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Routines |
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Used by the routines at SpawnEnemy and PrepareDraw.
Works out where an object is on the screen from its position on the planet and the position of the part of the planet that is on the screen. If the object is off the screen, its slot is freed. Otherwise the horizontal and vertical screen positions are stored at offsets 11 and 12, and the object is flagged as visible with bit 7 of offset 10. The wrap flag (bit 6 of offset 10) is set if the object has crossed the edge of the part of the planet that is on the screen.
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| FindObjectScreenPosition | ADD8 | LD A,(DE) | Read DE. | |||||||
| ADD9 | ADD A,A | Double A. | ||||||||
| ADDA | ADD A,A | Double A. | ||||||||
| ADDB | ADD A,A | Double A. | ||||||||
| ADDC | LD B,A | B=A. | ||||||||
| ADDD | INC DE | Advance DE. | ||||||||
| ADDE | LD A,(DE) | Read DE. | ||||||||
| ADDF | LD C,A | C=A. | ||||||||
| ADE0 | LD A,($CE45) | The position across. | ||||||||
| ADE3 | LD E,A | E=A. | ||||||||
| ADE4 | LD A,(IX+$02) | The horizontal position. | ||||||||
| ADE7 | AND %11111110 | Keep the bits of A that are set in FE. | ||||||||
| ADE9 | LD L,A | L=A. | ||||||||
| ADEA | LD A,($DDF7) | The next column. | ||||||||
| ADED | CP (IX+$01) | Jump to FindObjectScreenPosition_Edge if A is the byte at offset 01 of IX. | ||||||||
| ADF0 | JP Z,FindObjectScreenPosition_Edge | |||||||||
| ADF3 | LD A,($CE57) | The map position. | ||||||||
| ADF6 | CP (IX+$01) | Jump to FindObjectScreenPosition_Wrap if A is the byte at offset 01 of IX. | ||||||||
| ADF9 | JR Z,FindObjectScreenPosition_Wrap | |||||||||
| ADFB | LD A,($A1D7) | The rows of the sprite that was drawn. | ||||||||
| ADFE | AND A | Jump to FindObjectScreenPosition_Free if A is zero. | ||||||||
| ADFF | JP Z,FindObjectScreenPosition_Free | |||||||||
| AE02 | LD A,L | A=L. | ||||||||
| AE03 | ADD A,$17 | Add 17 to A. | ||||||||
| AE05 | ADD A,B | A gains B. | ||||||||
| AE06 | JP NC,FindObjectScreenPosition_Free | Jump to FindObjectScreenPosition_Free if the carry flag is clear. | ||||||||
| AE09 | LD A,(IX+$02) | The horizontal position. | ||||||||
| AE0C | CP $80 | Jump to FindObjectScreenPosition_Free if A is less than 80. | ||||||||
| AE0E | JP C,FindObjectScreenPosition_Free | |||||||||
| AE11 | LD A,E | A=E. | ||||||||
| AE12 | ADD A,$B0 | Add B0 to A. | ||||||||
| AE14 | LD E,A | E=A. | ||||||||
| AE15 | LD A,L | A=L. | ||||||||
| AE16 | SUB $30 | Subtract 30 from A. | ||||||||
| AE18 | LD L,A | L=A. | ||||||||
| FindObjectScreenPosition_Wrap | AE19 | LD A,L | A=L. | |||||||
| AE1A | CP E | Jump to FindObjectScreenPosition_Horizontal if A is E or more. | ||||||||
| AE1B | JR NC,FindObjectScreenPosition_Horizontal | |||||||||
| AE1D | ADD A,B | A gains B. | ||||||||
| AE1E | JR C,FindObjectScreenPosition_Row | Jump to FindObjectScreenPosition_Row if the carry flag is set. | ||||||||
| AE20 | DEC A | Step A back. | ||||||||
| AE21 | CP E | Return if A is less than E. | ||||||||
| AE22 | RET C | |||||||||
| FindObjectScreenPosition_Row | AE23 | LD A,E | A=E. | |||||||
| AE24 | SUB L | Subtract L from A. | ||||||||
| AE25 | SUB $09 | Subtract 09 from A. | ||||||||
| AE27 | JR C,FindObjectScreenPosition_Horizontal | Jump to FindObjectScreenPosition_Horizontal if the carry flag is set. | ||||||||
| AE29 | SET 6,(IX+$10) | Set bit 6 of offset 10 of IX. | ||||||||
