Martianoids: Routine at 45716

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 57344 to 61439. For each byte value 0 to 255 and each shift of 0 to 7 bits to the left, the byte is stored as a 16 bit number. The low byte of the number is stored in page 239 for a shift of 0, page 237 for a shift of 1, 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 61440 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 57344 to 61439. For each byte value 0 to 255 and each shift of 0 to 7 bits to the left, the byte is stored as a 16 bit number. The low byte of the number is stored in page 239 for a shift of 0, page 237 for a shift of 1, 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 61440 gives the value of each byte with its bits reversed, and is used to flip sprites.
BuildShiftTables	45716	LD L,0	Start with the byte value 0.
BuildShiftTables_NextValue	45718	LD D,0	Clear the high byte of the shifted value.
	45720	LD E,L	Copy the byte value into the low byte of the shifted value.
	45721	LD H,239	Point H at the first table page, 239.
	45723	LD B,8	There are 8 shifts to tabulate.
BuildShiftTables_Shift	45725	LD A,E	Store the low byte of the shifted value.
	45726	LD (HL),A	Store the low byte of the shifted value.
	45727	DEC H	Move to the page below.
	45728	LD A,D	Store the high byte of the shifted value.
	45729	LD (HL),A	Store the high byte of the shifted value.
	45730	DEC H	Move to the next page below.
	45731	SLA E	Shift the 16 bit value left by one bit.
	45733	RL D	Shift the 16 bit value left by one bit.
	45735	DJNZ BuildShiftTables_Shift	Loop back until all shifts have been stored.
	45737	INC L	Move on to the next byte value, and loop back until all 256 have been done.
	45738	JR NZ,BuildShiftTables_NextValue
	45740	LD HL,61440	Point HL at ReversedBytes.
BuildShiftTables_ReverseByte	45743	LD A,L	Take the byte value to reverse.
	45744	LD B,8	There are 8 bits to move.
BuildShiftTables_ReverseBit	45746	RRA	Move the lowest bit out into the carry flag.
	45747	RL E	Move the carry flag in at the bottom of E, so the bits end up reversed.
	45749	DJNZ BuildShiftTables_ReverseBit	Loop back until all 8 bits have been moved.
	45751	LD (HL),E	Store the reversed byte.
	45752	INC L	Move on to the next byte value, and loop back until all 256 have been done.
	45753	JR NZ,BuildShiftTables_ReverseByte
	45755	RET	Return.