Routines
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E48B: Draw One Byte Rows
Draws a piece of terrain that is one byte wide, going up the screen two pixel rows at a time, using a word of the picture for each pair of rows. The entry point at DrawOneByteRows moves one column to the right first.
Input
A Number of pairs of pixel rows to draw
HL Screen address of the bottom of the piece
SP Address of the picture
DrawOneByteRows E48B INC L Advance L.
DrawOneByteRows_Start E48C LD B,A B=A.
DrawOneByteRows_Loop E48D POP DE Bring DE back off the stack.
E48E LD (HL),E Write E where HL points.
E48F DEC H Step H back.
E490 LD (HL),D Write D where HL points.
E491 DEC H Step H back.
E492 LD A,H A=H.
E493 INC A Advance A.
E494 AND %00000111 Keep the bits of A that are set in 07.
E496 JR NZ,DrawOneByteRows_Next Jump to DrawOneByteRows_Next if the zero flag is clear.
E498 LD A,L A=L.
E499 SUB $20 Subtract 20 from A.
E49B LD L,A L=A.
E49C JR C,DrawOneByteRows_Next Jump to DrawOneByteRows_Next if the carry flag is set.
E49E LD A,H A=H.
E49F ADD A,$08 Add 08 to A.
E4A1 LD H,A H=A.
DrawOneByteRows_Next E4A2 DJNZ DrawOneByteRows_Loop Decrease B and jump to DrawOneByteRows_Loop until none remain.
E4A4 JP FinishStep Jump to FinishStep.
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