reversed.
	B2B5	DJNZ BuildShiftTables_ReverseBit	Loop back until all 08 bits have been moved.
	B2B7	LD (HL),E	Store the reversed byte.
	B2B8	INC L	Move on to the next byte value, and loop back until all 0100 have been done.
	B2B9	JR NZ,BuildShiftTables_ReverseByte
	B2BB	RET	Return.