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Routines |
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Sorts the draw list at DrawList into the order in which the items must be drawn, and builds the sorted list at SortedDrawList. This is a painter's algorithm: an item can only be drawn after every item which is behind it. Each item in the lists has three bytes: flags, and the address of its record. Bit 0 of the flags is set if the record is an entity, and reset if it is a compact record with the three coordinates at offsets 02 04 and 06 which is given the size 06 06 01. Bit 7 is set once the item has been moved to the sorted list. . The routine takes the first item which has not been moved, and compares it with each of the other items which have not been moved. The result for each pair is a number in the range 00-1A made from the relation along the first, second and third coordinates (00 for 'the other item is before this one', 01 for 'they overlap' and 02 for 'the other item is after this one'). The number selects a routine from the table at DrawOrderTable. If the other item has to be drawn first then it becomes the item under consideration (unless it has been tried already, which would be a loop). When an item has no other item that has to be drawn before it, it is moved to the sorted list and the whole process starts again.
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| SortDrawList | 9F99 | LD HL,$AF8F | Point HL at the sorted list. | |
| 9F9C | LD ($A17A),HL | Store it as the address of the next free place in the sorted list. | ||
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This entry point is used by the routine at MoveItemToSortedList.
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| SortDrawList_Next | 9F9F | LD DE,$AFFB | Point DE at the start of the draw list. | |
| SortDrawList_FirstLoop | 9FA2 | LD A,(DE) | Fetch the flags of the next item. | |
| 9FA3 | INC DE | Move onto the address of the record. | ||
| 9FA4 | AND A | Test the flags. | ||
| 9FA5 | JP Z,EndSortedList | Jump to EndSortedList if this is the end of the list. | ||
| 9FA8 | BIT 7,A | Test whether the item has been moved already. | ||
| 9FAA | LD L,A | Keep the flags in L. | ||
| 9FAB | LD A,(DE) | Fetch the first byte of the address of the record. | ||
| 9FAC | LD C,A | Keep it in C. | ||
| 9FAD | INC DE | Move onto the second byte of the address. | ||
| 9FAE | LD A,(DE) | Fetch it. | ||
| 9FAF | LD B,A | Keep it in B. | ||
| 9FB0 | INC DE | Move onto the next item. | ||
| 9FB1 | JR NZ,SortDrawList_FirstLoop | Jump back to look at the next item if this one has been moved already. | ||
| 9FB3 | LD ($A1AE),DE | Store the address of the next item as that of the item under consideration. | ||
| 9FB7 | PUSH BC | Copy the address of the record into IX (using the stack). | ||
| 9FB8 | POP IX | |||
| 9FBA | RR L | Rotate bit 0 of the flags into the carry flag. | ||
| 9FBC | JR C,SortDrawList_SecondLoop | Jump to SortDrawList_SecondLoop if the record is an entity. | ||
| 9FBE | LD L,(IX+$02) | Fetch the first byte of the first coordinate of the record. | ||
| 9FC1 | LD H,(IX+$03) | Fetch the second byte of the first coordinate of the record. | ||
| 9FC4 | LD ($9F7B),HL | Store the first coordinate in the first scratch item. | ||
| 9FC7 | LD L,(IX+$04) | Fetch the first byte of the second coordinate of the record. | ||
| 9FCA | LD H,(IX+$05) | Fetch the second byte of the second coordinate of the record. | ||
| 9FCD | LD ($9F7E),HL | Store the second coordinate in the first scratch item. | ||
| 9FD0 | LD L,(IX+$06) | Fetch the first byte of the third coordinate of the record. | ||
| 9FD3 | LD H,(IX+$07) | Fetch the second byte of the third coordinate of the record. | ||
| 9FD6 | LD ($9F81),HL | Store the third coordinate in the first scratch item. | ||
| 9FD9 | LD IX,$9F79 | Point IX at ScratchItems. | ||
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This entry point is used by the routines at IgnoreItem, IgnoreOverlap, DrawOtherItemFirst and ResolveFullOverlap.
