Routines
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58507: 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 58507 INC L Advance L.
DrawOneByteRows_Start 58508 LD B,A B=A.
DrawOneByteRows_Loop 58509 POP DE Bring DE back off the stack.
58510 LD (HL),E Write E where HL points.
58511 DEC H Step H back.
58512 LD (HL),D Write D where HL points.
58513 DEC H Step H back.
58514 LD A,H A=H.
58515 INC A Advance A.
58516 AND %00000111 Keep the bits of A that are set in 7.
58518 JR NZ,DrawOneByteRows_Next Jump to DrawOneByteRows_Next if the zero flag is clear.
58520 LD A,L A=L.
58521 SUB 32 Subtract 32 from A.
58523 LD L,A L=A.
58524 JR C,DrawOneByteRows_Next Jump to DrawOneByteRows_Next if the carry flag is set.
58526 LD A,H A=H.
58527 ADD A,8 Add 8 to A.
58529 LD H,A H=A.
DrawOneByteRows_Next 58530 DJNZ DrawOneByteRows_Loop Decrease B and jump to DrawOneByteRows_Loop until none remain.
58532 JP FinishStep Jump to FinishStep.
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