Knight Lore: Routine at D7AA

D7AA: Draw Lines

Used by the routine at PrintSprite. Draws the lines of a sprite one after the other. For each line the code jumps into the unrolled code, at a place that depends on the width of the sprite, and that is set by the jump that is changed by PrintSprite. The code for a sprite that is not on a byte boundary takes a pair of bytes off the stack, which are the mask and the data, shifts them across the two bytes of the buffer that they fall into with the table of shifted bytes in H, and keeps what is left of the second byte to go with the next pair.
DrawLines	D7AA	EX AF,AF'	Stash the number of lines left.
	D7AB	LD A,(BC)	Fetch the first byte of the buffer.
DrawLines_Jump	D7AC	JR DrawLines_Jump	Jump into the unrolled code. The jump is changed to go to the right place.
The first pair of a sprite that has five.
	D7AE	POP DE	Take the mask and the data of the pair off the stack.
	D7AF	LD L,E	Combine the byte that is carried with the first part of the shifted mask and data.
	D7B0	AND (HL)
	D7B1	LD L,D
	D7B2	XOR (HL)
	D7B3	CPL
	D7B4	LD (BC),A	Store the byte in the buffer and move on to the next one.
	D7B5	INC BC	Store the byte in the buffer and move on to the next one.
	D7B6	INC H	Switch to the table of the part that spills over into the next byte.
	D7B7	LD L,E
	D7B8	LD A,(BC)	Fetch the next byte of the buffer.
	D7B9	AND (HL)	Combine it with the second part of the shifted mask and data.
	D7BA	LD L,D
	D7BB	XOR (HL)
	D7BC	CPL
	D7BD	DEC H	Switch back to the first table.
The second pair of a sprite that has four or five.
	D7BE	POP DE	Take the mask and the data of the pair off the stack.
	D7BF	LD L,E	Combine the byte that is carried with the first part of the shifted mask and data.
	D7C0	AND (HL)
	D7C1	LD L,D
	D7C2	XOR (HL)
	D7C3	CPL
	D7C4	LD (BC),A	Store the byte in the buffer and move on to the next one.
	D7C5	INC BC	Store the byte in the buffer and move on to the next one.
	D7C6	INC H	Switch to the table of the part that spills over into the next byte.
	D7C7	LD L,E
	D7C8	LD A,(BC)	Fetch the next byte of the buffer.
	D7C9	AND (HL)	Combine it with the second part of the shifted mask and data.
	D7CA	LD L,D
	D7CB	XOR (HL)
	D7CC	CPL
	D7CD	DEC H	Switch back to the first table.
The third pair of a sprite that has three or more.
	D7CE	POP DE	Take the mask and the data of the pair off the stack.
	D7CF	LD L,E	Combine the byte that is carried with the first part of the shifted mask and data.
	D7D0	AND (HL)
	D7D1	LD L,D
	D7D2	XOR (HL)
	D7D3	CPL
	D7D4	LD (BC),A	Store the byte in the buffer and move on to the next one.
	D7D5	INC BC	Store the byte in the buffer and move on to the next one.
	D7D6	INC H	Switch to the table of the part that spills over into the next byte.
	D7D7	LD L,E
	D7D8	LD A,(BC)	Fetch the next byte of the buffer.
	D7D9	AND (HL)	Combine it with the second part of the shifted mask and data.
	D7DA	LD L,D
	D7DB	XOR (HL)
	D7DC	CPL
	D7DD	DEC H	Switch back to the first table.
The fourth pair of a sprite that has two or more.
	D7DE	POP DE	Take the mask and the data of the pair off the stack.
	D7DF	LD L,E	Combine the byte that is carried with the first part of the shifted mask and data.
	D7E0	AND (HL)
	D7E1	LD L,D
	D7E2	XOR (HL)
	D7E3	CPL
	D7E4	LD (BC),A	Store the byte in the buffer and move on to the next one.
	D7E5	INC BC	Store the byte in the buffer and move on to the next one.
	D7E6	INC H	Switch to the table of the part that spills over into the next byte.
	D7E7	LD L,E
	D7E8	LD A,(BC)	Fetch the next byte of the buffer.
	D7E9	AND (HL)	Combine it with the second part of the shifted mask and data.
	D7EA	LD L,D
	D7EB	XOR (HL)
	D7EC	CPL
	D7ED	DEC H	Switch back to the first table.
The last pair.
	D7EE	POP DE	Take the mask and the data of the pair off the stack.
	D7EF	LD L,E	Combine the byte that is carried with the first part of the shifted mask and data.
	D7F0	AND (HL)
	D7F1	LD L,D
	D7F2	XOR (HL)
	D7F3	CPL
	D7F4	LD (BC),A	Store the byte in the buffer and move on to the next one.
	D7F5	INC BC	Store the byte in the buffer and move on to the next one.
	D7F6	INC H	Switch to the table of the part that spills over into the next byte.
	D7F7	LD L,E
	D7F8	LD A,(BC)	Fetch the next byte of the buffer.
	D7F9	AND (HL)	Combine it with the second part of the shifted mask and data.
	D7FA	LD L,D
	D7FB	XOR (HL)
	D7FC	CPL
	D7FD	DEC H	Switch back to the first table.
	D7FE	LD (BC),A	Store the last byte of the line.
This entry point is used by the routine at DrawAlignedColumns.
DrawLines_Next	D7FF	LD A,C	Fetch the low byte of the address in the buffer.
	D800	ADD A,$00	Add the step to the start of the next line, which is changed for the width of the sprite.
	D802	LD C,A	Store it.
	D803	LD A,B	Fetch the high byte of the address in the buffer.
	D804	ADC A,$00	Add the carry, as the step is a single byte.
	D806	LD B,A	Store it.
	D807	EX AF,AF'	Restore the number of lines left.
	D808	DEC A	Count down the lines.
	D809	JP NZ,DrawLines	Jump to DrawLines until all of the lines are drawn.
	D80C	LD SP,($5BA9)	Restore the stack pointer from SavedStackPointer.
	D810	RET	Return.
View the equivalent code in; Alien 8 Pentagram