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| SortDrawList_SecondLoop | 9FDD | LD A,(DE) | Fetch the flags of the next item to compare. | |
| 9FDE | INC DE | Move onto the address of the record. | ||
| 9FDF | AND A | Test the flags. | ||
| 9FE0 | JP Z,MoveItemToSortedList | Jump to MoveItemToSortedList if the end of the list has been reached, as no other item has to go first. | ||
| 9FE3 | BIT 7,A | Test whether the item has been moved already. | ||
| 9FE5 | LD L,A | Keep the flags in L. | ||
| 9FE6 | LD A,(DE) | Fetch the first byte of the address of the record. | ||
| 9FE7 | LD C,A | Keep it in C. | ||
| 9FE8 | INC DE | Move onto the second byte of the address. | ||
| 9FE9 | LD A,(DE) | Fetch it. | ||
| 9FEA | LD B,A | Keep it in B. | ||
| 9FEB | INC DE | Move onto the next item. | ||
| 9FEC | JR NZ,SortDrawList_SecondLoop | Jump back to look at the next item if this one has been moved already. | ||
| 9FEE | LD ($A1B0),DE | Store the address of the next item as that of the other item. | ||
| 9FF2 | PUSH BC | Copy the address of the record into IY (using the stack). | ||
| 9FF3 | POP IY | |||
| 9FF5 | RR L | Rotate bit 0 of the flags into the carry flag. | ||
| 9FF7 | JR C,SortDrawList_Compare | Jump to SortDrawList_Compare if the record is an entity. | ||
| 9FF9 | LD L,(IY+$02) | Fetch the first byte of the first coordinate of the record. | ||
| 9FFC | LD H,(IY+$03) | Fetch the second byte of the first coordinate of the record. | ||
| 9FFF | LD ($9F8B),HL | Store the first coordinate in the second scratch item. | ||
| A002 | LD L,(IY+$04) | Fetch the first byte of the second coordinate of the record. | ||
| A005 | LD H,(IY+$05) | Fetch the second byte of the second coordinate of the record. | ||
| A008 | LD ($9F8E),HL | Store the second coordinate in the second scratch item. | ||
| A00B | LD L,(IY+$06) | Fetch the first byte of the third coordinate of the record. | ||
| A00E | LD H,(IY+$07) | Fetch the second byte of the third coordinate of the record. | ||
| A011 | LD ($9F91),HL | Store the third coordinate in the second scratch item. | ||
| A014 | LD IY,$9F89 | Point IY at ScratchItems_Second. | ||
| A018 | LD BC,$9F89 | Use it as the address to compare against. | ||
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Skip the pair if both are the same item.
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| SortDrawList_Compare | A01B | PUSH IX | Copy the address of the item under consideration into HL (using the stack). | |
| A01D | POP HL | |||
| A01E | AND A | Compare it with the address of the other item. | ||
| A01F | SBC HL,BC | |||
| A021 | JR Z,SortDrawList_SecondLoop | Jump to SortDrawList_SecondLoop if they are the same item. | ||
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Compare the third coordinate of the two items, and put the result in C: 00 if the other item is entirely before this one, 01 if they overlap, and 02 if the other item is entirely after this one.
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| A023 | EXX | Switch to the alternate registers. | ||
| A024 | LD C,$00 | Clear the result. | ||
| A026 | LD H,(IY+$08) | Fetch the first byte of the third coordinate of the other item. | ||
| A029 | LD L,(IY+$09) | Fetch the second byte of the third coordinate of the other item. | ||
| A02C | LD D,$00 | Clear D. | ||
| A02E | LD E,(IY+$0D) | Fetch the third size of the other item. | ||
| A031 | DEC E | Subtract one to make the last position it covers. | ||
| A032 | ADD HL,DE | Find the last position that the other item covers. | ||
| A033 | LD D,(IX+$08) | Fetch the first byte of the third coordinate of this item. | ||
| A036 | LD E,(IX+$09) | Fetch the second byte of the third coordinate of this item. | ||
| A039 | SBC HL,DE | Subtract the position of this item. | ||
| A03B | JR C,SortDrawList_Second | Jump to SortDrawList_Second if the other item ends before this one begins. | ||
| A03D | LD L,(IX+$0D) | Fetch the third size of this item. | ||
| A040 | LD H,$00 | Clear H. | ||
| A042 | ADD HL,DE | Find the position just beyond this item. | ||
| A043 | EX DE,HL | Keep it in DE. | ||
| A044 | LD H,(IY+$08) | Fetch the first byte of the third coordinate of the other item. | ||
| A047 | LD L,(IY+$09) | Fetch the second byte of the third coordinate of the other item. | ||
| A04A | AND A | Subtract the position beyond this item. | ||
| A04B | SBC HL,DE | |||
| A04D | JR C,SortDrawList_ThirdAdd | Jump to SortDrawList_ThirdAdd if the other item starts before the end of this one. | ||
| A04F | INC C | Add one to the result. | ||
| SortDrawList_ThirdAdd | A050 | INC C | Add one to the result. | |
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Do the same for the second coordinate, adding 03 to the result for an overlap and 06 for the other item being entirely after this one.
