Underwurlde: Routine at 38419

Used by the routine at Select_Game_Options.

Builds the tables that the sprite routines use to shift a byte of sprite data to any pixel position.

For every byte value L from 0 to 255 the table holds eight words, which are the byte shifted left by zero to seven bits. For a shift of n bits, the low byte of the word is stored at 65280 less 512 times n, plus L, and the high byte is stored in the page below it. The tables use the pages from 240 to 255.

The sprite routines select a table with B (which is always an even page) and read the high byte with B and the low byte with B+1.

Used by the routine at Select_Game_Options. Builds the tables that the sprite routines use to shift a byte of sprite data to any pixel position. For every byte value L from 0 to 255 the table holds eight words, which are the byte shifted left by zero to seven bits. For a shift of n bits, the low byte of the word is stored at 65280 less 512 times n, plus L, and the high byte is stored in the page below it. The tables use the pages from 240 to 255. The sprite routines select a table with B (which is always an even page) and read the high byte with B and the low byte with B+1.
BuildShiftTable	38419	LD L,0	Start with the byte value 0.
BuildShiftTable_Value	38421	LD D,0	Clear the high byte of the shifted word.
	38423	LD E,L	Start the low byte of the shifted word with the byte value.
	38424	LD H,255	Start at the page of the first low byte (255).
	38426	LD B,8	Set the number of pixel offsets to build to 8.
BuildShiftTable_Offset	38428	LD (HL),E	Store the low byte of the shifted word.
	38429	DEC H	Move down to the page of the high byte.
	38430	LD (HL),D	Store the high byte of the shifted word.
	38431	DEC H	Move down to the page of the next low byte.
	38432	SLA E	Shift the word left by one bit.
	38434	RL D	Shift the word left by one bit.
	38436	DJNZ BuildShiftTable_Offset	Decrease the shift counter and loop back until all eight shifts are stored.
	38438	INC L	Move on to the next byte value.
	38439	JR NZ,BuildShiftTable_Value	Jump back to BuildShiftTable_Value until all of the byte values have been done.
	38441	RET	Return.
View the equivalent code in; Sabre Wulf