| AE2D | LD A,$01 | A=01. | ||||||||
| AE2F | LD ($A2FA),A | The sprite reaches past the right of the screen. | ||||||||
| FindObjectScreenPosition_Horizontal | AE32 | LD A,($CE57) | The map position. | |||||||
| AE35 | CP (IX+$01) | Jump to FindObjectScreenPosition_Same if A is the byte at offset 01 of IX. | ||||||||
| AE38 | JR Z,FindObjectScreenPosition_Same | |||||||||
| AE3A | LD A,($DDF7) | The next column. | ||||||||
| AE3D | CP (IX+$01) | Jump to FindObjectScreenPosition_Same if A is the byte at offset 01 of IX. | ||||||||
| AE40 | JR Z,FindObjectScreenPosition_Same | |||||||||
| AE42 | LD A,L | A=L. | ||||||||
| AE43 | SUB E | Subtract E from A. | ||||||||
| AE44 | ADD A,$08 | Add 08 to A. | ||||||||
| AE46 | LD L,A | L=A. | ||||||||
| AE47 | JR FindObjectScreenPosition_StoreX | Jump to FindObjectScreenPosition_StoreX. | ||||||||
| FindObjectScreenPosition_Same | AE49 | LD A,($CE41) | The horizontal offset. | |||||||
| AE4C | ADD A,L | A gains L. | ||||||||
| FindObjectScreenPosition_StoreX | AE4D | LD ($A1D5),A | The horizontal screen position. | |||||||
| AE50 | LD (IX+$11),A | Write the horizontal screen position. | ||||||||
| AE53 | JP FindObjectScreenPosition_Vertical | Jump to FindObjectScreenPosition_Vertical. | ||||||||
| FindObjectScreenPosition_Edge | AE56 | LD A,L | A=L. | |||||||
| AE57 | CP E | Return if A is E or more. | ||||||||
| AE58 | RET NC | |||||||||
| AE59 | ADD A,$E0 | Add E0 to A. | ||||||||
| AE5B | LD L,A | L=A. | ||||||||
| AE5C | JR FindObjectScreenPosition_Horizontal | Jump to FindObjectScreenPosition_Horizontal. | ||||||||
| FindObjectScreenPosition_Vertical | AE5E | LD L,(IX+$05) | The vertical position. | |||||||
| AE61 | LD A,($CE46) | The position down. | ||||||||
| AE64 | LD E,A | E=A. | ||||||||
| AE65 | LD A,($CE58) | The map row. | ||||||||
| AE68 | CP (IX+$04) | Jump to FindObjectScreenPosition_NextZone if A is not the byte at offset 04 of IX. | ||||||||
| AE6B | JR NZ,FindObjectScreenPosition_NextZone | |||||||||
| AE6D | LD A,L | A=L. | ||||||||
| AE6E | DEC A | Step A back. | ||||||||
| AE6F | CP E | Return if A is less than E. | ||||||||
| AE70 | RET C | |||||||||
| AE71 | ADD A,$61 | Add 61 to A. | ||||||||
| AE73 | LD L,A | L=A. | ||||||||
| FindObjectScreenPosition_StoreY | AE74 | LD A,($CE42) | The vertical offset. | |||||||
| AE77 | ADD A,L | A gains L. | ||||||||
| AE78 | LD ($A1D6),A | The vertical screen position. | ||||||||
| AE7B | LD (IX+$12),A | Write the vertical screen position. | ||||||||
| AE7E | SET 7,(IX+$10) | Set bit 7 of offset 10 of IX. | ||||||||
| AE82 | RET | Return. | ||||||||
| FindObjectScreenPosition_NextZone | AE83 | INC A | Advance A. | |||||||
| AE84 | CP (IX+$04) | Jump to FindObjectScreenPosition_Free if A is not the byte at offset 04 of IX. | ||||||||
| AE87 | JP NZ,FindObjectScreenPosition_Free | |||||||||
| AE8A | LD A,L | A=L. | ||||||||
| AE8B | SUB C | Subtract C from A. | ||||||||
| AE8C | JR C,FindObjectScreenPosition_StoreY | Jump to FindObjectScreenPosition_StoreY if the carry flag is set. | ||||||||
| AE8E | CP E | Return if A is E or more. | ||||||||
| AE8F | RET NC | |||||||||
| AE90 | JP FindObjectScreenPosition_StoreY | Jump to FindObjectScreenPosition_StoreY. | ||||||||
| FindObjectScreenPosition_Free | AE93 | LD (IX+$00),$FF | Store FF at offset 00 of IX. | |||||||
| AE97 | RET | Return. | ||||||||
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