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Routines |
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Used by the routine at CheckTerrainCollision.
Adds the horizontal speed of the ship to the fraction in HorizontalScrollFraction and works out how many whole pixels of horizontal scroll that makes, limited by the value in HorizontalLimit in the same way as UpdateVerticalScrollSpeed for the vertical scroll. The result is stored in HorizontalSpeed.
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| UpdateHorizontalScrollSpeed | C81F | LD DE,($C7FA) | Fetch the word at HorizontalScrollFraction into DE. | |||||
| C823 | ADD HL,DE | HL gains DE. | ||||||
| C824 | LD ($C7FA),HL | Update the horizontal scroll fraction. | ||||||
| C827 | LD A,H | A=H. | ||||||
| C828 | SUB D | Subtract D from A. | ||||||
| C829 | ADD A,A | Double A. | ||||||
| C82A | LD H,A | H=A. | ||||||
| C82B | LD A,($5C3E) | The horizontal scroll limit. | ||||||
| C82E | BIT 7,H | Jump to UpdateHorizontalScrollSpeed_Negative if bit 7 of H is set. | ||||||
| C830 | JR NZ,UpdateHorizontalScrollSpeed_Negative | |||||||
| C832 | ADD A,$02 | Add 02 to A. | ||||||
| C834 | LD B,A | B=A. | ||||||
| C835 | JR UpdateHorizontalScrollSpeed_Limit | Jump to UpdateHorizontalScrollSpeed_Limit. | ||||||
| UpdateHorizontalScrollSpeed_Negative | C837 | NEG | Negate A. | |||||
| C839 | SUB $02 | Subtract 02 from A. | ||||||
| C83B | LD B,A | B=A. | ||||||
| UpdateHorizontalScrollSpeed_Limit | C83C | LD A,H | A=H. | |||||
| C83D | BIT 7,A | Jump to UpdateHorizontalScrollSpeed_Fast if bit 7 of A is set. | ||||||
| C83F | JR NZ,UpdateHorizontalScrollSpeed_Fast | |||||||
| C841 | CP B | Jump to UpdateHorizontalScrollSpeed_Store if A is not B. | ||||||
| C842 | JR NZ,UpdateHorizontalScrollSpeed_Store | |||||||
| C844 | LD A,($5C3E) | The horizontal scroll limit. | ||||||
| UpdateHorizontalScrollSpeed_Fast | C847 | CP B | Jump to UpdateHorizontalScrollSpeed_Store if A is not B. | |||||
| C848 | JR NZ,UpdateHorizontalScrollSpeed_Store | |||||||
| C84A | LD A,($5C3E) | The horizontal scroll limit. | ||||||
| C84D | NEG | Negate A. | ||||||
| UpdateHorizontalScrollSpeed_Store | C84F | LD ($CCBB),A | Update the horizontal scroll speed. | |||||
| C852 | CP $04 | Jump to UpdateHorizontalScrollSpeed_Halve if A is 04. | ||||||
| C854 | JR Z,UpdateHorizontalScrollSpeed_Halve | |||||||
| C856 | CP $FC | Return if A is not FC. | ||||||
| C858 | RET NZ | |||||||
| UpdateHorizontalScrollSpeed_Halve | C859 | LD A,($CE41) | The horizontal offset. | |||||
| C85C | AND %00000010 | Keep the bits of A that are set in 02. | ||||||
| C85E | RET Z | Return if the zero flag is set. | ||||||
| C85F | LD A,($CCBB) | The horizontal scroll speed. | ||||||
| C862 | SRA A | Shift A right keeping the sign. | ||||||
| C864 | LD ($CCBB),A | Update the horizontal scroll speed. | ||||||
| C867 | RET | Return. | ||||||
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