Routines
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E249: Draw Flat Rows
Draws a flat piece of terrain, three bytes wide, going up the screen two pixel rows at a time. The words of the picture are popped from the stack (which DrawTerrain has pointed at the picture) and stored in a zigzag, so that a word is two screen bytes next to each other. The entry point at DrawFlatRows moves one column to the right and carries on at DrawFlatRows_Start. The three JR instructions at DrawFlatRows_Next, DrawFlatRows_PatchSecond and DrawFlatRows_PatchThird have their displacements set by DrawTerrain to choose how much of the three bytes is drawn. When one is changed to 33 or to EE the row is drawn from DrawFlatRows_NextColumn or DrawFlatRows, and when DrawFlatRows_PatchSecond or DrawFlatRows_PatchThird is changed to 24 or 26 the word is skipped (DrawFlatRows_SkipWord and DrawFlatRows_SkipOne).
Input
A Number of pairs of pixel rows to draw
HL Screen address of the bottom left of the piece
SP Address of the picture
DrawFlatRows E249 INC L Advance L.
E24A JP DrawFlatRows_Start Jump to DrawFlatRows_Start.
DrawFlatRows_Normal E24D LD B,A B=A.
E24E LD A,($CE49) The segment screen address.
E251 ADD A,$03 Add 03 to A.
E253 LD ($CE49),A Update the segment screen address.
E256 INC L Advance L.
E257 INC L Advance L.
E258 INC L Advance L.
DrawFlatRows_Next E259 JR DrawFlatRows_Next Skip ahead by the displacement that DrawTerrain sets.
DrawFlatRows_Word1 E25B POP DE Bring DE back off the stack.
E25C LD (HL),E Write E where HL points.
DrawFlatRows_PatchSecond E25D JR DrawFlatRows_PatchSecond Skip ahead by the displacement that DrawTerrain sets.
DrawFlatRows_Word2 E25F INC L Advance L.
E260 LD (HL),D Write D where HL points.
DrawFlatRows_PatchThird E261 JR DrawFlatRows_PatchThird Skip ahead by the displacement that DrawTerrain sets.
DrawFlatRows_Word3 E263 INC L Advance L.
E264 POP DE Bring DE back off the stack.
E265 LD (HL),E Write E where HL points.
E266 DEC H Step H back.
E267 LD (HL),D Write D where HL points.
E268 DEC L Step L back.
DrawFlatRows_Word4 E269 POP DE Bring DE back off the stack.
E26A LD (HL),E Write E where HL points.
E26B DEC L Step L back.
DrawFlatRows_Store E26C LD (HL),D Write D where HL points.
DrawFlatRows_Up E26D DEC H Step H back.
E26E LD A,H A=H.
E26F INC A Advance A.
E270 AND %00000111 Keep the bits of A that are set in 07.
E272 JR NZ,DrawFlatRows_Loop Jump to DrawFlatRows_Loop if the zero flag is clear.
E274 LD A,L A=L.
E275 SUB $20 Subtract 20 from A.
E277 LD L,A L=A.
E278 JR C,DrawFlatRows_Loop Jump to DrawFlatRows_Loop if the carry flag is set.
E27A LD A,H A=H.
E27B ADD A,$08 Add 08 to A.
E27D LD H,A H=A.
DrawFlatRows_Loop E27E DJNZ DrawFlatRows_Next Decrease B and jump to DrawFlatRows_Next until none remain.
E280 JP FinishStep_Exchange Jump to FinishStep_Exchange.
DrawFlatRows_SkipWord E283 DEC H Step H back.
E284 POP DE Bring DE back off the stack.
E285 POP DE Bring DE back off the stack.
E286 JP DrawFlatRows_Store Jump to DrawFlatRows_Store.
DrawFlatRows_SkipOne E289 DEC H Step H back.
E28A POP DE Bring DE back off the stack.
E28B JP DrawFlatRows_Word4 Jump to DrawFlatRows_Word4.
DrawFlatRows_NextColumn E28E INC A Advance A.
E28F AND %00011111 Keep the bits of A that are set in 1F.
E291 JR NZ,DrawFlatRows_Column Jump to DrawFlatRows_Column if the zero flag is clear.
E293 LD A,L A=L.
E294 SUB $20 Subtract 20 from A.
E296 LD ($CE49),A Update the segment screen address.
DrawFlatRows_Column E299 LD A,L A=L.
E29A AND %11100000 Keep the bits of A that are set in E0.
E29C INC A Advance A.
E29D LD L,A L=A.
DrawFlatRows_Start E29E LD A,$4C A=4C.
E2A0 LD ($E25A),A Skip the rest of the flat row.
E2A3 INC SP Advance SP.
DrawFlatRows_PatchFourth E2A4 JR DrawFlatRows_PatchFourth Skip ahead by the displacement that DrawTerrain sets.
DrawFlatRows_Pop E2A6 INC SP Advance SP.
E2A7 POP DE Bring DE back off the stack.
E2A8 LD (HL),E Write E where HL points.
DrawFlatRows_PatchFifth E2A9 JR DrawFlatRows_PatchFifth Skip ahead by the displacement that DrawTerrain sets.
DrawFlatRows_Right E2AB INC L Advance L.
E2AC LD (HL),D Write D where HL points.
E2AD POP DE Bring DE back off the stack.
E2AE DEC H Step H back.
E2AF LD (HL),E Write E where HL points.
E2B0 DEC L Step L back.
E2B1 LD (HL),D Write D where HL points.
E2B2 POP DE Bring DE back off the stack.
E2B3 JP DrawFlatRows_Up Jump to DrawFlatRows_Up.
DrawFlatRows_Left E2B6 DEC H Step H back.
E2B7 LD (HL),D Write D where HL points.
E2B8 POP DE Bring DE back off the stack.
E2B9 POP DE Bring DE back off the stack.
E2BA JP DrawFlatRows_Up Jump to DrawFlatRows_Up.
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