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| SortDrawList_Second | A051 | LD H,(IY+$05) | Fetch the first byte of the second coordinate of the other item. | |
| A054 | LD L,(IY+$06) | Fetch the second byte of the second coordinate of the other item. | ||
| A057 | LD D,$00 | Clear D. | ||
| A059 | LD E,(IY+$0C) | Fetch the second size of the other item. | ||
| A05C | DEC E | Subtract one to make the last position it covers. | ||
| A05D | ADD HL,DE | Find the last position that the other item covers. | ||
| A05E | LD D,(IX+$05) | Fetch the first byte of the second coordinate of this item. | ||
| A061 | LD E,(IX+$06) | Fetch the second byte of the second coordinate of this item. | ||
| A064 | SBC HL,DE | Subtract the position of this item. | ||
| A066 | JR C,SortDrawList_First | Jump to SortDrawList_First if the other item ends before this one begins. | ||
| A068 | LD L,(IX+$0C) | Fetch the second size of this item. | ||
| A06B | LD H,$00 | Clear H. | ||
| A06D | ADD HL,DE | Find the position just beyond this item. | ||
| A06E | EX DE,HL | Keep it in DE. | ||
| A06F | LD H,(IY+$05) | Fetch the first byte of the second coordinate of the other item. | ||
| A072 | LD L,(IY+$06) | Fetch the second byte of the second coordinate of the other item. | ||
| A075 | AND A | Subtract the position beyond this item. | ||
| A076 | SBC HL,DE | |||
| A078 | LD A,C | Fetch the result so far. | ||
| A079 | JR C,SortDrawList_SecondAdd | Jump to SortDrawList_SecondAdd if the other item starts before the end of this one. | ||
| A07B | ADD A,$03 | Add 03 to the result. | ||
| SortDrawList_SecondAdd | A07D | ADD A,$03 | Add 03 to the result. | |
| A07F | LD C,A | Store the result. | ||
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Do the same for the first coordinate, adding 09 to the result for an overlap and 12 for the other item being entirely after this one.
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| SortDrawList_First | A080 | LD H,(IY+$02) | Fetch the first byte of the first coordinate of the other item. | |
| A083 | LD L,(IY+$03) | Fetch the second byte of the first coordinate of the other item. | ||
| A086 | LD D,$00 | Clear D. | ||
| A088 | LD E,(IY+$0B) | Fetch the first size of the other item. | ||
| A08B | DEC E | Subtract one to make the last position it covers. | ||
| A08C | ADD HL,DE | Find the last position that the other item covers. | ||
| A08D | LD D,(IX+$02) | Fetch the first byte of the first coordinate of this item. | ||
| A090 | LD E,(IX+$03) | Fetch the second byte of the first coordinate of this item. | ||
| A093 | SBC HL,DE | Subtract the position of this item. | ||
| A095 | JR C,SortDrawList_Dispatch | Jump to SortDrawList_Dispatch if the other item ends before this one begins. | ||
| A097 | LD L,(IX+$0B) | Fetch the first size of this item. | ||
| A09A | LD H,$00 | Clear H. | ||
| A09C | ADD HL,DE | Find the position just beyond this item. | ||
| A09D | EX DE,HL | Keep it in DE. | ||
| A09E | LD H,(IY+$02) | Fetch the first byte of the first coordinate of the other item. | ||
| A0A1 | LD L,(IY+$03) | Fetch the second byte of the first coordinate of the other item. | ||
| A0A4 | AND A | Subtract the position beyond this item. | ||
| A0A5 | SBC HL,DE | |||
| A0A7 | LD A,C | Fetch the result so far. | ||
| A0A8 | JR C,SortDrawList_FirstAdd | Jump to SortDrawList_FirstAdd if the other item starts before the end of this one. | ||
| A0AA | ADD A,$09 | Add 09 to the result. | ||
| SortDrawList_FirstAdd | A0AC | ADD A,$09 | Add 09 to the result. | |
| A0AE | LD C,A | Store the result. | ||
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Use the result to jump to the routine in the table at DrawOrderTable.
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| SortDrawList_Dispatch | A0AF | LD A,C | Fetch the result. | |
| A0B0 | ADD A,A | Double it to make the offset into the table. | ||
| A0B1 | EXX | Switch back to the normal registers. | ||
| A0B2 | LD HL,$A0BE | Point HL at DrawOrderTable. | ||
| A0B5 | LD C,A | Copy the offset into C. | ||
| A0B6 | LD B,$00 | Clear B. | ||
| A0B8 | ADD HL,BC | Add the offset to find the entry. | ||
| A0B9 | LD A,(HL) | Fetch the first byte of the address. | ||
| A0BA | INC HL | Move onto the second byte. | ||
| A0BB | LD H,(HL) | Fetch it. | ||
| A0BC | LD L,A | Combine them into HL. | ||
| A0BD | JP (HL) | Jump to the routine. | ||
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